Nested subqueries contain a subquery within another subquery to retrieve data from a table. Correlated subqueries reference columns from the outer query so the inner query is executed once for each row returned by the outer query. This document provides examples of using nested and correlated subqueries to find the third highest salary or details of employees with specific highest salaries from an employee table.

1. SUBQUERIES:
1. Nested sub queries
2. Correlated sub queries
Nested sub queries:
Query within another sub queries is called nested queries
It contains three select statements they are
1. Main query
2. Sub query
3. Nested sub query
TO GET THIRD HIGHEST SALARY FROM EMPLOYEE TABLE
SELECT MAX(SAL) FROM EMPLOYEE WHERE SAL<MAIN QUERY
(SELECT MAX(SAL)FROM EMPLOYEE WHERE SAL<SUB QUERY
(SELECT MAX(SAL)FROM EMPLOYEE))NESTED SUB QUERY
TO GET DETAILS OF EMPLOYEE WITH SECOND HIGHEST SALARY
SELECT * FROM EMPLOYEE WHERE SAL>
(SELECT MAX(SAL)FROM EMPLOYEE WHERE SAL>
(SELECT MAX(SAL)FROM EMPLOYEE))
[EXECUTION STARTS FROM 3RD LINE]
TO GET THE DETAILS OF THE EMPLOYEE WITH 4TH HIGHEST SALARY
SELECT * FROM EMPLOYEE WHERE SAL>
(SELECT TOP1 SAL FROM(SELECT DISTINCT TOP4
SAL FROM EMPLOYEE ORDERBY SAL DESC)S
ORDERBY SAL)
NOTE:
Temporary Table Name S Contains the Result of Nested Sub query I.E. Four Highest Salaries from
Employee Table.

2. CORRELLATED SUBQUERIES:
1. Main Query Will Execute First And The Value Of The Main Query Will Be Used By Correlated Sub
query Where Main Query Is Executed.
2.The Copy Of The Table Will Be Stored In Memory And Correlated Sub query Will Checks The Values
In Memory And Will Give Ranks For The Values In Memory And According To The Ranks The Resul ts
Will Be Displayed.
3. Sub Queries Will Work Fast And Correlated Sub Queries Will Work Slow Because Correlated Sub
Query Will Have Repetition.
TO GET DETAILS OF EMPLOYEE WITH FIRST HIGHEST SALARY
SELECT * FROM EMPLOYEE E WHERE O=
(SELECT COUNT(*) FROM EMPLOYEE M
WHERE M.SAL>E.SAL)
TO DISPLAY DUPLICATE RECORDS FROM EMPLOYEE TABLE
SELECT ENO,ENAME,SAL COUNT(*) FROM EMPLOYEE
GROUPBY ENO,ENAME,SAL
HAVING COUNT(*)>1