(mysql.info) insert-on-duplicate

 
 13.2.4.3 `INSERT ... ON DUPLICATE KEY UPDATE' Syntax
 ....................................................
 
 If you specify `ON DUPLICATE KEY UPDATE', and a row is inserted that
 would cause a duplicate value in a `UNIQUE' index or `PRIMARY KEY', an
 `UPDATE' of the old row is performed. For example, if column `a' is
 declared as `UNIQUE' and contains the value `1', the following two
 statements have identical effect:
 
      INSERT INTO table (a,b,c) VALUES (1,2,3)
        ON DUPLICATE KEY UPDATE c=c+1;
 
      UPDATE table SET c=c+1 WHERE a=1;
 
 The rows-affected value is 1 if the row is inserted as a new record and
 2 if an existing record is updated.
 
 If column `b' is also unique, the `INSERT' is equivalent to this
 `UPDATE' statement instead:
 
      UPDATE table SET c=c+1 WHERE a=1 OR b=2 LIMIT 1;
 
 If `a=1 OR b=2' matches several rows, only _one_ row is updated. In
 general, you should try to avoid using an `ON DUPLICATE KEY' clause on
 tables with multiple unique indexes.
 
 You can use the `VALUES(COL_NAME)' function in the `UPDATE' clause to
 refer to column values from the `INSERT' portion of the `INSERT ...
 UPDATE' statement. In other words, `VALUES(COL_NAME)' in the `UPDATE'
 clause refers to the value of COL_NAME that would be inserted, had no
 duplicate-key conflict occurred. This function is especially useful in
 multiple-row inserts. The `VALUES()' function is meaningful only in
 `INSERT ... UPDATE' statements and returns `NULL' otherwise. Example:
 
      INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
        ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);
 
 That statement is identical to the following two statements:
 
      INSERT INTO table (a,b,c) VALUES (1,2,3)
        ON DUPLICATE KEY UPDATE c=3;
      INSERT INTO table (a,b,c) VALUES (4,5,6)
        ON DUPLICATE KEY UPDATE c=9;
 
 The `DELAYED' option is ignored when you use `ON DUPLICATE KEY UPDATE'.
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