Question

I am having a difficult time forming a conditional INSERT

I have x_table with columns (instance, user, item) where instance ID is unique. I want to insert a new row only if the user already does not have a given item.

For example trying to insert instance=919191 user=123 item=456

Insert into x_table (instance, user, item) values (919191, 123, 456) 
    ONLY IF there are no rows where user=123 and item=456 

Any help or guidance in the right direction would be much appreciated.

Answer 1

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.