Question

I have the following query

SELECT COUNT(*) 
FROM Table1 
WHERE Column1 IN
(SELECT  Column1 FROM Table2)

Actually there is no column named Column1 in the Table2.
So if we execute only the sub query

 SELECT  Column1 FROM Table2

it will throw the error – Invalid column name ‘Column1’.

But if I Execute the full query , I am not getting any error. It is returing the total row count in Table1.

So I would like to know the reason why this query it not giving any error in this case And the way the IN clause works in this scenario.

Answer 1

Within a subquery, you’re allowed to access columns from the outer query. And that’s what’s happening here:

SELECT COUNT(*) 
FROM Table1 
WHERE Column1 IN
(SELECT  Column1 FROM Table2)

Could also be written as:

SELECT COUNT(*) 
FROM Table1 
WHERE Column1 IN
(SELECT Table1.Column1 FROM Table2)

Provided that there’s at least one row in Table2, the subquery will always return the Column1 value from Table1, and the IN() will be successful.

This is one reason I usually recommend using aliases:

SELECT COUNT(*) 
FROM Table1 t1
WHERE t1.Column1 IN
(SELECT t2.Column1 FROM Table2 t2)

will produce an error if there’s no Column1 in Table2 – or, if you do reference t1 inside the subquery, it’s more obvious that it’s deliberate.