Question

Say I have 3 tables, each with a field foo. How do I get the following records:

1. All records where the value of foo exist in all 3 tables?
2. All records where the value of foo exist in any 2 tables?
3. All records where the value of foo exist in 1 table only?

foo is UNIQUE.

My attempt for #1:

SELECT a.foo
FROM   a
WHERE  a.foo IN (SELECT b.foo
                 FROM   b
                 WHERE  b.foo IN (SELECT c.foo
                                  FROM   c)) 

For #2, I think I need to do something similar to #1, but for 2 tables at a time {a,b}, {b,c}, {a,c} – and then UNION it? Not sure.

For #3, no clue.

UPDATE: Sample Data

Table1
 - foo-a
 - foo-b
 - foo-c

Table2
 - foo-a
 - foo-b

Table3
 - foo-a
 - foo-c
 - foo-d

For #1 question, there is 1 record that exist in all 3 tables: foo-a. For #2 question, there are 2 records that exist in 2 tables: foo-b (in Table1 and Table2) and foo-c (in Table1 and Table3). For #3 question, there is 1 record that exist in only 1 table: foo-d.

Answer 1

Since FOO is unique,

For No. 1,

SELECT Foo
FROM
    (
        SELECT Foo FROM table1
        UNION ALL
        SELECT Foo FROM table2
        UNION ALL
        SELECT Foo FROM table3
    ) a
GROUP BY Foo
HAVING COUNT(*) = 3

For No. 2,

SELECT Foo
FROM
    (
        SELECT Foo FROM table1
        UNION ALL
        SELECT Foo FROM table2
        UNION ALL
        SELECT Foo FROM table3
    ) a
GROUP BY Foo
HAVING COUNT(*) = 2

For No. 3,

SELECT Foo
FROM
    (
        SELECT Foo FROM table1
        UNION ALL
        SELECT Foo FROM table2
        UNION ALL
        SELECT Foo FROM table3
    ) a
GROUP BY Foo
HAVING COUNT(*) = 1