sql与众不同，获取2列

Question

I'm trying to create a simple message feature for my website but I couldn't get distinct datas from 2 columns ( **from** column and **to** column)

you see examples datas on the picture

how can I get the return "1,4,23,45,345"?

Answer 1

You should to union both columns and then filter for distinct values:

select distinct T.from_to from
( select `from` as from_to
  from messages
  union
  select `to` as from_to
  from messages
) T

if you really need all in a comma separate string, use GROUP_CONCAT([DISTINCT] aggregation function.

EDITED :

You should mark as solution Gerald answer. After test union operator and reread mysql union operator documentation , by default, mysql filter for distinct values:

mysql> create table ta( a int );
Query OK, 0 rows affected (0.05 sec)

mysql> insert into ta values (1),(1),(2);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from ta
    -> union
    -> select * from ta;
+------+
| a    |
+------+
|    1 |
|    2 |
+------+
2 rows in set (0.00 sec)

then, the final query is:

  select `from` as from_to
  from messages
  union distinct
  select `to` as from_to
  from messages

Notice that distinct is not mandatory.

Only if you need a comma sparate string the first solution is necessary:

select distinct GROUP_CONCAT( distinct T.from_to from )
( select `from` as from_to
  from messages
  union
  select `to` as from_to
  from messages
) T

Answer 2

select from as ft from messages
union
select to as ft from messages

In MySQL union selects distinct rows by default. You would use UNION ALL to allow for duplicates.