SQL: create new field, populate with query results

Question

I have two tables: tbl_listings, with the column: prop_id; and another table: tbl_bookings, with the columns: prop_id, booking_date.

I want to write a query that counts all the times prop_id appears in tbl_bookings, and then populates a new column in tbl_listings with the results of that query.

my query looks like:

ALTER TABLE tbl_listings
ADD COLUMN prop_count INT

UPDATE tbl_listings
    SET prop_count =
    (SELECT COUNT(*)
    FROM tbl_bookings
    GROUP BY prop_id)

but for some reason I get an error saying: Subquery returns more than 1 row. How do I fix this?

Answer 1

That sub-query gives you a count for each distinct prop_id. You can only assign one count value to prop_count. If you intended to update the prop_count in multiple rows that correspond to prop_ids, you will need to add a correlated subquery to your update that correlates the prop_id in tbl_bookings with the corresponding prop_id in tbl_listings.

As I think about your question more, I am wondering if you meant to insert into an empty tbl_listings table rather than update. You can do that with this command:

INSERT INTO tbl_listings(prop_id,prop_count)
SELECT prop_id, COUNT(*) as prop_count
FROM tbl_bookings
GROUP BY prop_id

If you really meant to update and assuming that each prop_id is present in your tbl_listings table, you can issue the following update:

UPDATE tbl_listings
SET prop_count=(SELECT COUNT(*) 
                FROM tbl_bookings AS TB
                WHERE TB.prop_id=TL.prop_id)
FROM tbl_listings AS TL

If you want to update tbl_listings by inserting new prop_ids from tbl_bookings and their respective counts, you can do:

INSERT INTO tbl_listings(prop_id,prop_count)
SELECT prop_id, COUNT(*) as prop_count
FROM tbl_bookings AS TB
WHERE NOT EXISTS(SELECT prop_id -- Insert only new prop_ids/counts
                 FROM tbl_listings AS TL 
                 WHERE TL.prop_id=TB.prop_id)
GROUP BY prop_id