MySQL 5.7

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create table tbl (
  id int unsigned auto_increment primary key
, a int
, b int
, c int
);
insert into tbl (a,b,c) values
(1,2,3),(4,5,6);
select a,b,c, a+b+c
  from tbl;


a
b
c
a+b+c


1
2
3
6

4
5
6
15


select a,b,c, 'a'+'b'+'c'
  from tbl;


a
b
c
'a'+'b'+'c'


1
2
3
0

4
5
6
0