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SQL EXISTS Operator

EXISTS Operator

The EXISTS operator is used to check if a subquery returns any records. It returns True if one or more records exist, otherwise it returns False.

SQL EXISTS Syntax

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

Demonstration Database

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

Below are the data from the "Websites" table:

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

Below are the data from the "access_log" website access records 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 if there are any websites with total 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 subquery:

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:

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