如何在同一列中包含两个表中的值

Question

I have the following query:

select
    t1.x,
    t2.y

from table1 t1
    join table2 t2
        on t1.x = t2.x

which is producing the following results:

x   y1
x   y2
x   y3

I, however, want it to produce the following, where the value from t1.x is also in the second column:

x   x
x   y1
x   y2
x   y3

Is there an easy way to do this? I'm trying to achieve this in PostgreSQL but I would also be interested in any solution in MySQL.

Thanks in advance!

Answer 1

You seem to want:

select t1.x, t1.x as y
from table1 t1
union all
select t2.x, t2.y
from table2 t2;

The join is only needed if you want to filter (or multiply) the rows, based on x being in both tables.

Answer 2

I am not sure if that is what you need. But I suspect you just need LEFT JOIN with some tweak:

SELECT
    t1.x,
    COALESCE(t2.y, t1.x)
FROM table1 t1
LEFT JOIN table2 t2
ON t1.x = t2.x

Answer 3

You can use UNION:

select
    x,
    x as y
from table1
union
select
    t1.x,
    t2.y
from table1 t1
    join table2 t2
on t1.x = t2.x
order by x

or

select
    t1.x,
    t1.x
from table1 t1
    join table2 t2
on t1.x = t2.x
union
select
    t1.x,
    t2.y
from table1 t1
    join table2 t2
on t1.x = t2.x
order by x