Question

I have a query that is returning the correct data to me, but being a developer rather than a DBA I’m wondering if there is any reason to convert it to joins rather than nested selects and if so, what it would look like.

My code currently is

select * from adjustments where store_id in (
    select id from stores where original_id = (
        select original_id from stores where name ='abcd'))

Any references to the better use of joins would be appreciated too.

Answer 1

Besides any likely performance improvements, I find following much easier to read.

SELECT  * 
FROM    adjustments a
        INNER JOIN stores s ON s.id = a.store_id
        INNER JOIN stores s2 ON s2.original_id = s.original_id
WHERE   s.name = 'abcd'        

Test script showing my original fault in ommitting original_id

DECLARE @Adjustments TABLE (store_id INTEGER)
DECLARE @Stores TABLE (id INTEGER, name VARCHAR(32), original_id INTEGER)

INSERT INTO @Adjustments VALUES (1), (2), (3)
INSERT INTO @Stores VALUES (1, 'abcd', 1), (2, '2', 1), (3, '3', 1)

/* 
   OP's Original statement returns store_id's 1, 2 & 3 
   due to original_id being all the same
*/
SELECT  * FROM @Adjustments WHERE store_id IN (
  SELECT id FROM @Stores WHERE original_id = (
    SELECT original_id FROM @Stores WHERE name ='abcd'))

/* 
   Faulty first attempt with removing original_id from the equation
   only returns store_id 1
*/
SELECT  a.store_id
FROM    @Adjustments a
        INNER JOIN @Stores s ON s.id = a.store_id
WHERE   s.name = 'abcd'