The difference between Replace into and insert into

The difference between Replace into and insert into:

The replace operation is similar to insert. With one exception, 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.

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 insert, because no index is used to determine whether the new row has replicated other rows.

The values of all columns are taken from the values that are specified in the Replace statement. All missing columns are set to their default values, which is the same as insert. 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", the reference to the column name on the right is treated as default (col_name). Therefore, the assignment is equivalent to set col_name = DEFAULT (col_name) + 1.

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. The number is the number of rows that are 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 the table contains more than one unique index, and the new row replicates the values of the different old rows in different unique indexes, it is possible that a single row has replaced multiple old rows.

The number of rows affected can easily determine if replace adds only one row, or if replace replaces other rows: Check whether the number is 1 (added) or larger (replace).

If you are using the C API, you can use the Mysql_affected_rows () function to get the number of rows affected.

Currently, you cannot change from one table to another in a subquery and select from the same table.

The following is a detailed description of the algorithm (this algorithm is also used for load DATA ...). REPLACE):

1. Try inserting a new row into the table

2. When an insert fails because of a duplicate keyword error for a primary key or a unique keyword:

A. Removing conflicting rows from a table that contain duplicate key values

B. Try again to insert a new row into the table

Use the following 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 ...

The difference between Replace into and insert into