MySQL only insert new row if combination of columns (which allow duplicates) is unique

Question

Since IF EXISTS isn\'t supported by MySQL I am struggling to think of the syntax for doing something like the following pseudo in MySQL:

IF ((select count(*)         


        

Answer 1

Create a composite unique index. This will allow any number of duplicates in the individual fields, but the combination needs to be unique.

CREATE UNIQUE INDEX ix_uq ON test (field1, field2, field3);

...and use INSERT IGNORE to insert if the unique index is not violated. If it is, just ignore the insert.

INSERT IGNORE INTO test (field1,field2,field3) VALUES (1,1,1);

An SQLfiddle for testing.

If you want to insert unless there's a duplicate, and update if there is, you can also use INSERT INTO ... ON DUPLICATE KEY UPDATE;

INSERT INTO test (field1, field2, field3) VALUES (1,1,1)
  ON DUPLICATE KEY UPDATE field4=field4+1;

Another SQLfiddle.