SQL EXISTS and NOT EXISTS
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Introduction
In this article, we are going to see how the SQL EXISTS operator works and when you should use it.
Although the EXISTS operator has been available since SQL:86, the very first edition of the SQL Standard, I found that there are still many application developers who don’t realize how powerful SQL subquery expressions really are when it comes to filtering a given table based on a condition evaluated on a different table.
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.
YouTube Video
I also published a YouTube video about EXISTS and NOT EXISTS conditional operators, so if you’re interested in this topic, you might want to watch this video lesson as well.
SQL NOT EXISTS
Let’s consider we want to select all students that have no grade lower than 9. For this, we can use NOT EXISTS, which negates the logic of the EXISTS operator.
Therefore, the NOT EXISTS operator returns true
if the underlying subquery returns no record. However, if a single record is matched by the inner subquery, the NOT EXISTS operator will return false
, and the subquery execution can be stopped.
To match all student
records that have no associated student_grade
with a value lower than 9, we can run the following SQL query:
SELECT id, first_name, last_name FROM student WHERE NOT EXISTS ( SELECT 1 FROM student_grade WHERE student_grade.student_id = student.id AND student_grade.grade < 9 ) ORDER BY id
When running the query above, we can see that only the Alice record is matched:
| id | first_name | last_name | |----|------------|-----------| | 1 | Alice | Smith |
Cool, right?
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Conclusion
The advantage of using the SQL EXISTS and NOT EXISTS operators is that the inner subquery execution can be stopped as long as a matching record is found.
If the subquery requires to scan a large volume of records, stopping the subquery execution as soon as a single record is matched can greatly speed up the overall query response time.
