How to select same table in update statement with MySQL

Question

In this table, there is a column called 'position'. This column counts up from 0. (First row has position 0, second has 1 etc.)

Now, if the row with position 4, moves up, I want to set the row above (row with position 3) to the current position. (So position 3 becomes position 4.)

So for example:

I have:

id = 12, position = 2
id = 11, position = 3
id = 9, position = 4
id = 8, position = 5

After the update I want:

id = 12, position = 2
id = 11, position = 4
id = 9, position = 3
id = 8, position = 5

I want to use the id of the row, to update the position.

This is what I've got, but it doesn't work on MySQL:

UPDATE message
SET position = position + 1
WHERE position = (SELECT position - 1 FROM message WHERE id = 11);

When I try to run it, it gives me this error: 'SQL Error(1093): You can't specify target table 'message' for update in FROM clause'.

How can I fix this?

Answer 1

EDIT - after UPDATE with sample data from OP:

I assume you know the ID (=11) and want the data to change as illustrated in your updated question... you need to update 2 rows to achieve that:

UPDATE message m SET
m.position = (CASE WHEN m.id = 11 THEN m.position + 1 ELSE m.position - 1 END)
WHERE m.id = 11 OR m.id =
(SELECT z.li FROM (SELECT MAX (l.id) li FROM message h INNER JOIN message l ON h.position = (l.position - 1) WHERE h.id = 11) z);

Answer 2

Add ORDER BY position DESC to the query to do it from the bottom up and avoid having the same position twice at the same time.

Edit: I posted this before the OP said what his error was - I just guessed.

By far the simplest solution here is just get the position in the parent code, and don't do in in MySQL.

Answer 3

Think we need to follow this through (I think I've understood the idea, but correct me if not):

Consider two rows:

Before row change

ID=10, Position=3
ID=11, Position=4

We move ID=11 up:

ID=10, Position=3
ID=11, Position=3

We need

ID=10, Position=4
ID=11, Position=3

Therefore the update should be:

UPDATE message
SET position = position + 1
WHERE position = (SELECT position FROM (select id, position from message) WHERE id = 11)
    and id<>11