MySQL Replace into

In order to be able to use replace, you must have both insert and delete permissions for the table.

The Replace statement returns a number that indicates the number of rows affected. This number is the same as the number of rows that were deleted and inserted. If the number is 1 for a single line replace, the row is inserted and no rows are deleted. If the number is greater than 1, one or more old rows are deleted before the new row is inserted. If a table contains more than one unique index, and a new row duplicates a value from a different old row in a different unique index, it is possible that a single row replaces more than one old row.

Note that it is meaningless to use a Replace statement unless the table has a primary key or a unique index. The statement is the same as the insert, because no indexes are used to determine whether the new row replicates other rows.

The run of replace is very similar to insert. Except for one point, if an old record in the table has the same value as a new record for primary key or a unique index, the old record is deleted before the new record is inserted

Values are taken from the value specified in the Replace statement. All missing columns are set to their default values, which are the same as inserts. You cannot reference a value from the current row, nor can you use a value in a new row. If you use an assignment such as "SET col_name = col_name + 1", a reference to the column name on the right side is treated as default (col_name). Therefore, the assignment is equivalent to set col_name = DEFAULT (col_name) +

Use format:

REPLACE [Low_priority | Delayed]
[Into] tbl_name [(Col_name,...)]
VALUES ({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 ...