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

What you want is INSERT INTO table (...) SELECT ... WHERE .... from MySQL 5.6 manual.

In you case it's:

INSERT INTO x_table (instance, user, item) SELECT 919191, 123, 456 
WHERE (SELECT COUNT(*) FROM x_table WHERE user=123 AND item=456) = 0

Or maybe since you're not using any complicated logic to determiante whether to run the INSERT or not you could just set a UNIQUE key on the combination of these two columns and then use INSERT IGNORE.

Answer 2

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 3

So this one stands for PostgreSQL

INSERT INTO x_table
SELECT NewRow.*
FROM (SELECT 919191 as instance, 123 as user, 456 as item) AS NewRow
LEFT JOIN x_table
ON x_table.user = NewRow.user AND x_table.item = NewRow.item
WHERE x_table.instance IS NULL

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 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.

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.

Answer 6

In case you don't want to set a unique constraint, this works like a charm :

INSERT INTO `table` (`column1`, `column2`) SELECT 'value1', 'value2' FROM `table` WHERE `column1` = 'value1' AND `column2` = 'value2' HAVING COUNT(`column1`) = 0

Hope it helps !