SQL SubQueries Basic- What are they? How to use them ?

sql-subqueries
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What is subquery?

Subqueries in SQL are queries inside queries. Users can specify a Query in either the FROM clause or WHERE clause. The internal query filters rows and makes it easier for the external query to operate. You can bypass the use of subquery but it will make the query long with less readability.

Syntax of Subqueries:

SELECT *
FROM demo_tab
WHERE ( SELECT *
	FROM demo_tab
	CLAUSE
	) ;

Here in the above syntax used in an demo example

SELECT col_1, col_2, col_3
FROM demo_table
WHERE ( get some value using SELECT Statement
);

here is our demo table named MARKS_DATA

Example 1 : Using Subquery to get value of average marks
SELECT *
FROM MARKS_DATA
WHERE MARKS > (SELECT AVG(MARKS) as AVG
	FROM MARKS_DATA
	)

result of the inside query is

SELECT AVG(MARKS) as AVG
FROM MARKS_DATA ;

As you can see here ,all the students have scored more than the average(i.e. 84) of marks

Thanks For Reading !

SOURCE: Microsoft T-SQL Documentation For SubQueries

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