presto：如何将 2 列的值合并为一列

Question

I am a beginner of SQL. I have a table like the following:

Id start_place  end_place
1   a            b
2   b            c 
3   d            e
4   a            e

desired output:

 place
 a
 b
 c
 d
 e

I want to find combine the distinct values and distinct values from start_place and end_place and combine them into a column (if there are duplicates, eg, b are in both start and end place, show b only once).

Answer 1

You can use union :

select start_place
from t
union   -- on purpose to remove duplicates
select end_place
from t;

Note that union removes duplicates. Under most circumstances, you want union all so you don't incur the overhead of removing duplicates. In this case, though, duplicate remove is what you really want.