MySQL LIKE operator with wildcard and backslash

Question

It's frustrated with MySQL's pattern escaping used in LIKE operator.

root@dev> create table foo(name varchar(255));
Query OK, 0 rows affected (0.02 sec)

root@dev> insert into foo values('with\\slash');
Query OK, 1 row affected (0.00 sec)

root@dev> insert into foo values('\\slash');
Query OK, 1 row affected (0.00 sec)

root@dev> select * from foo where name like '%\\\\%';
Empty set (0.01 sec)

root@dev> select * from foo;
+------------+
| name       |
+------------+
| with\slash | 
| \slash     | 
+------------+
2 rows in set (0.00 sec)

root@dev> select * from foo where name like '%\\\\%';
Empty set (0.00 sec)

root@dev> select * from foo where name like binary '%\\\\%';
+------------+
| name       |
+------------+
| with\slash | 
| \slash     | 
+------------+
2 rows in set (0.00 sec)

According to MySQL docs: http://dev.mysql.com/doc/refman/5.5/en/string-comparison-functions.html#operator_like %\\\\% is the right operand, but why it yields no result?

EDIT: The database I'm testing that in has character_set_database set to utf8. To further my investigation, I created the same setup in a database that has character_set_database set to latin1, and guess what, '%\\\\%' works!

EDIT: The problem can be reproduced and it's the field collation problem. Details: http://bugs.mysql.com/bug.php?id=63829

Answer 1

In MySQL 5.6.10, with the text field collation utf8mb4_unicode_520_ci this can be achieved by using 5 backslash characters instead of 4, i.e:

select * from foo where name like binary '%\\\\\%';

Somehow, against all expectations, this properly finds all rows with backslashes. At least this should work until the MySQL field collation bug above is fixed. Considering it's been more than 5 years since the bug is discovered, any app designed with this may outlive its usefulness before MySQL is even fixed - so should be a pretty reliable workaround.