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MySQL EXISTS

MySQL EXISTS Clause

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

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

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.

We have two tables:

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

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

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.