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.

Answer 2

Think of it this way:

For 'each' row from Suppliers, check if there 'exists' a row in the Order table that meets the condition Suppliers.supplier_id (this comes from Outer query current 'row') = Orders.supplier_id. When you find the first matching row, stop right there - the WHERE EXISTS has been satisfied.

The magic link between the outer query and the subquery lies in the fact that Supplier_id gets passed from the outer query to the subquery for each row evaluated.

Or, to put it another way, the subquery is executed for each table row of the outer query.

It is NOT like the subquery is executed on the whole and gets the 'true/false' and then tries to match this 'true/false' condition with outer query.

Answer 3

You can produce identical results using either JOIN, EXISTS, IN, or INTERSECT:

SELECT s.supplier_id
FROM suppliers s
INNER JOIN (SELECT DISTINCT o.supplier_id FROM orders o) o
    ON o.supplier_id = s.supplier_id

SELECT s.supplier_id
FROM suppliers s
WHERE EXISTS (SELECT * FROM orders o WHERE o.supplier_id = s.supplier_id)

SELECT s.supplier_id 
FROM suppliers s 
WHERE s.supplier_id IN (SELECT o.supplier_id FROM orders o)

SELECT s.supplier_id
FROM suppliers s
INTERSECT
SELECT o.supplier_id
FROM orders o

Answer 4

If you had a where clause that looked like this:

WHERE id in (25,26,27) -- and so on

you can easily understand why some rows are returned and some are not.

When the where clause is like this:

WHERE EXISTS (select * from orders where suppliers.supplier_id = orders.supplier_id);

it just means : return rows that have an existing record in the orders table with te same id.

Answer 5

What you describe is a so called query with a correlated subquery.

(In general) it's something that you should try to avoid by writing the query by using a join instead:

SELECT suppliers.* 
FROM suppliers 
JOIN orders USING supplier_id
GROUP BY suppliers.supplier_id

Because otherwise, the subquery will be executed for each row in the outer query.

Answer 6

EXISTS means that the subquery returns at least one row, that's really it. In that case, it's a correlated subquery because it checks the supplier_id of the outer table to the supplier_id of the inner table. This query says, in effect:

SELECT all suppliers For each supplier ID, see if an order exists for this supplier If the supplier is not present in the orders table, remove the supplier from the results RETURN all suppliers who have corresponding rows in the orders table

You could do the same thing in this case with an INNER JOIN.

SELECT suppliers.* 
  FROM suppliers 
 INNER 
  JOIN orders 
    ON suppliers.supplier_id = orders.supplier_id;

Ponies comment is correct. You'd need to do grouping with that join, or select distinct depending on the data you need.