MySQL LIKE operator with wildcard and backslash - mysql

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.

Answer 2

With MySQL 5.0.12 dev on Windows 10 I got the following results when I changed the query from
SELECT * FROM `foo` WHERE `name` LIKE '%http:\/\/%'
to
SELECT * FROM `foo` WHERE `name` LIKE '%http:\\\\\\\%'
it works and yet the first string with forward slashes was the original field content. It seems to have interpreted forward slashes as backslashes.

Answer 3

It seems it has some relation to that MySQL bug: http://bugs.mysql.com/bug.php?id=46659
I think you connect to mysql not specifying correct --character-set-server option (which defaults to latin1 with collation latin1_swedish_ci), and having utf-8 as the current charset of the console. That causes incorrect char conversions and comparisons when you deal with data which supposed to be converted to the utf8 from the charset of --character-set-server.