A subquery is a SELECT statement embedded within another SQL statement, usually in the WHERE clause. Subqueries allow data to be returned from multiple tables in an alternate way. They can be used with SELECT, INSERT, UPDATE, and DELETE statements along with comparison operators. Subqueries execute first before the main query uses their results. There are two types: single-row subqueries return one row while multiple-row subqueries return more than one row.

1. SQL SUBQUERY
S.M. TOWHIDUL ISLAM
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2. WHAT IS A SUBQUERY?
 A subquery is a SELECT statement embedded in a clause of another SQL statement (SELECT, INSERT,
UPDATE, DELETE).
 SQL subquery is usually added in the WHERE Clause of the SQL statement.
SELECT . . .
FROM . . .
WHERE . . .
(SELECT . . .
FROM . . .
WHERE . . .)
Main
Query
Subquery

3. SUBQUERY
 Subquery or Inner query or Nested query.
 An alternate way of returning data from multiple tables.
 Subqueries can be used with the following SQL statements along with the comparison operators like =,
<, >, >=, <= etc.
 Subqueries can also be used inside the WHERE or HAVING clause.
SELECT select_list
From table
WHERE OOP operator SELECT select_list
From table

4. SUB-QUERY SYNTAX
The subquery (inner query) executes once
before the main query.
 The result of the subquery is used by the
main query (outer query).

5. TYPES OF SUBQUERIES

6. TYPES OF SUBQUERIES
 Single-row subqueries: Queries that return only one row from the inner SELECT statement.

7. TYPES OF SUBQUERIES
 Multiple-row subqueries: Queries that return more than one row from the inner SELECT statement.

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