Syntax:
TRUNCATE [TABLE] tbl_name

TRUNCATE TABLE empties a table completely. It requires the DROP
privilege as of MySQL 5.1.16. (Before 5.1.16, it requires the DELETE
privilege.

Logically, TRUNCATE TABLE is equivalent to a DELETE statement that
deletes all rows, but there are practical differences under some
circumstances.

For an InnoDB table, InnoDB processes TRUNCATE TABLE by deleting rows
one by one if there are any FOREIGN KEY constraints that reference the
table. If there are no FOREIGN KEY constraints, InnoDB performs fast
truncation by dropping the original table and creating an empty one
with the same definition, which is much faster than deleting rows one
by one. The AUTO_INCREMENT counter is reset to zero by TRUNCATE TABLE,
regardless of whether there is a FOREIGN KEY constraint.

In the case that FOREIGN KEY constraints reference the table, InnoDB
deletes rows one by one and processes the constraints on each one. If
the FOREIGN KEY constraint specifies DELETE CASCADE, rows from the
child (referenced) table are deleted, and the truncated table becomes
empty. If the FOREIGN KEY constraint does not specify CASCADE, the
TRUNCATE TABLE statement deletes rows one by one and stops if it
encounters a parent row that is referenced by the child, returning this
error:

ERROR 1451 (23000): Cannot delete or update a parent row: a foreign
key constraint fails (`test`.`child`, CONSTRAINT `child_ibfk_1`
FOREIGN KEY (`parent_id`) REFERENCES `parent` (`id`))

This is the same as a DELETE statement with no WHERE clause.

The count of rows affected by TRUNCATE TABLE is accurate only when it
is mapped to a DELETE statement.

For other storage engines, TRUNCATE TABLE differs from DELETE in the
following ways in MySQL 5.1:

o Truncate operations drop and re-create the table, which is much
  faster than deleting rows one by one, particularly for large tables.

o Truncate operations cause an implicit commit.

o Truncation operations cannot be performed if the session holds an
  active table lock.

o Truncation operations do not return a meaningful value for the number
  of deleted rows. The usual result is "0 rows affected," which should
  be interpreted as "no information."

o As long as the table format file tbl_name.frm is valid, the table can
  be re-created as an empty table with TRUNCATE TABLE, even if the data
  or index files have become corrupted.

o The table handler does not remember the last used AUTO_INCREMENT
  value, but starts counting from the beginning. This is true even for
  MyISAM and InnoDB, which normally do not reuse sequence values.

o When used with partitioned tables, TRUNCATE TABLE preserves the
  partitioning; that is, the data and index files are dropped and
  re-created, while the partition definitions (.par) file is
  unaffected.

o Since truncation of a table does not make any use of DELETE, the
  TRUNCATE TABLE statement does not invoke ON DELETE triggers.

URL: http://dev.mysql.com/doc/refman/5.1/en/truncate-table.html

