Exists operator
The exists operator is used to determine whether there are records in the query clause. 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 runoob sample database.
Here is the data selected 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 | rookie course| http://www.runoob.com/ | 4689 | CN |
|4 | microblog| http://weibo.com/ | 20 | CN |
| 5 | Facebook | https://www.facebook.com/ | 3 | USA |
+----+--------------+---------------------------+-------+---------+Here’s “access.”_ Log “data of website access record 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 instance
Now we want to find out if a website with a total number of visits (count field) greater than 200 exists.
We use the following SQL statement:
SELECT Websites.name, Websites.url
FROM Websites
WHERE EXISTS (SELECT count FROM access_log WHERE Websites.id = access_log.site_id AND count > 200);After executing the above SQL, the output is as follows:
Exists can be used with not to find records that do not match the query statement
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);After executing the above SQL, the output is as follows:
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