SQL EXISTSOperators

EXISTS Operator

The EXISTS operator is used to determine whether a query clause has records. If one or more records exist, it returns True; otherwise, it returns False.

SQL EXISTS Syntax

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

Demo Database

In this tutorial, we will use the EXAMPLE sample database.

The following is the data selected from the "Websites" table:

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘宝       | https://www.taobao.com/   | 13    | CN      |
| 3  | Example | http://www.example.com/    | 4689  | CN      |
| 4  | 微博       | http://weibo.com/         | 20    | CN      |
| 5  | Facebook     | https://www.facebook.com/ | 3     | USA     |
+----+--------------+---------------------------+-------+---------+

The following is the data from the "access_log" website access log table:

mysql> SELECT * FROM access_log;
+-----+---------+-------+------------+
| aid | site_id | count | date       |
+-----+---------+-------+------------+
|   1 |       1 |    45 | 2016-05-10 |
|   2 |       3 |   100 | 2016-05-13 |
|   3 |       1 |   230 | 2016-05-14 |
|   4 |       2 |    10 | 2016-05-14 |
|   5 |       5 |   205 | 2016-05-14 |
|   6 |       4 |    13 | 2016-05-15 |
|   7 |       3 |   220 | 2016-05-15 |
|   8 |       5 |   545 | 2016-05-16 |
|   9 |       3 |   201 | 2016-05-17 |
+-----+---------+-------+------------+
9 rows in set (0.00 sec)

SQL EXISTS Example

Now we want to find out whether there are websites with a total number of visits (count field) greater than 200.

We use the following SQL statement:

Example

SELECT Websites.name, Websites.url FROM Websites WHERE EXISTS (SELECT count FROM access_log WHERE Websites.id = access_log.site_id AND count > 200);

Executing the above SQL outputs the following results:

EXISTS can be used with NOT to find records that do not match the query statement:

Example

SELECT Websites.name, Websites.url FROM Websites WHERE NOT EXISTS (SELECT count FROM access_log WHERE Websites.id = access_log.site_id AND count > 200);

Executing the above SQL outputs the following results:

Other Extensions