MySQL Conditional Insert

You can also use INSERT IGNORE which silently ignores the insert instead of updating or inserting a row when you have a unique index on (user, item).

The query will look like this:

INSERT IGNORE INTO x_table(instance, user, item) VALUES (919191, 123, 456)

You can add the unique index with CREATE UNIQUE INDEX user_item ON x_table (user, item).

If your DBMS does not impose limitations on which table you select from when you execute an insert, try:

INSERT INTO x_table(instance, user, item) 
    SELECT 919191, 123, 456
        FROM dual
        WHERE NOT EXISTS (SELECT * FROM x_table
                             WHERE user = 123 
                               AND item = 456)

In this, dual is a table with one row only (found originally in Oracle, now in mysql too). The logic is that the SELECT statement generates a single row of data with the required values, but only when the values are not already found.

Alternatively, look at the MERGE statement.