Join tables SQL Server

Question

I want to join two tables but I want the result set to only show matches that arent in the right side.

Example:

LeftTable

• leftID | PK
• value |

RightTable

• rightID |PK

• leftID |FK

  select l.value

  from LeftTable l

  join RightTable r

  on l.leftID = r.leftID

I know this wont give me what I want, but I am just trying to find out which items in the left table DO NOT appear in the right side byusing the leftID foreign key relationship.

Any ideas?

Answer 1

What if we do

select LT.value
from LeftTable LT
left outer join RightTable RT
on LT.leftID = RT.leftID
Where RT.leftId is null

SO join will return all the matching as well as those rows which are in left but not in right. With where clause we only get those rows for which right table left id is null.

Answer 2

select * from LeftTable where leftID not in (select leftID from RightTable)

Answer 3

You nearly have it, just a couple of minor changes - the join should be a LEFT JOIN, not an [INNER] JOIN, and you need to only return the rows where the right table is missing, i.e. its fields are NULL:

SELECT T1.value
FROM LeftTable T1
LEFT JOIN RightTable T2
ON T1.leftID = T2.leftID
WHERE T2.leftID IS NULL

Answer 4

SELECT LeftID
FROM LeftTable 
    LEFT OUTER JOIN RightTable ON LeftTable.LeftID = RightTable.LeftID
WHERE RightTable.RightId IS NULL

Answer 5

An alternative using exists, as a preference to using where not in. The left outer join with a check on the null state of the right is probably still optimal though.

select
        left.*
    from
        LeftTable as left
    where
        not exists (
            select
                    *
                from
                    RightTable as right
                where
                    right.RightID = left.LeftID
            )