Postgres update from left join

Question

I'm new in PostgreSQL and trying to convert a query from SQL Server.

I have a table Users with, among others, the columns bUsrActive, bUsrAdmin and sUsrClientCode. I want to update Users and set bUsrActive = false if there does not exist a another user with the same sUsrClientCode where bUsrAdmin = true and bUsrActive = true.

In SQL Server I have this query

UPDATE u SET u.bUsrActive = 0
FROM Users u
LEFT JOIN Users u2 ON u.sUsrClientCode = u2.sUsrClientCode AND u2.bUsrAdmin = 1 AND u2.bUsrActive = 1
WHERE u.bUsrAdmin = 0 AND u.bUsrActive = 1 AND u2.nkUsr IS NULL

I'm trying to convert this to postgres. I wrote 3 approaches.

1) My first attempt. Obviously not working.

UPDATE Users u
    SET bUsrActive = false
FROM Users u2
WHERE u.sUsrClientCode = u2.sUsrClientCode AND u2.bUsrAdmin = true AND u2.bUsrActive = true
AND u.bUsrAdmin = false AND u.bUsrActive = true AND u2.nkUsr IS NULL;

2) I understand why it's not working (it updates all users). I just can't figure out how can I reference table Users u in the UPDATE ... SET part.

UPDATE Users
    SET bUsrActive = false
FROM Users u
LEFT JOIN Users u2 ON u.sUsrClientCode = u2.sUsrClientCode AND u2.bUsrAdmin = true AND u2.bUsrActive = true
WHERE u.bUsrAdmin = false AND u.bUsrActive = true AND u2.nkUsr IS NULL;

3) The following is working, but not using join.

UPDATE Users
    SET bUsrActive = false
WHERE  NOT EXISTS (
    SELECT 1
    FROM Users u
    WHERE u.sUsrClientCode = Users.sUsrClientCode AND u.bUsrAdmin = true AND u.bUsrActive = true
) AND Users.bUsrAdmin = false AND Users.bUsrActive = true;

I'll probably go with the last solution. I just wanted to know if it's possible to do what I want using a left join.

Answer 1

Here's a generic way to transform this update query from SQL-server form to PostgreSQL:

UPDATE Users
 SET bUsrActive = false
WHERE
 ctid IN (
   SELECT u.ctid FROM Users u
      LEFT JOIN Users u2 ON u.sUsrClientCode = u2.sUsrClientCode AND u2.bUsrAdmin = 1 AND u2.bUsrActive = 1
    WHERE u.bUsrAdmin = 0 AND u.bUsrActive = 1 AND u2.nkUsr IS NULL
)

ctid is a pseudo-column that points to the unique location of a row. You could use instead the primary key of the table if it had one.

The query #2 from the question doesn't do what you expect because the updated table Users is never joined to the same table Users u in the FROM clause. Just as when you put a table name twice in a FROM clause, they don't get implicitly joined or bound together, they are considered as two independant sets of rows.

Answer 2

I think this is the correct way of doing 2) I belive it's more optimal/efficient than doing a sub select.

UPDATE Users uOrig
    SET bUsrActive = false
FROM Users u
      LEFT JOIN Users u2 ON u.sUsrClientCode = u2.sUsrClientCode AND u2.bUsrAdmin = 1 AND u2.bUsrActive = 1
WHERE u.bUsrAdmin = 0 AND u.bUsrActive = 1 AND u2.nkUsr IS NULL
    and uOrig.sUsrClientCode = u.sUsrClientCode;