Syntax:
REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name [(col_name,...)]
    {VALUES | VALUE} ({expr | DEFAULT},...),(...),...

Or:

REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name
    SET col_name={expr | DEFAULT}, ...

Or:

REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name [(col_name,...)]
    SELECT ...

REPLACE works exactly like INSERT, except that if an old row in the
table has the same value as a new row for a PRIMARY KEY or a UNIQUE
index, the old row is deleted before the new row is inserted. See [HELP
INSERT].

REPLACE is a MySQL extension to the SQL standard. It either inserts, or
deletes and inserts. For another MySQL extension to standard SQL ---
that either inserts or updates --- see
http://dev.mysql.com/doc/refman/5.1/en/insert-on-duplicate.html.

Note that unless the table has a PRIMARY KEY or UNIQUE index, using a
REPLACE statement makes no sense. It becomes equivalent to INSERT,
because there is no index to be used to determine whether a new row
duplicates another.

Values for all columns are taken from the values specified in the
REPLACE statement. Any missing columns are set to their default values,
just as happens for INSERT. You cannot refer to values from the current
row and use them in the new row. If you use an assignment such as SET
col_name = col_name + 1, the reference to the column name on the right
hand side is treated as DEFAULT(col_name), so the assignment is
equivalent to SET col_name = DEFAULT(col_name) + 1.

To use REPLACE, you must have both the INSERT and DELETE privileges for
the table.

URL: http://dev.mysql.com/doc/refman/5.1/en/replace.html

