Question

What is the difference in ‘%’ and ‘%%’, when used in mysql where clause with ‘LIKE’ ?

select * from `wp_users` u where u.user_nicename like "%lastuser%"

VS

select * from `wp_users` u where u.user_nicename like "%%lastuser%%"

Answer 1

There is no difference between %% and % when it comes to pattern matching in mysql.

I’ve seen developers get confused over this when they try to match a literal % and therefor write %%. This is most often because of the fact that format-strings often use a double % to indicate that you’d like it to be treated as an exact literal.

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MySQL documentation of LIKE

• MySQL 5.0 Reference Manual :: 11.5.1 String Comparison Functions :: LIKE

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What’s the origin of the string, and where is it going?

If the string is passed to a function such as sprintf the format-string rule I mentioned earlier is present, though there is no confusion in that case.

The developer want it to be a single % in the string passed to mysql, and therefor wrote %%.

$query = sprintf (
  "SELECT ... FROM ... WHERE id <> %d AND data LIKE '%%hello world%%'",
  50
);

// $query => "SELECT ... FROM ... WHERE id <> 50 AND data LIKE '%hello world%'";

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A few sample SELECTs using the LIKE operator

mysql> SELECT 'abc' LIKE 'ab%';
+------------------+
| 'abc' LIKE 'ab%' |
+------------------+
|                1 |
+------------------+
1 row in set (0.01 sec)

mysql> SELECT 'abc' LIKE 'ab%%';
+-------------------+
| 'abc' LIKE 'ab%%' |
+-------------------+
|                 1 |
+-------------------+
1 row in set (0.00 sec)

mysql> SELECT 'abc' LIKE 'ab\%';
+-------------------+
| 'abc' LIKE 'ab\%' |
+-------------------+
|                 0 |
+-------------------+
1 row in set (0.00 sec)

mysql> SELECT 'ab%' LIKE 'ab\%';
+-------------------+
| 'ab%' LIKE 'ab\%' |
+-------------------+
|                 1 |
+-------------------+
1 row in set (0.00 sec)