Question

I have the query below that when run it says that 325 rows were affected. However, if I only run the SELECT portion of the query, I get 331 results returned. This means that when the whole query runs, only 325 rows are getting updated and not 331. I’m not sure what the discrepancy could be. I’d like it to update all the rows that it finds in the SELECT portion of the query

This update results in 325 rows updated

UPDATE A
SET
A.status = X.c
FROM 
tableA A INNER JOIN (
   SELECT  tableB.c_id, COUNT(*) as c
   FROM    tableB
   LEFT JOIN tableC
        ON tableB.c_id = tableC.c_id
   WHERE tableC.c_id != tableC.c_id_update
   GROUP BY tableB.c_id) X
ON A.c_id = X.c_id

And running only the SELECT portion of the query returns 331 rows

SELECT  tableB.c_id, COUNT(*) as c
FROM    tableB
LEFT JOIN tableC
    ON tableB.c_id = tableC.c_id
WHERE tableC.c_id != tableC.c_id_update
GROUP BY tableB.c_id

I’m really stuck on figuring out why there’s 6 (331-325) records that don’t get updated?! Thanks for your help.

Answer 1

Its probably that your addition join to TableA is removing 6 records. Look for it like this:

select x.* from
(
SELECT  tableB.c_id, COUNT(*) as c
FROM    tableB
LEFT JOIN tableC
    ON tableB.c_id = tableC.c_id
WHERE tableC.c_id != tableC.c_id_update
GROUP BY tableB.c_id
) X
left join tableA A on A.c_id = X.c_id
where A.C_id is null

Essentially this is a left join, and you only return the rows where the left join is absent. It should tell you which 6 rows are missing from TableA