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 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

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...

Answer 2

If you add a constraint that (x_table.user, x_table.item) is unique, then inserting another row with the same user and item will fail.

eg:

mysql> create table x_table ( instance integer primary key auto_increment, user integer, item integer, unique (user, item));
Query OK, 0 rows affected (0.00 sec)

mysql> insert into x_table (user, item) values (1,2),(3,4);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> insert into x_table (user, item) values (1,6);
Query OK, 1 row affected (0.00 sec)

mysql> insert into x_table (user, item) values (1,2);
ERROR 1062 (23000): Duplicate entry '1-2' for key 2

Answer 3

Although it's good to check for duplication before inserting your data I suggest that you put a unique constraint/index on your columns so that no duplicate data can be inserted by mistake.

Answer 4

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 elsewhere 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.

Answer 5

With a UNIQUE(user, item), do:

Insert into x_table (instance, user, item) values (919191, 123, 456) 
  ON DUPLICATE KEY UPDATE user=123

the user=123 bit is a "no-op" to match the syntax of the ON DUPLICATE clause without actually doing anything when there are duplicates.