Best way to test if a row exists in a MySQL table

Question

I\'m trying to find out if a row exists in a table. Using MySQL, is it better to do a query like this:

SELECT COUNT(*) AS total FROM table1 WHERE ...
         


        

Answer 1

A COUNT query is faster, although maybe not noticeably, but as far as getting the desired result, both should be sufficient.

Answer 2

Or you can insert raw sql part to conditions so I have 'conditions'=>array('Member.id NOT IN (SELECT Membership.member_id FROM memberships AS Membership)')

Answer 3

A short example of @ChrisThompson's answer

Example:

mysql> SELECT * FROM table_1;
+----+--------+
| id | col1   |
+----+--------+
|  1 | foo    |
|  2 | bar    |
|  3 | foobar |
+----+--------+
3 rows in set (0.00 sec)

mysql> SELECT EXISTS(SELECT 1 FROM table_1 WHERE id = 1);
+--------------------------------------------+
| EXISTS(SELECT 1 FROM table_1 WHERE id = 1) |
+--------------------------------------------+
|                                          1 |
+--------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT EXISTS(SELECT 1 FROM table_1 WHERE id = 9);
+--------------------------------------------+
| EXISTS(SELECT 1 FROM table_1 WHERE id = 9) |
+--------------------------------------------+
|                                          0 |
+--------------------------------------------+
1 row in set (0.00 sec)

Using an alias:

mysql> SELECT EXISTS(SELECT 1 FROM table_1 WHERE id = 1) AS mycheck;
+---------+
| mycheck |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

Answer 4

You could also try EXISTS:

SELECT EXISTS(SELECT * FROM table1 WHERE ...)

and per the documentation, you can SELECT anything.

Traditionally, an EXISTS subquery starts with SELECT *, but it could begin with SELECT 5 or SELECT column1 or anything at all. MySQL ignores the SELECT list in such a subquery, so it makes no difference.

Answer 5

I feel it is worth pointing out, although it was touched on in the comments, that in this situation:

SELECT 1 FROM my_table WHERE *indexed_condition* LIMIT 1

Is superior to:

SELECT * FROM my_table WHERE *indexed_condition* LIMIT 1

This is because the first query can be satisfied by the index, whereas the second requires a row look up (unless possibly all the table's columns are in the index used).

Adding the LIMIT clause allows the engine to stop after finding any row.

The first query should be comparable to:

SELECT EXISTS(SELECT * FROM my_table WHERE *indexed_condition*)

Which sends the same signals to the engine (1/* makes no difference here), but I'd still write the 1 to reinforce the habit when using EXISTS:

SELECT EXISTS(SELECT 1 FROM my_table WHERE *indexed_condition*)

It may make sense to add the EXISTS wrapping if you require an explicit return when no rows match.

Answer 6

For non-InnoDB tables you could also use the information schema tables:

http://dev.mysql.com/doc/refman/5.1/en/tables-table.html