Question

I have this query to simulate full join of three tables in mysql,

SELECT a. * , b. * , c. *
FROM tbl_1 a
LEFT OUTER JOIN tbl_2 b ON a.num = b.num
LEFT OUTER JOIN tbl_3 c ON a.num = c.num
UNION
SELECT a. * , b. * , c. *
FROM tbl_1 a
RIGHT OUTER JOIN tbl_2 b ON a.num = b.num
RIGHT OUTER JOIN tbl_3 c ON a.num = c.num

But the result it generates is not what i am expecting, the tables that i am going to join are as follows,

Table1-
num |info
----------
1   |aaaaa  
2   |bbbb
3   |ccc
3   |ccc
4   |dddd

Table2-
num |info
----------
1   |aaaaa
3   |ccc
4   |dddd
5   |eeee
6   |ffff

Table3-
num |info
----------
1   |aaaaa
6   |ffff
2   |bbbb

And the result I am expecting is,

Required result 1-
num |info   num |info   num |info
----------- ----------- ----------
1   |aaaaa  1   |aaaaa  1   |aaaaa
2   |bbbb   NULL|NULL   2   |bbbb
3   |ccc    3   |ccc    NULL|NULL
3   |ccc    3   |ccc    NULL|NULL
4   |dddd   4   |dddd   NULL|NULL
NULL|NULL   5   |eeee   NULL|NULL
NULL|NULL   6   |ffff   6   |ffff

OR

Required result 2-
num |info   num |info   num |info
----------- ----------- ----------
1   |aaaaa  1   |aaaaa  1   |aaaaa
2   |bbbb   NULL|NULL   2   |bbbb
3   |ccc    3   |ccc    NULL|NULL
3   |ccc    3   |ccc    NULL|NULL
4   |dddd   4   |dddd   NULL|NULL
NULL|NULL   6   |ffff   6   |ffff

But what i get is

Result i get-
num |info   num |info   num |info
----------- ----------- ----------
1   |aaaaa  1   |aaaaa  1   |aaaaa
2   |bbbb   NULL|NULL   2   |bbbb
3   |ccc    3   |ccc    NULL|NULL
3   |ccc    3   |ccc    NULL|NULL
4   |dddd   4   |dddd   NULL|NULL
NULL|NULL   NULL|NULL   6   |ffff
NULL|NULL   NULL|NULL   2   |bbbb

May i know what is wrong over here, i’ve been stuck with this for 2 days and couldn’t resolve..may i know the query to obtain the result i want?

Thank you very much 🙂

Answer 1

This works as shown in John Woo’s comment with sql fiddle. (the original answer contained an error but it should now work)

SELECT a. * , b. * , c. *
FROM tbl_1 a
LEFT OUTER JOIN tbl_2 b USING (num)
LEFT OUTER JOIN tbl_3 c USING (num)
UNION
SELECT a. * , b. * , c. *
FROM tbl_2 b
LEFT OUTER JOIN tbl_1 a USING (num)
LEFT OUTER JOIN tbl_3 c USING (num)
UNION
SELECT a. * , b. * , c. *
FROM tbl_3 c
LEFT OUTER JOIN tbl_1 a USING (num)
LEFT OUTER JOIN tbl_2 b USING (num)