MacLochlainns Weblog

It’s always interesting when I answer questions. This question was how to you perform a correlated UPDATE statement. My answer was assuming you’re updating the rating_id foreign key column in the rating table with the value from an item_rating column in the item table where on or another column value in the rating table match the item_rating column value in the item table match, you would write a correlated UPDATE statement like:

UPDATE item i
SET    i.rating_id = r.rating_id
WHERE  EXISTS
        (SELECT NULL
         FROM   rental r
         WHERE  r.rating = i.item_rating
         OR     r.description = i.item_rating);

This works in Oracle, MySQL, MariaDB, and MS SQL Server. I thought my work was done but I was wrong. The individual was trying to write a correlated UPDATE statement for PostgreSQL. The statement returned the following error:

ERROR:  syntax error at or near "WHERE"
LINE 3: WHERE  EXISTS
       ^

I did didn’t find an article to point the individual to after quick Google and DuckDuckGo searches. So, I thought I’d provide how you do it in PostgreSQL:

UPDATE item i
SET    rating_id = r.rating_id
FROM   rating r
WHERE  r.rating = i.item_rating
OR     r.description = i.item_rating;

In short, PostgreSQL doesn’t do what most expect because the UPDATE statement supports a FROM clause. Let’s give them the prize for different dialect. While I hope that I’m not a syntax bigot because I use MySQL more, I think the default syntax should always be supported in SQL dialects. After all, MySQL has a far superior named-notation INSERT statement alternative to the standard with the assignment method but MySQL also supports the standard syntax.

While I’ve shown you how to do it in PostgreSQL, what do you think? Should PostgreSQL be as responsible as MySQL is in maintaining standard SQL approaches?