sql server - Check if column value exists in another column in SQL

Question

I need a SQL query that compares two values and returns an ID.

I have this table:

ID  Calling_ID  Called_ID
1   27          10
2   15          20
3   80          90
4   90          88
5   60          30
6   88          40
7   15          60
8   30          40
9   27          95
10  40          30

How do I check if each value in the Calling_ID column exists in the Called_ID column and then return the ID? The above data would return 88, 30, 40.

Answer 1

This should work:

SELECT ID FROM [TableName]
WHERE Calling_ID IN
(
SELECT Called_ID FROM [TableName]
)