MySQL EXISTS: Definition, Syntax, and Examples

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MySQL EXISTS Clause

The EXISTS operator in MySQL is used to check if a subquery returns any results.

It returns TRUE if the subquery finds at least one row.
It returns FALSE if the subquery returns no rows.

It is commonly used in WHERE clauses to perform conditional checks.

EXISTS Syntax

SELECT column_name(s)
FROM table_name
WHERE EXISTS (
  SELECT 1
  FROM another_table
  WHERE condition
);

EXISTS → Checks if the subquery has any rows.
SELECT 1 → It is a convention to use SELECT 1 instead of selecting actual data, as it improves performance.
WHERE condition → Condition for the subquery.

MySQL EXISTS Examples

We have two tables:

students Table

student_id	name	gender	city	age	score	course_id	email	phone_number
1	Alice	Female	Delhi	20	85	101	NULL	1234567890
2	Bob	Male	Mumbai	22	75	NULL	bob@email.com	NULL
3	Charlie	Male	Delhi	21	95	102	NULL	NULL
4	David	Male	Bangalore	23	65	101	david@email.com	NULL
5	Eve	Female	Mumbai	20	80	103	NULL	NULL
6	Frank	Male	Delhi	22	90	103	NULL	NULL
7	Alice	Female	Mumbai	19	60	102	NULL	NULL

courses Table

course_name	course_name
101	Math
102	Science
103	History
104	English

The following SQL statement find students enrolled in courses.

Example

SELECT name

FROM students s

WHERE EXISTS (

SELECT 1

FROM courses c

WHERE c.course_id = s.course_id

);

Try it yourself

Output:

name
Alice
Bob
Charlie
David
Eve
Frank

Explanation:

EXISTS checks if the student’s course_id exists in the courses table.
Since all course IDs in students exist in courses, all students are returned.

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