对多列求和并在mysql中求和它们的别名

Question

Is it possible to sum the columns and sum again?

select 
id,
sum(t2.col) as cola,
sum(t3.col) as colb,
sum(t4.col) as colc,
sum(cola + colb + colc) as total
from t1
left join t2 on t2.id = t1.t2fk
left join t3 on t3.id = t1.t3fk
left join t4 on t4.id = t1.t4fk
group by t1.id

something like this

sum(cola + colb + colc) as total

Answer 1

You can't do it with aliases, but you can do it by re-writing the SELECT clauses, like this:

select 
id,
sum(t2.col) as cola,
sum(t3.col) as colb,
sum(t4.col) as colc,
sum(t2.col) + sum(t3.col) + sum(t4.col) as total
from t1
left join t2 on t2.id = t1.t2fk
left join t3 on t3.id = t1.t3fk
left join t4 on t4.id = t1.t4fk
group by t1.id

There will be no performance penalty (ie MySQL will not "re-summarize" the three columns to produce the total)

Answer 2

Yes, this is possible by using a nested query.

SELECT id, cola, colb, colc, SUM(cola + colb + colc)
  FROM  (
      SELECT id, SUM(a) cola, SUM(b) colb, SUM(c) colc
        FROM whatever ...
       GROUP BY id
        ) subquery_alias
 GROUP BY id

Most servers' query planning modules do a good job optimizing these sorts of queries built from sub queries.

Pro tip Format these nested queries carefully in your source code so you, or somebody else, can read them easily years from now.

Answer 3

For MySql 8.0+ you can use a CTE:

with cte as (
  select id, sum(t2.col) as cola, sum(t3.col) as colb, sum(t4.col) as colc
  from t1
  left join t2 on t2.id = t1.t2fk
  left join t3 on t3.id = t1.t3fk
  left join t4 on t4.id = t1.t4fk
  group by t1.id
)
select c.*, null as total
from cte as c
union all
select null, null, null, null, sum(cola + colb + colc)
from cte

For earlier versions:

select id, sum(t2.col) as cola, sum(t3.col) as colb, sum(t4.col) as colc, null as total
from t1
left join t2 on t2.id = t1.t2fk
left join t3 on t3.id = t1.t3fk
left join t4 on t4.id = t1.t4fk
group by t1.id
union all
select null, null, null, null, sum(t2.col + t2.col + t2.col)
from t1
left join t2 on t2.id = t1.t2fk
left join t3 on t3.id = t1.t3fk
left join t4 on t4.id = t1.t4fk

In case there are null s in the columns t2.col , t3.col and t4.col , instead of

sum(t2.col + t2.col + t2.col)

use:

sum(t2.col) + sum(t2.col) + sum(t2.col)