Scalar subquery

sql

SELECT employee_id, salary,
       (SELECT AVG(salary) FROM employees) AS avg_salary
FROM employees;

Explanation

A scalar subquery must return at most one row.

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Same sheet · prioritizing Subqueries

Filter rows based on results from another query.

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Reference outer query columns inside the subquery.

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Use a derived table as a query source.

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Return rows when a related row exists.

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Return rows with no related matches.

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Pre-aggregate a table and join the result.

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