Question

I have a query

SELECT * 
FROM table1
WHERE
documentId in
(
   --select items from a second table where a third column happens to be null
   select documentId from table2 t2 inner join table1 t1
   on t1.documentId = t2.documentId and t1.itemId = t2.ItemId
   WHERE t1.someOtherColumn is null
)
and itemId in
(
   --similar query as above, just selecting itemId now
   select itemId from table2 t2 inner join table1 t1
   on t1.documentId = t2.documentId and t1.itemId = t2.ItemId
   WHERE t1.someOtherColumn is null
)
order by 1

Given that table1 has composite PK = documentId + itemId, is this sufficient to select only values from table1 that are unique? My fear is that a situation may occur where documentId exists and itemId exists, but since they are not looked at together then incorrect selections can be made.

For example,

Let’s assume that there exists a value

documentId = 1 and itemId = 1.

Let’s assume there is no composite key

documentId = 1 and itemId = 1.

I do not want composite key

[documentId = 1 and itemId = 3]

to be included.

I also do not want composite key

[documentId = 2 and itemId = 1]

to be included.

If there were a composite key added later (it’s not there now)

[documentId = 1 and itemId = 1]

then it should be included.

Answer 1

This would do the same without having to use a join nor two INs

SELECT *
FROM table1
WHERE exists
(
    SELECT 1
    FROM table2
    WHERE someOtherColumn is null
        AND table1.documentId = table2.documentId
        AND table1.itemId = table2.ItemId
)
ORDER BY 1