Question

I am trying to duplicate rows in the same table. Table A has no auto_increment (unfortunately).

INSERT INTO A (id, foreign_id, value)
    SELECT (SELECT MAX(id)+1 FROM A), 2, value
        FROM A WHERE foreign_id = 1;

But (SELECT MAX(id)+1 FROM A) is calculated once, so it will always return the same ID. At the second row insertion the query will fail (duplicate primary key).

I tried that first:

INSERT INTO A (id, foreign_id, value)
    SELECT MAX(id)+1, 2, value
        FROM A WHERE foreign_id = 1;

But the MAX(id)+1 takes into account WHERE foreign_id = 1, so there’s no guarantee that I’ll get a non-used ID.

How can I proceed to either:

• Have a subquery that is re-calculated for each INSERT
• Or more generally: duplicate rows in my table given there’s no auto_increment

Answer 1

Have you tried using variables?

SET @max=SELECT MAX(id);
INSERT INTO A (id, foreign_id, value)
    SELECT @max:=@max+1, 2, value
        FROM A WHERE foreign_id = 1;