Question

The following code fetches data if the SubjectB column from Table 2 matches the Subject A column from Table 1.

SELECT
UID
FROM
Table1
WHERE EXISTS (
    SELECT 1
    FROM Table2
    WHERE SubjectB = SubjectA
)

I want to add an additional condition after the closed parentheses:

AND another_column_from_table1 > 10

But the sytax does not seem valid. What is the correct way to have a query with WHERE EXISTS … AND conditions?

Answer 1

Given your initial query, you should be able to write:

SELECT UID
  FROM Table1
 WHERE EXISTS (SELECT 1
                 FROM Table2
                WHERE SubjectB = SubjectA
              )
   AND another_column_from_table1 > 10;

There shouldn’t be a syntax error from simply adding the condition to the end of the query. From the comments, it seems that this is syntactically accepted, but generates no output. The obvious technique to debug this is:

SELECT UID, another_column_from_table1
  FROM Table1
 WHERE EXISTS (SELECT 1
                 FROM Table2
                WHERE SubjectB = SubjectA
              )

This will show you the values in the column; presumably, since the result set is empty when you add the extra filter condition, the values in the second column of this select will all be less than 10 (or null).

If there’s a one-to-one relationship between entries in Table1 and Table2, then you can use a JOIN instead:

SELECT t1.UID
  FROM Table1 AS t1
  JOIN Table2 AS t2 ON t2.SubjectB = t1.SubjectA
 WHERE t1.another_column_from_table1 > 10;

If there could be several rows in Table2 for each row in Table1, then you would need to add DISTINCT:

SELECT DISTINCT t1.UID
  FROM Table1 AS t1
  JOIN Table2 AS t2 ON t2.SubjectB = t1.SubjectA
 WHERE t1.another_column_from_table1 > 10;