There are two types of subqueries: scalar and multi-row. Scalar subqueries return a single value or row/column, while multi-row subqueries return multiple rows or columns. Correlated subqueries rely on information from the outer query. Subqueries can reside in the WHERE, FROM, or SELECT clauses and provide an alternative to complex joins/unions while making queries more readable by isolating each part. Correlated subqueries may be evaluated once per outer row and use outer query values, while non-correlated subqueries execute once independently of the outer query.

1. Oracle Sub Queries

2. Type of Sub - Queries
• There are two type of sub – queries
• Scalar subqueries(Single Row Subqueries) return a single value, or exactly
one row and exactly one column.
• Multi row subqueries return either:
• One column with multiple rows (i.e. a list of values), or
• Multiple columns with multiple rows (i.e. tables).
• Correlated subqueries, where the inner query relies on information obtained
from the outer query.

3. What is sub-queries
• subquery is a query within a query.
• You can create subqueries within your SQL statements.
• These subqueries can reside in the WHERE clause, the FROM clause,
or the SELECT clause.

4. Advantages of Oracle subqueries
• Provide an alternative way to query data that would require
complex joins and unions.
• Make the complex queries more readable.
• Allow a complex query to be structured in a way that it is possible to
isolate each part.

5. What is co-related sub-queries
• A correlated subquery is a subquery that uses values from the outer
query.
• A correlated subquery may be evaluated once for each row selected
by the outer query.

6. Difference between sub-queries and
correlated sub-queries
Sub-Queries
1. The inner query is executed only once.
2. The inner query will get executed first and
based on inner query results out query gets
executed.
3. Inner query is not dependent on outer query
Correlated Sub-Queries
1. Inner query is executed multiple time based on
no of rows of outer query.
2. The outer query will get executed first and for
every row of outer query, inner query will get
executed.
3. Inner query is dependent on outer query.

7. Example
• Sub-Queries Where Clouse
SELECT first_name, last_name
FROM collectors
WHERE id IN (
SELECT collector_id
FROM sales);

8. Example
• Correlated Sub-Queries Where Clouse
Select *
From Employee
Where Sal > (SELECT AVG(SAL)
FROM EMPLOYEE);

9. Example
• Correlated Sub-Queries From Clouse
Select EMPNO, ENAME, SAL, DEPTNO, (SELECT DNAME FROM
DEPARTMENT WHERE DEPTNO = A.DEPTNO)
From Employee A;

10. Example
• Correlated Sub-Queries in case of update
UPDATE DEPARTMENT A
SET TOTAL_SAL = (SELECT SUM(SAL)
FROM EMPLOYEE
WHERE DEPTNO = A.DEPTNO)

11. Example
• Use of Correlated Sub-Queries in case of deleting duplicate rows
DELETE FROM EMPLOYEE A
WHERE ROWNUM <> (SELECT MIN(ROWNUM)
FROM EMPLOYEE
WHERE EMPNO = A.EMPNO)