在MySQL中完全外部联接三个表

Question

I have following tables and desired output. I tried like full join based on value and group by subject but its still not showing the desired output

Any help or guidance will be much appreciated. Below is the MySql query:

select t1.*,t2.value from Table1 as t1 left join Table2 as t2
on RIGHT(t2.value,1) =RIGHT(t1.value,1) 

union all 
select t1.*,t2.value from Table1 as t1 right join Table2 as t2
on RIGHT(t2.value,1) =RIGHT(t1.value,1)

DDL:

CREATE TABLE Table1 (`subject` varchar(30), `value` varchar(30));
CREATE TABLE Table2 (`subject` varchar(30), `value` varchar(30));
CREATE TABLE Table3 (`subject` varchar(30), `value` varchar(30));

INSERT INTO Table1
(`subject`, `value`)
VALUES
('subject1', 'ValueA1'),
('subject1', 'ValueA2'),
('subject1', 'ValueA3'),
('subject2', 'ValueA4'),
('subject2', 'ValueA5'),
('subject3', 'ValueA6'),
('subject3', 'ValueA7');

INSERT INTO Table2
(`subject`, `value`)
VALUES
('subject1', 'ValueB1'),
('subject1', 'ValueB2'),
('subject2', 'ValueB3'),
('subject1', 'ValueB4'),
('subject2', 'ValueB5'),
('subject3', 'ValueB6');

INSERT INTO Table3
(`subject`, `value`)
VALUES
('subject1', 'ValueC1'),
('subject2', 'ValueC2'),
('subject2', 'ValueC3'),
('subject2', 'ValueC4'),
('subject2', 'ValueC5'),
('subject3', 'ValueC6');

Answer 1

You need to use NOT EXISTS along with the UNION ALL .

I think it's bit complicated with three tables, but here:

select *
from (
    select coalesce(t.subject, t3.subject) subject,
        t.value1,
        t.value2,
        t3.value value3
    from (
        select coalesce(t1.subject, t2.subject) subject,
            t1.value value1,
            t2.value value2
        from Table1 as t1
        left join Table2 as t2 on RIGHT(t2.value, 1) = RIGHT(t1.value, 1)
            and t1.subject = t2.subject

        union all

        select coalesce(t1.subject, t2.subject),
            t1.value,
            t2.value
        from Table1 as t1
        right join Table2 as t2 on RIGHT(t2.value, 1) = RIGHT(t1.value, 1)
            and t1.subject = t2.subject
        where not exists (
                select 1
                from table1 t
                where t.subject = t2.subject
                    and right(t.value, 1) = right(t2.value, 1)
                )
        ) t
    right join table3 t3 on t.subject = t3.subject
        and right(coalesce(t.value1, t.value2), 1) = right(t3.value, 1)

    union all

    select coalesce(t.subject, t3.subject) subject,
        t.value1,
        t.value2,
        t3.value value3
    from (
        select coalesce(t1.subject, t2.subject) subject,
            t1.value value1,
            t2.value value2
        from Table1 as t1
        left join Table2 as t2 on RIGHT(t2.value, 1) = RIGHT(t1.value, 1)
            and t1.subject = t2.subject

        union all

        select coalesce(t1.subject, t2.subject),
            t1.value,
            t2.value
        from Table1 as t1
        right join Table2 as t2 on RIGHT(t2.value, 1) = RIGHT(t1.value, 1)
            and t1.subject = t2.subject
        where not exists (
                select 1
                from table1 t
                where t.subject = t2.subject
                    and right(t.value, 1) = right(t2.value, 1)
                )
        ) t
    left join table3 t3 on t.subject = t3.subject
        and right(coalesce(t.value1, t.value2), 1) = right(t3.value, 1)
    where not exists (
            select 1
            from table3 t2
            where t2.subject = t.subject
                and right(t2.value, 1) = right(coalesce(t.value1, t.value2), 1)
            )
    ) t
order by subject,
    value1,
    value2,
    value3;

Demo

Answer 2

I think the best way to emulate a full join in MySQL is to select all the key values first and then use left join . However, you are not looking for a full join . That would produce a Cartesian product for each subject.

You want to "list" each subject independently in each column. For that, you can use union all , group by , and variables:

select subject,
       max(value1) as value1,
       max(value2) as value2,
       max(value3) as value3
from ((select t1.subject, t1.value as value1, NULL as value2, NULL as value3
              (@rn1 := if(@s1 = subject, @rn1 + 1,
                          if(@s1 := subject, 1, 1)
                         )
              ) as rn
       from table1 t1 cross join
            (select @rn1 := 0, @s1 := '') params
      ) union all
      (select t2.subject, NULL, t2.value, NULL as value3
              (@rn2 := if(@s2 = subject, @rn2 + 1,
                          if(@s2 := subject, 1, 1)
                         )
              ) as rn
       from table2 t2 cross join
            (select @rn2 := 0, @s2 := '') params
      ) union all
      (select t3.subject, NULL as value1, NULL as value2, t3.value as value3
              (@rn3 := if(@s3 = subject, @rn3 + 1,
                          if(@s3 := subject, 1, 1)
                         )
              ) as rn
       from table3 t3 cross join
            (select @rn3 := 0, @s3 := '') params
      )
     )
group by subject, rn;