MySQL Conditional Insert

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

Answer 1

Insert into x_table (instance, user, item) values (919191, 123, 456) 
    where ((select count(*) from x_table where user=123 and item=456) = 0);

The syntax may vary depending on your DB...