SQL

https://learnsql.com/blog/sql-window-functions-cheat-sheet/

SQL Server DateTime Best Practices-
https://www.mssqltips.com/sqlservertip/5206/sql-server-datetime-best-practices/

how to use the GROUP BY clause or ROW_NUMBER() function to find duplicate values in SQL Server. 

The query for finding the duplicate values in multiple columns using the GROUP BY clause :

SELECT col1,col2,... COUNT(*) FROM table_name GROUP BY col1,col2,... HAVING COUNT(*) > 1;

IMP-
SELECT a.*
FROM Dup_Practice a
JOIN (SELECT name, id, COUNT(*)as bb
FROM [Dup_Practice]
GROUP BY name,id
HAVING count(*) > 1) B
ON a.name = B.name and a.id=b.id

Using ROW_NUMBER() function to find duplicates in a table
The following statement uses the ROW_NUMBER() function to find duplicate rows based on both a and b columns:

WITH cte AS (
    SELECT 
        a, 
        b, 
        ROW_NUMBER() OVER (
            PARTITION BY a,b
            ORDER BY a,b) rownum
    FROM 
        t1
) 
SELECT 
  * 
FROM 
    cte 
WHERE 
    rownum > 1;

Here is the result:

How it works:

First, the ROW_NUMBER() distributes rows of the t1 table into partitions by values in the a and b columns. The duplicate rows will have repeated values in the a and b columns, but different row numbers as shown in the following picture:

Second, the outer query removes the first row in each group.

Generally, This statement uses the ROW_NUMBER() function to find the duplicate values in one column of a table:

WITH cte AS (
    SELECT 
        col,
        ROW_NUMBER() OVER (
            PARTITION BY col
            ORDER BY col) row_num
    FROM 
        t1
) 
SELECT * FROM cte 
WHERE row_num > 1;

FUNCTIONS:

It is a predefined program which returns one value, returned value can be of numeric, string data,

data type functions can be used for calculations

SINGLE ROW FUNCTION: This function will process one row at a time and returns one value.

 Mathematical Functions

 String Functions

 Date & Time Functions

 Data Conversion {Type Casting Functions}

 Ranking Functions {Windows Functions}

 Meta Data Functions

 Other Functions

MULTIPLE ROW FUNCTION: This function will process multiple rows at a time and returns one value.

 Aggregate Functions

SQL SERVER supports to work with 12 categories of Functions and they are found at following paths

 Click on Object Explorer – Data Bases

 Select the Data Base - TESTDB

 Check under Programmability Functions.

 Check under System Functions.

MATHEMATICAL FUNCTIONS:

This function will take input as numbers and will return output as numbers

ABS (NUMBER) - returns unsigned value of a given number (Number - Argument or Parameter.

SELECT ABS(-890), ABS(17)

SQRT (NUMBER) - Returns square root of a given positive number.

SQUARE (NUMBER) - Returns square of a given value

POWER ( NUMBER(Base), NUMBER(Exponent)) - It will find the power of a given number.

SELECT POWER (2,5)

SIGN ( NUMBER )

Returns 1 if a number is positive

Returns -1 if a number is negative

Returns 0 if a number is zero

PI() - Returns PI value

SIN (NUMBER) - By default this function will take input given in radians, hence radians should be

converted to degrees by a standard formula is PI ()/180

SELECT SIN (30* PI()/180)

Built In Functions

Or

Predefined Functions

Table Valued

Functions

Scalar

Functions

Multiple Row Functions

Or

Group Functions

Single Row Functions

Or

Individual Functions

User Defined Functions

Or

Stored Functions

**Functions used with in other function refers to FUNCTION CASCADING

ROUND ( NUMBER, NUMBER , [NUMBER])

Argument1, Argument2, Argument3 (optional for TRUNCATE)

By default Argument3 is ZERO

ROUND(1234.5678,2,1) --- 1234.5600

ROUND(1234.5678,2) ----- 1234.5700

ROUND(1234.5638,2) ----- 1234.5600

ROUND(1234.5678,1) ----- 1234.6000

ROUND(1234.5678,0) ----- 1235.0000

ROUND(1234.2678,0) ----- 1234.0000

ROUND(1234.5678,-2)----- 1200.0000

ROUND(5678.123,-2) ----- 5700.0000

ROUND(1234.5678,2,1) --- 1234.5600

ROUND(1234.5678,1,1) --- 1234.5000

ROUND(1234.5678,0,1) --- 1234.0000

ROUND(5/2.0,0) --- 3.00

ROUND(5/2.0,0,1) --- 2

This function is used to round a given number with respect to Integer value or Decimal value based

on user required number of digits. If 3rd argument is specified other than zero it works as TRUNCATE.

CEILING(NUMBER) – This function will increment a given number to its nearest integer. Based on

any digit in decimal points is greater than zero.

CEILING (123.000) ---- 123

CEILING (123.010) ---- 124

CEILING (123.456) ---- 124

FLOOR (NUMBER) – It decreases a nearest integer

FLOOR (-123.456) ---- {-124}

STRING FUNCTIONS: This are used to perform different operations on STRING DATA TYPE.

LEN(TEXT) - Returns number of characters.

SELECT ENAME, LEN(ENAME) FROM EMP WHERE LEN(ENAME) > 5

LOWER(STRING) – Converts characters to Lower case.

UPPER(STRING) – Converts characters to Upper case.

ASCII(STRING) – returns ASCII code of the given character

ASCII('A')----65

ASCII('P')----80

UNICODE( STRING ) – Returns UNICODE of a given string.

CHAR ( NUMBER ) – Returns character of a given ASCII code

CHAR (65) –- A { Only 0-255 characters are available}

NCHAR(NUMBER)

NCHAR(256) {1-65535 Characters are available}

LEFT(STRING,NUMBER) – Extracts number of characters from left side of a string

LEFT('COMPUTER',3) ---- COM

RIGHT( STRING, NUMBER) – Extracts number of characters from right side of a string

RIGHT('COMPUTER',3)-----TER

SUBSTRING (STRING, STARTPOSITION, NUMBER OF CHAR’S) – It extracts required string

from a given string based on start position and number of characters.

SUBSTRING ('COMPUTER',4,3)—PUT

LTRIM(STRING) - Removes blanks spaces from left side of a string.

RTRIM(STRING)- Removes blank spaces from right side of a string.

REPLICATE(STRING, NUMBER) - Displays a given string for n times.

REPLICATE('COMM',2)

REVERSE(STRING) - It will reverse a given string.

SELECT REVERSE('COMM')--- MMOC

REPLACE (STRING1, STRING2, STRING3)

String1 ---- given string, String2 ---- search string, String3 ---- replace string

REPLACE ('COMPUTER','UT','IR')

It will search for String2 in String1 and replaces with String3 at all occurrences of String 2 in String1

STUFF(STRING1, NUMBER1, NUMBER2, STRING2)

String1 --- Given String, Number 1--- Start Position, Number2 --- Number of Characters

String2 --- Replace

STUFF('COMPUTER',5,2,'IL') -- COMPILER

CHARINDEX(STRING1, STRING2, [NUMBER])

String1 --- Search String, String2 --- Given String, Number --- Position

It will search for string1 in string2 and returns position of string1, if it exists else returns zero, by

default it will search from first character. It also supports to search for character/string after a

specific position

SELECT CHARINDEX('R','COMPUTER',7)

SELECT REPLICATE ('*', GRADE) FROM SALGRADE

Q) Display all the employees from the EMP table with half of the string in upper case and next string

in lower case

SELECT LEFT (ENAME, CEILING (LEN (ENAME)/2.0)) +

LOWER(RIGHT (ENAME, CEILING (LEN (ENAME)/2))) FROM EMP

Q) Display all Employees whose name contains 3rd Character as ‘L’.

SELECT ENAME FROM EMP WHERE SUBSTRING (ENAME, 3,1)= 'L'

SELECT ENAME FROM EMP WHERE CHARINDEX ('L', ENAME, 3) = 3

Q) Display all Employees whose name contains A at second occurance.

SELECT ENAME FROM EMP

WHERE CHARINDEX ('A', ENAME, CHARINDEX ('A', ENAME) +1 !=0

DATE & TIME FUNCTIONS:

These functions are used to perform operations on date, time data type. When date type data is

retrieved it will be in the following format.

CENTURY: YEAR: MONTH: DATE: HOURS: MINUTES: SECONDS

GETDATE(): It returns current system DATE & TIME of the Server.

SELECT GETDATE()

ARITHMETIC OPERATIONS on data & time data type “+”, “-“ are the operations which can be used

for performing operations on date type data.

DATE + NUMBER = DATE

DATE – NUMBER = DATE

DATE – DATE = NUMBER [Number of days]

SELECT HIREDATE, HIREDATE+15, HIREDATE-28 FROM EMP

DAY (DATE) -------- extracts days from date

MONTH (DATE) ----- extracts months from date

YEAR (DATE) ------ extracts year from date

SELECT DAY(GETDATE())

SELECT MONTH(GETDATE())

SELECT YEAR(GETDATE())

DATEADD(DATEPART , NUMBER , DATE) - It is used to add number of days, months, years to a

given date.

DATEPART can be specified with this format - DD: MM: YY: HH: MI: SS: DW

SELECT DATEADD(DAY,10,GETDATE())

SELECT DATEADD(DD,10,GETDATE())

SELECT DATEADD(MONTH,10,GETDATE())

SELECT DATEADD(MM,10,GETDATE())

SELECT DATEADD(YEARS,10,GETDATE())

SELECT DATEADD(YY,10,GETDATE())

DATEDIFF( DATEPART, STARTDATE, ENDDATE) - It returns difference between 2 dates in

terms of days, months & years.

SELECT ENAME, DATEDIFF(DD,HIREDATE,GETDATE()) DAYS FROM EMP

DATEPART(DATEPART,DATE)- It extracts individual parts of a table.

DATEPART(DD,GETDATE()) – MM: YY: HH: MI: SS: DW

DATENAME(DATEPART,DATE) - In this function month name, day name will be extracted other

date parts providing same output.

DATENAME(MM,GETDATE()) ---- JUNE(Based on the current server date)

DATENAME(DW,GETDATE()) ---- MON

Q) Display all those Employees data that have been hired on Sunday.

SELECT ENAME, HIREDATE FROM EMP WHERE DATENAME(DW,HIREDATE) = 'SUNDAY'

Q) Display all those Employees who have been hired in the month of December.

SELECT ENAME,HIREDATE FROM EMP WHERE DATENAME(MM,HIREDATE)= 'DECEMBER'

Q) Display all those Employees who have been hired in second half of the week.

SELECT ENAME, HIREDATE FROM EMP

WHERE DATEPART (DW, HIREDATE) > = ROUND(7/2.0,0)

DATA CONVERSION:

 This function will support to convert the data stored with one type to the other type.

 SQL SERVER will convert the data with the support of following two functions.

CAST(SOURCE_DATA AS TARGET_DATA_TYPE)

CONVERT(TARGET_DATA_TYPE, SOURCE_DATA [NUMBER])

SELECT ENAME + ' WORKING AS ' + JOB + ' IN DEPT ' + CAST(DEPTNO AS

VARCHAR(3)) FROM EMP

** String and String can only be concatenated.

** String and Integer cannot be concatenated.

SELECT 5/ CAST(2 AS FLOAT)

SELECT 5/ CONVERT(FLOAT,2)

SELECT CONVERT(VARCHAR(30), GETDATE(),0)

SELECT CONVERT(VARCHAR(30), GETDATE(),8) for time

** 0 stands for Date formats, there are more than 100 Date formats.

** 8 & 108 are particularly assigned for time format

RANKING OR WINDOW FUNCTIONS

 These functions have been introduced from SQL 2005.

 This are used to calculate ranks on any type of data.

 These functions require a support of OVER clause, which requires a specification of ORDER

BY clause, followed by ORDER type.

 This function cannot be used at WHERE clause.

SQL SERVER will support to work with following Rank functions

ROW_NUMBER() – Provides unique row number for each row

RANK() – It calculates ranks with gaps

DENSE_RANK() – It calculates ranks with out gaps

NTILE(INT) – Provides the data in to groups / blocks. It will group the data based on the user

specified number and number of rows available in a table.

SELECT EMPNO, ENAME, SAL, ROW_NUMBER() OVER ( ORDER BY EMPNO ASC) RN

FROM EMP

SELECT EMPNO, ENAME, SAL, RANK() OVER(ORDER BY SAL DESC) RNK FROM EMP

SELECT EMPNO, ENAME, SAL, DENSE_RANK() OVER(ORDER BY SAL DESC) RNK

FROM EMP

SELECT EMPNO, ENAME, SAL, NTILE(4) OVER( ORDER BY EMPNO) NT FROM EMP

**The data will be grouped into four groups based on given NTILE number.

OTHER FUNCTIONS

ISNULL(EXPRESSION1, EXPRESSION2)

This Function will search for NULL values in Expression1 and Substitutes Expression2 at all the rows

of Expression1 where NULL values are found, Expression2 is depended on data type of Expression1.

SELECT ENAME, SAL, COMM, ISNULL(COMM,0) RS FROM EMP

SELECT ENAME, SAL, COMM, ISNULL(ENAME,'UNKNOWN') UN FROM EMP

SELECT ENAME ,SAL, COMM, ISNULL ( CAST(COMM AS VARCHAR(4),'NC') RS

FROM EMP

SELECT ENAME,SAL,COMM,SAL+ISNULL(COMM,0) RS FROM EMP

COALESCE(EXPRESSION1,EXPRESSION2……EXPRESSIONN) - It will search for NULL values and

Substitutes Expression2 at all rows of Expression1 where NULL values are found. It Substitutes

Expression3 if Expression2 contains NULL values so on it can have any number of expressions.

SELECT EMPNO,ENAME,MGR,SAL,COMM COALESCE(COMM,SAL,MGR,EMPNO,25) RS

FROM EMP

NULLIF( EXPRESSION1, EXPRESSION2) - It is used to compare two expressions of any data

type. If equal it returns NULL, if not equal returns value of Expression1

SELECT NULLIF(10,10) -----NULL

SELECT NULLIF(10,90) -----10

SELECT NULLIF(‘X’,’Y’)--- X

SELECT ENAME,LEN(ENAME)L1,JOB,LEN(JOB)L2 NULLIF(LEN(ENAME),LEN(JOB))RES

FROM EMP

CASE EXPRESSION

It supports to specify multiple conditions provided with their respective results. If no condition is

satisfied then it will provide default result.

Syntax:

CASE EXPRESSION

WHEN CONDITION1 THEN RESULT1

[ WHEN CONDITION2 THEN RESULT2 ]

ELSE

DEFAULT_RESULT

END [ALIAS_NAME]

Q) Display ENAME, SAL and INCREMENT the salary with difference %’s based on category of JOB.

Manager – 18%, Clerk – 15%, Others – 12%

SELECT ENAME,SAL,JOB,

CASE JOB

WHEN 'MANAGER' THEN SAL*18/100

WHEN 'CLERK' THEN SAL * 15/100

ELSE

SAL*12/100

END INCR

FROM EMP

Q) Display ENAME, DEPTNO, SAL and INCREMENT the Salary with different %’s on category of Dept

No 10 – 20%, Dept no 20 – 18%, Dept no 30 – 15%

SELECT ENAME, DEPTNO, SAL,

CASE DEPTNO

WHEN 10 THEN SAL* 20/100

WHEN 20 THEN SAL * 18/100

WHEN 30 THEN SAL*15/100

END INCR

FROM EMP

MULTIROW FUNCTIONS OR GROUP FUNCTIONS

AGGREGATE FUNCTIONS:

 This function will process multiple rows at a time and returns one value.

 SQL SERVER supports to work with following functions

 SUM (EXPRESSION) - finds the sum of values in given expression.

 AVG (EXPRESSION) - first finds the sum and then divide with number of values in the

expression.

 MAX (EXPRESSION) - finds the maximum value in the given expression.

 MIN (EXPRESSION) - finds the minimum value in the given expression.

 COUNT (EXPRESSION) --- returns number of values in a expression including duplicates.

 COUNT (DISTINCT (EXPRESSION) - returns number of values in an expression excluding

duplicates.

 COIUNT (*) - returns number of rows.

 COUNT_BIG (EXPRESSION) - returns number of values.

 GROUPING (EXPRESSION) - returns zero or one.

 Returns ZERO if there is no group

 Returns ONE if there is a group data

POINTS TO BE REMEMBERED

 Aggregate Function will exclude NULL values.

 When a SELECT statement is enclosed with Aggregate Functions, along with them writing

expressions that returns multiple rows is invalid.

 Aggregate functions cannot be used in where clause.

 In SQL SERVER Aggregate Functions cannot be cascaded.

SELECT SUM(SAL) FROM EMP

SELECT AVG(SAL) FROM EMP

SELECT SUM(COMM) R1, AVG(COMM) R2, COUNT(COMM)R3 FROM EMP

SELECT SUM(SAL), MAX(SAL) FROM EMP WHERE DEPTNO = 30

SELECT COUNT(SAL) FROM EMP

SELECT COUNT(DISTINCT(SAL) FROM EMP

SELECT COUNT(*) FROM EMP

To Count the Records

SP_SPACEUSED EMP

SELECT @@ROWCOUNT

SELECT SUM(SAL) , COUNT(*) FROM EMP WHERE JOB = 'SALESMAN'

SELECT DEPTNO,SUM(SAL),COUNT(*) FROM EMP GROUP BY DEPTNO

SELECT DEPTNO, JOB,SUM(SAL), COUNT(*) FROM EMP GROUP BY JOB, DEPTNO

GROUP BY: This clause performs two tasks

 Eliminates duplicates and always arranges the data in ascending order.

 The column used in group by clause, the same column can be used for displaying the output.

 When multiple columns are used it will eliminate if all column contains duplicates.

SELECT SUM(SAL), COUNT(*) FROM EMP GROUP BY DEPTNO

SELECT DEPTNO, SUM(SAL), COUNT(*) FROM EMP GROUP BY DEPTNO

SELECT JOB, SUM(SAL), COUNT(*) FROM EMP GROUP BY JOB

SELECT MGR, COUNT(*) FROM EMP GROUP BY MGR

SELECT DEPTNO, JOB, SUM(SAL), COUNT(*) FROM EMP GROUP BY JOB , DEPTNO

Q) Display DEPTNO, total salary and number of employee’s related to Dept No 20.

SELECT DEPTNO, SUM(SAL), COUNT(*) FROM EMP

WHERE DEPTNO = 20 GROUP BY DEPTNO

ROLLUP AND CUBE OPERATORS

ROLLUP:

 This operator should be used only at GROUP BY clause.

 It will calculate individual totals, group totals, and additional row is generated for grand

totals.

SELECT DEPTNO, SUM(SAL), COUNT(*) FROM EMP GROUP BY DEPTNO WITH ROLLUP

SELECT DEPTNO, JOB, SUM(SAL), COUNT(*) FROM EMP

GROUP BY DEPTNO, JOB WITH ROLLUP

SELECT DEPTNO, JOB, SUM(SAL),COUNT(*) , GROUPING(JOB) FROM EMP

GROUP BY DEPTNO, JOB WITH ROLLUP

SELECT DEPTNO ,

CASE GROUPING(JOB)

WHEN 0 THEN JOB

WHEN 1 THEN “ALL JOBS” -- Replaces NULLS With The Value

END DEP, SUM(SAL), COUNT(*)

FROM EMP GROUP BY DEPTNO, JOB WITH ROLLUP

CUBE:

 This operator is also used only at GROUP BY clause.

 It will support to perform individual totals, group totals, regroup totals and additional row is

generated for grand totals.

SELECT DEPTNO, JOB, SUM(SAL), COUNT(*) FROM EMP

GROUP BY DEPTNO, JOB WITH CUBE

Q) Display total salaries of individual departments in a single row.

SELECT

SUM (CASE DEPTNO WHEN 10 THEN SAL END) “D10”,

SUM (CASE DEPTNO WHEN 20 THEN SAL END) “D20”,

SUM (CASE DEPTNO WHEN 30 THEN SAL END) “D30”

FROM EMP

SELECT DISTINCT JOB,

SUM (CASE DEPTNO WHEN 10 THEN SAL END) “D10”,

SUM (CASE DEPTNO WHEN 20 THEN SAL END) “D20”,

SUM (CASE DEPTNO WHEN 30 THEN SAL END) “D30”,

SUM (SAL) “TOTAL SAL”

FROM EMP GROUP BY JOB

Best sql question for interview  

Q #1) What is SQL?

Answer: Structured Query Language SQL is a database tool that is used to create and access the database to support software applications.

Q #2) What are tables in SQL?

Answer: The table is a collection of record and its information at a single view.

Q #3) What are the different types of statements supported by SQL?

Answer:

There are 3 types of SQL statements:

a) DDL (Data Definition Language): It is used to define the database structure such as tables. It includes three statements such as CREATE, ALTER, and DROP.

Some of the DDL Commands are listed below:

CREATE: It is used for creating the table.

CREATE TABLE table_name column_name1 data_type(size), column_name2 data_type(size), column_name3 data_type(size),

ALTER: The ALTER table is used for modifying the existing table object in the database.

ALTER TABLE table_name  ADD column_name datatype

OR

ALTER TABLE table_name DROP COLUMN column_name

b) DML (Data Manipulation Language): These statements are used to manipulate the data in records. Commonly used DML statements are INSERT, UPDATE, and DELETE.

The SELECT statement is used as a partial DML statement, used to select all or relevant records in the table.

c) DCL (Data Control Language): These statements are used to set privileges such as GRANT and REVOKE database access permission to the specific user.

Q #4) How do we use the DISTINCT statement? What is its use?

Answer: The DISTINCT statement is used with the SELECT statement. If the record contains duplicate values then the DISTINCT statement is used to select different values among duplicate records.

Syntax:

SELECT DISTINCT column_name(s)  FROM table_name;

Q #5) What are different Clauses used in SQL?

Answer:

WHERE Clause: This clause is used to define the condition, extract and display only those records which fulfill the given condition.

Syntax:

SELECT column_name(s)  FROM table_name  WHERE condition;

GROUP BY Clause: It is used with SELECT statement to group the result of the executed query using the value specified in it. It matches the value with the column name in tables and groups the end result accordingly.

Syntax:

SELECT column_name(s)  FROM table_name  GROUP BY column_name;

HAVING clause: This clause is used in association with the GROUP BY clause. It is applied to each group of results or the entire result as a single group. It is much similar as WHERE clause but the only difference is you cannot use it without GROUP BY clause

Syntax:

SELECT column_name(s)  FROM table_name  GROUP BY column_name  HAVING condition;

ORDER BY clause: This clause is used to define the order of the query output either in ascending (ASC) or in descending (DESC). Ascending (ASC) is set as the default one but descending (DESC) is set explicitly.

Syntax:

SELECT column_name(s)  FROM table_name  WHERE condition  ORDER BY column_name ASC|DESC;

USING clause: USING clause comes in use while working with SQL JOIN. It is used to check equality based on columns when tables are joined. It can be used instead of the ON clause in JOIN.

Syntax:

SELECT column_name(s)  FROM table_name  JOIN table_name  USING (column_name);

Q #6) Why do we use SQL constraints? Which constraints we can use while creating a database in SQL?

Answer: Constraints are used to set the rules for all records in the table. If any constraints get violated then it can abort the action that caused it.

Constraints are defined while creating the database itself with the CREATE TABLE statement or even after the table is created once with the ALTER TABLE statement.

There are 5 major constraints are used in SQL, such as

• NOT NULL: That indicates that the column must have some value and cannot be left NULL.
• UNIQUE: This constraint is used to ensure that each row and column has a unique value and no value is being repeated in any other row or column.
• PRIMARY KEY: This constraint is used in association with NOT NULL and UNIQUE constraints such as on one or the combination of more than one column to identify the particular record with a unique identity.
• FOREIGN KEY: It is used to ensure the referential integrity of data in the table. It matches the value in one table with another using the PRIMARY KEY.
• CHECK: It ensures whether the value in columns fulfills the specified condition.

Q #7) What are different JOINS used in SQL?

Answer:

4 major types of Joins are used while working on multiple tables in SQL databases:

INNER JOIN: It is also known as SIMPLE JOIN which returns all rows from BOTH tables when it has at least one matching column.

Syntax:

SELECT column_name(s)  FROM table_name1   INNER JOIN table_name2  ON column_name1=column_name2;

For Example,

In this example, we have a table Employee with the following data:

The second table's name is Joining.

Enter the following SQL statement:

SELECT Employee.Emp_id, Joining.Joining_Date   FROM Employee   INNER JOIN Joining   ON Employee.Emp_id = Joining.Emp_id   ORDER BY Employee.Emp_id;

There will be 4 records selected. Results are:

Employee and Orders tables have a matching customer_id value.

LEFT JOIN (LEFT OUTER JOIN): This join returns all rows from the LEFT table and its matched rows from a RIGHT table.

Syntax:

SELECT column_name(s)  FROM table_name1  LEFT JOIN table_name2  ON column_name1=column_name2;

For Example,

In this example, we have a table Employee with the following data:

The second table's name is Joining.

Enter the following SQL statement:

SELECT Employee.Emp_id, Joining.Joining_Date FROM Employee LEFT OUTER JOIN Joining ON Employee.Emp_id = Joining.Emp_id ORDER BY Employee.Emp_id;

There will be 4 records selected. You will see the following results:

RIGHT JOIN (RIGHT OUTER JOIN): This joins returns all rows from the RIGHT table and its matched rows from the LEFT table.

Syntax:

SELECT column_name(s) FROM table_name1 RIGHT JOIN table_name2 ON column_name1=column_name2;

For Example,

In this example, we have a table Employee with the following data:

The second table's name is Joining.

Enter the following SQL statement:

SELECT Employee.Emp_id, Joining.Joining_Date FROM Employee RIGHT JOIN Joining ON Employee.Emp_id = Joining.Emp_id ORDER BY Employee.Emp_id;

Output:

Emp_id	Joining_Date
E0012	2016/04/18
E0013	2016/04/19
E0014	2016/05/01

FULL JOIN (FULL OUTER JOIN): This joins returns all results when there is a match either in the RIGHT table or in the LEFT table.

Syntax:

SELECT column_name(s)  FROM table_name1  FULL OUTER JOIN table_name2  ON column_name1=column_name2;

For Example,

In this example, we have a table Employee with the following data:

The second table's name is Joining.

Enter the following SQL statement:

SELECT Employee.Emp_id, Joining.Joining_Date FROM Employee FULL OUTER JOIN Joining ON Employee.Emp_id = Joining.Emp_id ORDER BY Employee.Emp_id;

There will be 8 records selected. These are the results that you should see.

Q #8) What are transactions and their controls?

Answer: A transaction can be defined as the sequence task that is performed on databases in a logical manner to gain certain results. Operations like Creating, updating, deleting records performed in the database come from transactions.

In simple words, we can say that a transaction means a group of SQL queries executed on database records.

There are 4 transaction controls such as

• COMMIT: It is used to save all changes made through the transaction.
• ROLLBACK: It is used to roll back the transaction. All changes made by the transaction are reverted back and the database remains as before.
• SET TRANSACTION: Set the name of the transaction.
• SAVEPOINT: It is used to set the point where the transaction is to be rolled back.

Q #9) What are the properties of the transaction?

Answer: Properties of the transaction are known as ACID properties. These are:

• Atomicity: Ensures the completeness of all transactions performed. Checks whether every transaction is completed successfully or not. If not, then the transaction is aborted at the failure point and the previous transaction is rolled back to its initial state as changes are undone.
• Consistency: Ensures that all changes made through successful transactions are reflected properly on the database.
• Isolation: Ensures that all transactions are performed independently and changes made by one transaction are not reflected on others.
• Durability: Ensures that the changes made in the database with committed transactions persist as it is even after a system failure.

Q #10) How many Aggregate functions are available in SQL?

Answer: SQL Aggregate functions determine and calculate values from multiple columns in a table and return a single value.

There are 7 aggregate functions in SQL:

• AVG(): Returns the average value from specified columns.
• COUNT(): Returns number of table rows.
• MAX(): Returns the largest value among the records.
• MIN(): Returns smallest value among the records.
• SUM(): Returns the sum of specified column values.
• FIRST(): Returns the first value.
• LAST(): Returns last value.

Q #11) What are Scalar functions in SQL?

Answer: Scalar functions are used to return a single value based on the input values.

Scalar Functions are as follows:

• UCASE(): Converts the specified field in the upper case.
• LCASE(): Converts the specified field in lower case.
• MID(): Extracts and returns character from the text field.
• FORMAT(): Specifies the display format.
• LEN(): Specifies the length of the text field.
• ROUND(): Rounds up the decimal field value to a number.

Q #12) What are triggers?

Answer: Triggers in SQL is kind of stored procedures used to create a response to a specific action performed on the table such as INSERT, UPDATE or DELETE. You can invoke triggers explicitly on the table in the database.

Action and Event are two main components of SQL triggers. When certain actions are performed, the event occurs in response to that action.

Syntax:

CREATE TRIGGER name {BEFORE|AFTER} (event [OR..]} ON table_name [FOR [EACH] {ROW|STATEMENT}] EXECUTE PROCEDURE functionname {arguments}

Q #13) What is View in SQL?

Answer: A View can be defined as a virtual table that contains rows and columns with fields from one or more tables.

Syntax:

CREATE VIEW view_name AS SELECT column_name(s) FROM table_name WHERE condition

Q #14) How we can update the view?

Answer: SQL CREATE and REPLACE can be used for updating the view.

Execute the below query to update the created view.

Syntax:

CREATE OR REPLACE VIEW view_name AS  SELECT column_name(s)  FROM table_name  WHERE condition

Q #15) Explain the working of SQL Privileges?

Answer: SQL GRANT and REVOKE commands are used to implement privileges in SQL multiple user environments. The administrator of the database can grant or revoke privileges to or from users of database objects by using commands like SELECT, INSERT, UPDATE, DELETE, ALL, etc.

GRANT Command: This command is used to provide database access to users other than the administrator.

Syntax:

GRANT privilege_name  ON object_name  TO {user_name|PUBLIC|role_name}  [WITH GRANT OPTION];

In the above syntax, the GRANT option indicates that the user can grant access to another user too.

REVOKE command: This command is used to provide database deny or remove access to database objects.

Syntax:

REVOKE privilege_name  ON object_name  FROM {user_name|PUBLIC|role_name};

Q #16) How many types of Privileges are available in SQL?

Answer: There are two types of privileges used in SQL, such as

• System privilege: System privilege deals with the object of a particular type and provides users the right to perform one or more actions on it. These actions include performing administrative tasks, ALTER ANY INDEX, ALTER ANY CACHE GROUP CREATE/ALTER/DELETE TABLE, CREATE/ALTER/DELETE VIEW etc.
• Object privilege: This allows to perform actions on an object or object of another user(s) viz. table, view, indexes etc. Some of the object privileges are EXECUTE, INSERT, UPDATE, DELETE, SELECT, FLUSH, LOAD, INDEX, REFERENCES etc.

Q #17) What is SQL Injection?

Answer: SQL Injection is a type of database attack technique where malicious SQL statements are inserted into an entry field of database in a way that once it is executed, the database is exposed to an attacker for the attack. This technique is usually used for attacking data-driven applications to have access to sensitive data and perform administrative tasks on databases.

For Example,

SELECT column_name(s) FROM table_name WHERE condition;

Q #18) What is SQL Sandbox in SQL Server?

Answer: SQL Sandbox is a safe place in the SQL server environment where untrusted scripts are executed. There are 3 types of SQL sandbox:

• Safe Access Sandbox: Here a user can perform SQL operations such as creating stored procedures, triggers etc. but cannot have access to the memory as well as cannot create files.
• External Access Sandbox: Users can access files without having the right to manipulate the memory allocation.
• Unsafe Access Sandbox: This contains untrusted codes where a user can have access to memory.

Q #19) What is the difference between SQL and PL/SQL?

Answer: SQL is a Structured Query Language to create and access databases whereas PL/SQL comes with procedural concepts of programming languages.

Q #20) What is the difference between SQL and MySQL?

Answer: SQL is a Structured Query Language that is used for manipulating and accessing the relational database. On the other hand, MySQL itself is a relational database that uses SQL as the standard database language.

Q #21) What is the use of the NVL function?

Answer: NVL function is used to convert the null value to its actual value.

Q #22) What is the Cartesian product of the table?

Answer: The output of Cross Join is called a Cartesian product. It returns rows combining each row from the first table with each row of the second table. For Example, if we join two tables having 15 and 20 columns the Cartesian product of two tables will be 15×20=300 rows.

Q #23) What do you mean by Subquery?

Answer: Query within another query is called as Subquery. A subquery is called inner query which returns output that is to be used by another query.

Q #24) How many row comparison operators are used while working with a subquery?

Answer: There are 3-row comparison operators that are used in subqueries such as IN, ANY and ALL.

Q #25) What is the difference between clustered and non-clustered indexes?

Answer: The differences between the two are as follows:

• One table can have only one clustered index but multiple non-clustered indexes.
• Clustered indexes can be read rapidly rather than non-clustered indexes.
• Clustered indexes store data physically in the table or view whereas, non-clustered indexes do not store data in the table as it has separate structure from the data row.

Q #26) What is the difference between DELETE and TRUNCATE?

Answer: The differences are:

• The basic difference in both is DELETE command is DML command and the TRUNCATE command is DDL.
• DELETE command is used to delete a specific row from the table whereas the TRUNCATE command is used to remove all rows from the table.
• We can use the DELETE command with WHERE clause but cannot use the TRUNCATE command with it.

Q #27) What is the difference between DROP and TRUNCATE?

Answer: TRUNCATE removes all rows from the table which cannot be retrieved back, DROP removes the entire table from the database and it also cannot be retrieved back.

Q #28) How to write a query to show the details of a student from Students table whose
name start with K?

Answer: Query:

SELECT * FROM Student WHERE Student_Name like ‘K%’;

Here ‘like’ operator is used to perform pattern matching.

Q #29) What is the difference between Nested Subquery and Correlated Subquery?

Answer: Subquery within another subquery is called Nested Subquery.  If the output of a subquery depends on column values of the parent query table then the query is called Correlated Subquery.

SELECT adminid(SELEC Firstname+' '+Lastname  FROM Employee WHERE  empid=emp. adminid)AS EmpAdminId FROM Employee;

The result of the query is the details of an employee from the Employee table.

Q #30) What is Normalization? How many Normalization forms are there?

Answer: Normalization is used to organize the data in such a manner that data redundancy will never occur in the database and avoid insert, update and delete anomalies.

There are 5 forms of Normalization:

• First Normal Form (1NF): It removes all duplicate columns from the table. It creates a table for related data and identifies unique column values.
• First Normal Form (2NF): Follows 1NF and creates and places data subsets in an individual table and defines the relationship between tables using the primary key.
• Third Normal Form (3NF): Follows 2NF and removes those columns which are not related through the primary key.
• Fourth Normal Form (4NF): Follows 3NF and does not define multi-valued dependencies. 4NF is also known as BCNF.

Q #31) What is a Relationship? How many types of Relationships are there?

Answer: The relationship can be defined as the connection between more than one table in the database.

There are 4 types of relationships:

• One to One Relationship
• Many to One Relationship
• Many to Many Relationship
• One to Many Relationship

Q #32) What do you mean by Stored Procedures? How do we use it?

Answer: A stored procedure is a collection of SQL statements that can be used as a function to access the database. We can create these stored procedures earlier before using it and can execute them wherever required by applying some conditional logic to it. Stored procedures are also used to reduce network traffic and improve performance.

Syntax:

CREATE Procedure Procedure_Name  (  //Parameters  )  AS  BEGIN  SQL statements in stored procedures to update/retrieve records  END

Q #33) State some properties of Relational databases?

Answer: Properties are as follows:

• In relational databases, each column should have a unique name.
• The sequence of rows and columns in relational databases is insignificant.
• All values are atomic and each row is unique.

Q #34) What are Nested Triggers?

Answer: Triggers may implement data modification logic by using INSERT, UPDATE, and DELETE statements. These triggers that contain data modification logic and find other triggers for data modification are called Nested Triggers.

Q #35) What is a Cursor?

Answer: A cursor is a database object which is used to manipulate data in a row-to-row manner.

Cursor follows steps as given below:

• Declare Cursor
• Open Cursor
• Retrieve row from the Cursor
• Process the row
• Close Cursor
• Deallocate Cursor

Q #36) What is Collation?

Answer: Collation is a set of rules that check how the data is sorted by comparing it. Such as character data is stored using correct character sequence along with case sensitivity, type, and accent.

Q #37) What do we need to check in Database Testing?

Answer: In Database testing, the following thing is required to be tested:

• Database connectivity
• Constraint check
• Required application field and its size
• Data Retrieval and processing with DML operations
• Stored Procedures
• Functional flow

Q #38) What is Database White Box Testing?

Answer: Database White Box testing involves:

• Database Consistency and ACID properties
• Database triggers and logical views
• Decision Coverage, Condition Coverage, and Statement Coverage
• Database Tables, Data Model, and Database Schema
• Referential integrity rules

Q #39) What is Database Black Box Testing?

Answer: Database Black Box testing involves:

• Data Mapping
• Data stored and retrieved
• Use of Black Box testing techniques such as Equivalence Partitioning and Boundary Value Analysis (BVA)

Q #40) What are Indexes in SQL?

Answer: The index can be defined as the way to retrieve the data more quickly. We can define indexes using CREATE statements.

Syntax:

CREATE INDEX index_name  ON table_name (column_name)

Further, we can also create a Unique Index using the following syntax:

CREATE UNIQUE INDEX index_name  ON table_name (column_name)

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UPDATE: We have added few more short questions for practice.

Q #41) What does SQL stand for?

Answer: SQL stands for Structured Query Language.

Q #42) How to select all records from the table?

Answer: To select all the records from the table we need to use the following syntax:

Select * from table_name;

Q #43) Define join and name different types of joins?

Answer: Join keyword is used to fetch data from two or more related tables. It returns rows where there is at least one match in both the tables included in the join. Read more here.
Type of joins are:

1. Right join
2. Outer join
3. Full join
4. Cross join
5. Self join.

Q #44) What is the syntax to add a record to a table?

Answer: To add a record in a table INSERT syntax is used.

For Example,

INSERT into table_name VALUES (value1, value2..);

Q #45) How do you add a column to a table?

Answer: To add another column in the table, use the following command:

ALTER TABLE table_name ADD (column_name);

Q #46) Define the SQL DELETE statement.

Answer: DELETE is used to delete a row or rows from a table based on the specified condition.
The basic syntax is as follows:

DELETE FROM table_name WHERE <Condition>

Q #47) Define COMMIT?

Answer: COMMIT saves all changes made by DML statements.

Q #48) What is the Primary key?

Answer: A Primary key is a column whose values uniquely identify every row in a table. Primary key values can never be reused.

Q #49) What are Foreign keys?

Answer: When a table’s primary key field is added to related tables in order to create the common field which relates the two tables, it called a foreign key in other tables. Foreign key constraints enforce referential integrity.

Q #50) What is CHECK Constraint?

Answer: A CHECK constraint is used to limit the values or type of data that can be stored in a column. They are used to enforce domain integrity.

Q #51) Is it possible for a table to have more than one foreign key?

Answer: Yes, a table can have many foreign keys but only one primary key.

Q #52) What are the possible values for the BOOLEAN data field?

Answer: For a BOOLEAN data field, two values are possible: -1(true) and 0(false).

Q #53) What is a stored procedure?

Answer: A stored procedure is a set of SQL queries that can take input and send back output.

Q #54) What is identity in SQL?

Answer: An identity column in where SQL automatically generates numeric values. We can define a start and increment value of the identity column.

Q #55) What is Normalization?

Answer: The process of table design to minimize the data redundancy is called normalization. We need to divide a database into two or more table and define the relationship between them.

Q #56) What is a Trigger?

Answer: The Trigger allows us to execute a batch of SQL code when table event occurs (INSERT, UPDATE or DELETE commands are executed against a specific table).

Q #57) How to select random rows from a table?

Answer: Using a SAMPLE clause we can select random rows.

For Example,

SELECT * FROM table_name SAMPLE(10);

Q #58) Which TCP/IP port does SQL Server run?

Answer: By default SQL Server runs on port 1433.

Q #59) Write a SQL SELECT query that only returns each name only once from a table?

Answer: To get the result as each name only once, we need to use the DISTINCT keyword.

SELECT DISTINCT name FROM table_name;

Q #60) Explain DML and DDL?

Answer: DML stands for Data Manipulation Language. INSERT, UPDATE and DELETE  are DML statements.

DDL stands for Data Definition Language. CREATE, ALTER, DROP, RENAME are DDL statements.

Q #61) Can we rename a column in the output of the SQL query?

Answer: Yes, using the following syntax we can do this.

SELECT column_name AS new_name FROM table_name;

Q #62) Give the order of SQL SELECT?

Answer: Order of SQL SELECT clauses is: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY. Only the SELECT and FROM clauses are mandatory.

Q #63) Suppose a Student column has two columns, Name and Marks. How to get names and marks of the top three students.
Answer: SELECT Name, Marks FROM Student s1 where 3 <= (SELECT COUNT(*) FROM Students s2 WHERE s1.marks = s2.marks)

Q #64) What is SQL comments?

Answer: SQL comments can be inserted by adding two consecutive hyphens (–).

Q #65) Difference between TRUNCATE, DELETE and DROP commands?

Answer:

• DELETE removes some or all rows from a table based on the condition. It can be rolled back.
• TRUNCATE removes ALL rows from a table by de-allocating the memory pages. The operation cannot be rolled back
• DROP command removes a table from the database completely.

Q #66) What are the properties of a transaction?

Answer: Generally, these properties are referred to as ACID properties. They are:

1. Atomicity
2. Consistency
3. Isolation
4. Durability.

Q #67) What do you mean by ROWID?

Answer: It’s an 18 character long pseudo column attached with each row of a table.

Q #68) Define UNION, MINUS, UNION ALL, INTERSECT?

Answer:

• MINUS – returns all distinct rows selected by the first query but not by the second.
• UNION – returns all distinct rows selected by either query
• UNION ALL – returns all rows selected by either query, including all duplicates.
• INTERSECT – returns all distinct rows selected by both queries.

Q #69) What is a transaction?

Answer: A transaction is a sequence of code that runs against a database. It takes the database from one consistent state to another.

Q #70) What is the difference between UNIQUE and PRIMARY KEY constraints?

Answer: The differences are as follows:

• A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys.
• The primary key cannot contain Null values whereas the Unique key can contain Null values.

Q #71) What is a composite primary key?

Answer: The primary key created on more than one column is called composite primary key.

Q #72) What is an Index?

Answer: An Index is a special structure associated with a table to speed up the performance of queries. The index can be created on one or more columns of a table.

Q #73) What is the Subquery?

Answer: A Subquery is a subset of select statements whose return values are used in filtering conditions of the main query.

Q #74) What do you mean by query optimization?

Answer: Query optimization is a process in which a database system compares different query strategies and select the query with the least cost.

Q #75) What is Collation?

Answer: Set of rules that define how data is stored, how case-sensitivity and Kana character can be treated etc.

Q #76) What is Referential Integrity?

Answer: Set of rules that restrict the values of one or more columns of the tables based on the values of the primary key or unique key of the referenced table.

Q #77) What is the Case function?

Answer: Case facilitates if-then-else type of logic in SQL. It evaluates a list of conditions and returns one of the multiple possible result expressions.

Q #78) Define a temp table?

Answer: A temp table is a temporary storage structure to store the data temporarily.

Q #79) How can we avoid duplicating records in a query?

Answer: By using the DISTINCT keyword, duplication of records in a query can be avoided.

Q #80) Explain the difference between Rename and Alias?

Answer: Rename is a permanent name given to a table or column whereas Alias is a temporary name given to a table or column.

Q #81) What is a View?

Answer: A view is a virtual table that contains data from one or more tables. Views restrict data access of the table by selecting only required values and make complex queries easy.

Q #82) What are the advantages of Views?

Answer: Advantages of Views are:

• Views restrict access to the data because the view can display selective columns from the table.
• Views can be used to make simple queries to retrieve the results of complicated queries. For Example, views can be used to query information from multiple tables without the user knowing.

Q #83) List the various privileges that a user can grant to another user?

Answer:  SELECT, CONNECT, RESOURCES.

Q #84) What is schema?

Answer: A schema is a collection of database objects of a User.

Q #85) What is a Table?

Answer: A table is the basic unit of data storage in the database management system. Table data is stored in rows and columns.

Q #86) Does View contain Data?

Answer: No, Views are virtual structures.

Q #87) Can a View based on another View?

Answer: Yes, A View is based on another View.

Q #88) What is the difference between the HAVING clause and WHERE clause?

Answer: Both specify a search condition but Having clause is used only with the SELECT statement and typically used with GROUP BY clause.
If GROUP BY clause is not used then Having behaved like WHERE clause only.

Q #89) What is the difference between Local and Global temporary tables?

Answer: If defined inside a compound statement a local temporary table exists only for the duration of that statement but a global temporary table exists permanently in the DB but its rows disappear when the connection is closed.

Q #90) What is CTE?

Answer: A CTE or common table expression is an expression that contains temporary result set which is defined in a SQL statement.