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create table t (
id int not null primary key auto_increment,
a int,
b int,
c int
);
insert into t(a, b, c) values
( 1, 1, 1),
( 2, null, 2),
(null, 3, 3),
( 4, 4, 4);
select sum(value)
from (
select a as value from t union all
select b from t union all
select c from t
) as x;
| sum(value) |
|---|
| 25 |