How do SQL EXISTS statements work?

Question

I\'m trying to learn SQL and am having a hard time understanding EXISTS statements. I came across this quote about \"exists\" and don\'t understand something:

Answer 1

This is a very good question, so I decided to write a very detailed article about this topic on my blog.

Database table model

Let’s assume we have the following two tables in our database, that form a one-to-many table relationship.

The student table is the parent, and the student_grade is the child table since it has a student_id Foreign Key column referencing the id Primary Key column in the student table.

The student table contains the following two records:

| id | first_name | last_name | admission_score |
|----|------------|-----------|-----------------|
| 1  | Alice      | Smith     | 8.95            |
| 2  | Bob        | Johnson   | 8.75            |

And, the student_grade table stores the grades the students received:

| id | class_name | grade | student_id |
|----|------------|-------|------------|
| 1  | Math       | 10    | 1          |
| 2  | Math       | 9.5   | 1          |
| 3  | Math       | 9.75  | 1          |
| 4  | Science    | 9.5   | 1          |
| 5  | Science    | 9     | 1          |
| 6  | Science    | 9.25  | 1          |
| 7  | Math       | 8.5   | 2          |
| 8  | Math       | 9.5   | 2          |
| 9  | Math       | 9     | 2          |
| 10 | Science    | 10    | 2          |
| 11 | Science    | 9.4   | 2          |

SQL EXISTS

Let’s say we want to get all students that have received a 10 grade in Math class.

If we are only interested in the student identifier, then we can run a query like this one:

SELECT
    student_grade.student_id
FROM
    student_grade
WHERE
    student_grade.grade = 10 AND
    student_grade.class_name = 'Math'
ORDER BY
    student_grade.student_id

But, the application is interested in displaying the full name of a student, not just the identifier, so we need info from the student table as well.

In order to filter the student records that have a 10 grade in Math, we can use the EXISTS SQL operator, like this:

SELECT
    id, first_name, last_name
FROM
    student
WHERE EXISTS (
    SELECT 1
    FROM
        student_grade
    WHERE
        student_grade.student_id = student.id AND
        student_grade.grade = 10 AND
        student_grade.class_name = 'Math'
)
ORDER BY id

When running the query above, we can see that only the Alice row is selected:

| id | first_name | last_name |
|----|------------|-----------|
| 1  | Alice      | Smith     |

The outer query selects the student row columns we are interested in returning to the client. However, the WHERE clause is using the EXISTS operator with an associated inner subquery.

The EXISTS operator returns true if the subquery returns at least one record and false if no row is selected. The database engine does not have to run the subquery entirely. If a single record is matched, the EXISTS operator returns true, and the associated other query row is selected.

The inner subquery is correlated because the student_id column of the student_grade table is matched against the id column of the outer student table.