comprare aggregate sum function to number in postgres - postgresql

Question

I have the next query which does not work:
UPDATE item
SET popularity= (CASE
WHEN (select SUM(io.quantity) from item i NATURAL JOIN itemorder io GROUP BY io.item_id) > 3 THEN TRUE
ELSE FALSE
END);
Here I want to compare each line of inner SELECT SUM value with 3 and update popularity. But SQL gives error:
ERROR: more than one row returned by a subquery used as an expression
I understand that inner SELECT returns many values, but can smb help me in how to compare each line. In other words make loop.

Answer 1

When using a subquery you need to get a single row back, so you're effectively doing a query for each record in the item table.
UPDATE item i
SET popularity = (SELECT SUM(io.quantity) FROM itemorder io
WHERE io.item_id = i.item_id) > 3;
An alternative (which is a postgresql extension) is to use a derived table in a FROM clause.
UPDATE item i2
SET popularity = x.orders > 3
FROM (select i.item_id, SUM(io.quantity) as orders
from item i NATURAL JOIN itemorder io GROUP BY io.item_id)
as x(item_id,orders)
WHERE i2.item_id = x.item_id
Here you're doing a single group clause as you had, and we're joining the table to be updated with the results of the group.