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Tutorials > SQL > Subqueries

Subqueries

A subquery in SQL is a query that is nested inside another query. It is used to retrieve data that will be used in the main query as a condition. Subqueries can be used in several parts of a SQL statement, such as the SELECT, FROM, WHERE, and HAVING clauses.

Here are a few examples of using subqueries in SQL:

  • Using a subquery in the FROM clause:

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SELECT column1, column2
FROM (SELECT column3, column4
      FROM table2) AS subquery_alias
WHERE column1 = 'value';

  • Using a subquery in the WHERE clause:

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    SELECT column1, column2
FROM table1
WHERE column1 = (SELECT column3
                 FROM table2
                 WHERE column4 = 'value');

  • Using a subquery in the SELECT clause:

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    SELECT column1,
       (SELECT AVG(column3)
        FROM table2
        WHERE table2.column4 = table1.column2) AS avg_column3
FROM table1;

  • Using multiple subqueries in a single SQL statement:

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    SELECT column1, column2, column3,
       (SELECT COUNT(*)
        FROM table3
        WHERE table3.column4 = table1.column2) AS count_column4,
       (SELECT AVG(column5)
        FROM table4
        WHERE table4.column6 = table2.column3) AS avg_column5
FROM table1
INNER JOIN table2
ON table1.column1 = table2.column1;



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