Suppose you have a MySQL table containing an enum column. The enumeration allows the values "FOO", "BAR" and "BAZ". This is a production database, hooked up by a quite huge amount of programs inserting and deleting rows into that table.
Now suppose that, for some reason, one of these programs tries to insert the value "HELLO" into the enumeration column. You would expect an error, wouldn't you? Unfortunately not in MySQL: when you try to insert a value not defined in the enumeration group, it will insert instead a special value '' (empty string) which is always present in any enumeration, and associated with the index 0. You just obtain a warning.
mysql> insert into test (my_enum) values ("FOO"); Query OK, 1 row affected (0.00 sec)
mysql> insert into test (my_enum) values ("HELLO"); Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> select * from test; +---------+ | my_enum | +---------+ | FOO | | | +---------+ 2 rows in set (0.00 sec)
This means that you will get non-expected entries in the column (not within your enum expected values). Moreover, you have no way of tracing back who performed the wrong insertion, and what value he stored. Imagine there's just a mistyped "FO" instead of "FOO" somewhere: it will produce an empty column entry.
So, what can you do if you really want the enum values enforced? You switch on Strict mode, which, unfortunately, is server wise. Not table wise, not even database wise. It's server wise. Meaning that if your server is dealing with hundreds of databases, all of them will be under strict mode. Argh!
Glad to be proven wrong!