MySQL Subqueries

A subquery is a SELECT inside another query, written in brackets. The inner query runs first, and its result is used by the outer query.

Subquery with a comparison

Find the students whose marks are above the average.

SELECT name, marks
FROM students
WHERE marks > (SELECT AVG(marks) FROM students)
ORDER BY marks DESC;

The inner query returns the average, 75.5. The outer query then returns the students above it.

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Subquery with IN

SELECT name
FROM students
WHERE course_id IN (SELECT id FROM courses WHERE course_name = 'Python')
ORDER BY id;

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Subquery with EXISTS

EXISTS is true if the inner query returns at least one row.

SELECT c.course_name
FROM courses c
WHERE EXISTS (SELECT 1 FROM students s WHERE s.course_id = c.id)
ORDER BY c.id;

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Second highest marks

SELECT MAX(marks) AS second_highest
FROM students
WHERE marks < (SELECT MAX(marks) FROM students);

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Summary

• A subquery is a query in brackets inside another query.
• Use it with comparison operators, IN and EXISTS.
• Next: UNION.

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