Important note: If you upgrade to MySQL 4.1.1 or higher, it is difficult to downgrade back to 4.0 or 4.1.0! That is because, for earlier versions, InnoDB is not aware of multiple tablespaces.

Added support for SUM(DISTINCT), MIN(DISTINCT), and MAX(DISTINCT).

The KILL statement now takes CONNECTION and QUERY modifiers. The first is the same as KILL with no modifier (it kills a given connection thread). The second kills only the statement currently being executed by the connection.

Added TIMESTAMPADD() and TIMESTAMPDIFF() functions.

Added WEEK and QUARTER values as INTERVAL arguments for the DATE_ADD() and DATE_SUB() functions.

New binary log format that enables replication of these session variables: sql_mode, SQL_AUTO_IS_NULL, FOREIGN_KEY_CHECKS (which was replicated since 4.0.14, but here it's done more efficiently and takes less space in the binary logs), UNIQUE_CHECKS. Other variables (like character sets, SQL_SELECT_LIMIT, ...) will be replicated in upcoming 5.0.x releases.

Implemented Index Merge optimization for OR clauses. See Sección 7.2.6, “Index Merge Optimization”.

Basic support for stored procedures (SQL:2003 style). See Capítulo 19, Procedimientos almacenados y funciones.

Added SELECT INTO list_of_vars, which can be of mixed (that is, global and local) types. See Sección 19.2.9.3, “La sentencia SELECT ... INTO”.

Easier replication upgrade (5.0.0 masters can read older binary logs and 5.0.0 slaves can read older relay logs). See Sección 6.5, “Compatibilidad entre versiones de MySQL con respecto a la replicación” for more details). The format of the binary log and relay log is changed compared to that of MySQL 4.1 and older.