MySQL Best Practices

If you mention ON DUPLICATE KEY UPDATE when a row is inserted that is causing a duplicate value in a UNIQUE index or PRIMARY KEY then MySQL performs an UPDATE of the old row.

Suppose, in a table there are three columns i.e. a, b, c.
If column ‘a’ is declared as UNIQUE and contains the value 1 then the following two statements are similar effect:

INSERT INTO tbl_name (a,b,c) VALUES (1,2,3) ON DUPLICATE KEY UPDATE c = c+1;
UPDATE tbl_name SET c = c+1 WHERE a = 1;

A table containing an AUTO_INCREMENT column and INSERT … ON DUPLICATE KEY UPDATE inserts or updates a row then, the LAST_INSERT_ID() function returns the AUTO_INCREMENT value.

Read Also: Update from one table to another with condition in MySQL

The ON DUPLICATE KEY UPDATE clause may contain multiple column assignments i.e. separated by commas.

The DELAYED option is ignored while you are using ON DUPLICATE KEY UPDATE.

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