Syntax:
TRUNCATE [TABLE] tbl_name

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

For an InnoDB table before version 5.0.3, InnoDB processes TRUNCATE
TABLE by deleting rows one by one. As of MySQL 5.0.3, row by row
deletion is used only 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. (When fast truncation is used, it resets any
AUTO_INCREMENT counter to zero. From MySQL 5.0.13 on, 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 5.0:

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

o As of MySQL 5.0.8, truncate operations cause an implicit commit.
  Before 5.0.8, truncate operations are not transaction-safe; an error
  occurs when attempting one in the course of an active transaction.

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 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/5.0/en/truncate-table.html

