SQL Doubt

I am not sure I understand your question, but I am guessing a little:

You have table A with PK column ID and table B with column AID and you want to know if there are rows in B where AID is not in A?

Select * From B where AID not in (select ID from A)

The easiest way to do this that I know of would be to do a JOIN operation. You can use OUTER JOIN and look for NULL values. Something like:

SELECT x.ID
FROM dbo.TableY AS y
LEFT JOIN dbo.TableX AS x
ON y.ID = x.ID

You can add a WHERE clause that either filters for the parent table, Y, ID value, or just looks for IS NULL or IS NOT NULL on the child table, X, ID value.

A -- Primary Key table B -- Foreign Key table

Below query returns the values in primary table and which are not in foreign key table or child table.

Select * From A where ID not in (select ID from B)

If you want to check for a specified value then run the below by replace the question mark(?) with the value,

Select * From A where ID not in (select ID from B) and Id = ?