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

Or:

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

Or:

REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name
    [PARTITION (partition_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/5.6/en/insert-on-duplicate.html.

*Note*: REPLACE makes sense only if a table has a PRIMARY KEY or UNIQUE
index. Otherwise, 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.

Beginning with MySQL 5.6.2, REPLACE supports explicit partition
selection using the PARTITION option with a comma-separated list of
names of partitions, subpartitions, or both. As with INSERT, if it is
not possible to insert the new row into any of these partitions or
subpartitions, the REPLACE statement fails with the error Found a row
not matching the given partition set. See
http://dev.mysql.com/doc/refman/5.6/en/partitioning-selection.html, for
more information.

URL: http://dev.mysql.com/doc/refman/5.6/en/replace.html

