Which MySQL data type to use for storing boolean values

For MySQL 5.0.3 and higher, you can use BIT. The manual says:

As of MySQL 5.0.3, the BIT data type is used to store bit-field values. A type of BIT(M) enables storage of M-bit values. M can range from 1 to 64.

Otherwise, according to the MySQL manual you can use BOOL or BOOLEAN, which are at the moment aliases of tinyint(1):

Bool, Boolean: These types are synonyms for TINYINT(1). A value of zero is considered false. Non-zero values are considered true.

MySQL also states that:

We intend to implement full boolean type handling, in accordance with standard SQL, in a future MySQL release.

References: http://dev.mysql.com/doc/refman/5.5/en/numeric-type-overview.html

BOOL and BOOLEAN are synonyms of TINYINT(1). Zero is false, anything else is true. More information here.

This is an elegant solution that I quite appreciate because it uses zero data bytes:

some_flag CHAR(0) DEFAULT NULL

To set it to true, set some_flag = '' and to set it to false, set some_flag = NULL.

Then to test for true, check if some_flag IS NOT NULL, and to test for false, check if some_flag IS NULL.

(This method is described in "High Performance MySQL: Optimization, Backups, Replication, and More" by Jon Warren Lentz, Baron Schwartz and Arjen Lentz.)