Subquery error message while running query - sql

Question

UPDATE rm
SET rm.cost_sqft = ISNULL((SELECT
CASE
WHEN (rm.rm_cat IN ('8','NON-REPORT') AND rm.space_fee = 'No')
THEN '0'
ELSE rent_w.w * #rental_rate
END
FROM
#rental_weight AS rent_w, bl, rm
WHERE
rm.main_contact = rent_w.space_type
AND rm.description = rent_w.space_quality
AND bl.cost_type = rent_w.campuse_cost
AND bl.bl_id = rm.bl_id),'0')
WHERE rm.bl_id = #bl_id
AND rm.fl_id = #fl_id
AND rm.rm_id = #rm_id
While running this statement, I get this error:
Msg 512, Level 16, State 1, Procedure icat_rm_cost_sqft, Line 100 [Batch Start Line 2]
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Answer 1

The error means that the subquery (the select) is returning more than one row for a given key to be applied to the update.
Consider the scenario where the update becomes something like this very contrived scenario:
Sample data in a table called Customer:
cust_id cust_type name
1 X Fred
1 Y jack
And the query which will generate the error:
update customer
set name = (
select "MR " || name
from customer
where cust_id = 1 /* returns Fred and Jack */
)
where cust_id = 1 and cust_type = 'X';
What name should customer 1X be updated to? "Mr Fred" or "Mr Jack"? the RDBMS can't decide for you, you need to modify the query as a correlated subquery (add a join condition on cust_type) or a subquery that just returns one row.
Granted the above is very contrived, but that is what the error is telling you.