Question

I have a table with four columns, where col1 & col2 contain similar values (INT). I now want to know if there are duplicates in col1 and/or col2. i.e.,

col1 | col2
-----+-----
111  | 222
333  | 444
111  | 333
555  | 111

→ Duplicates: 111 (3x) and 333 (2x).

I am using SQLite, but I think this is a basic SQL question.

Answer 1

To get a count of each element use UNION ALL in a subquery, then GROUP BY on the result of that:

SELECT col, COUNT(*)
FROM
(
    SELECT col1 AS col FROM Table1
    UNION ALL
    SELECT col2 FROM Table1
) T1
GROUP BY col

Add HAVING COUNT(*) > 1 if you only wish to see the duplicated values.