Question

I have two tables with exactly the same columns

Table A

id   FLAG
1    Y
2    Y
3    N
4    N

Table B

id   FLAG
1    Y
2
3    N
4

I would like to write a select query in Oracle that will display all rows in Table A where the FLAG column is not the same as Table B but having matching ID’s columns

The result should be as follows.

id   FLAG
2    Y
4    N

Answer 1

“all rows in Table A … as Table B having matching ID’s columns”:

FROM a JOIN b USING (id)

“where the FLAG column is not the same”:

WHERE a.flag != b.flag 
   OR (a.flag IS NULL AND b.flag IS NOT NULL)
   OR (b.flag IS NULL AND a.flag IS NOT NULL)

So, the query would be

SELECT id, a.flag
  FROM a JOIN b USING (id)
WHERE a.flag != b.flag 
   OR (a.flag IS NULL AND b.flag IS NOT NULL)
   OR (b.flag IS NULL AND a.flag IS NOT NULL)

The WHERE-condition is ugly but necessary to catch the cases where table b has a NULL-value, but a not, or the other way round…