MySQL重用子查询的总和

Question

I have the following simplified query:

Select
(select sum(f1) from tableA a where a.id = t.id) sum1,
(select sum(f2) from tableB b where b.id = t.id) sum2,
t.*
from Table t;

My wish is to have sum1 and sum2 re-used without calculating them again:

Select
(select sum(f1) from tableA a where a.id = t.id) sum1,
(select sum(f2) from tableB b where b.id = t.id) sum2,
sum1 + sum2 `sum3`,    
t.*
from Table t;

Of course i can do the following query but this will unnecessary double the run time:

Select
(select sum(f1) from tableA a where a.id = t.id) sum1,
(select sum(f2) from tableB b where b.id = t.id) sum2,
(select sum(f1) from tableA a where a.id = t.id) + 
(select sum(f2) from tableB b where b.id = t.id) `sum3`,    
t.*
from Table t;

or even inserting the sum1 and sum2 results to a temporary table but can't imaging i'm overlooking something to have mysql do some efficient querying on summed fields.

Is there a better, more efficient way to re-use summed fields?

Answer 1

Try running this query,

select Resutl.*,Result.sum1+Result.sum2 as sum3 from(
       SELECT (SELECT SUM(f1) FROM tableA a WHERE a.id = t.id) sum1,
              (SELECT SUM(f2) FROM tableB b WHERE b.id = t.id) sum2,
              t.*
       FROM Table t)Result
)

Hope it will help.

Answer 2

My wish is to have sum1 and sum2 re-used without calculating them again

Whoa, steady on. The original query is not as efficient as it might be. Without knowing what the data distribution looks like its hard to advise what the most appropriate query is, but assuming that the tuples in A and B have a foreign key constraint (implicit or explicit) on table T but no implicit foreign key restraint on each other, that t.id is unique, and most of the rows in tableT have corresponding rows in tableA and tableB then....

SELECT t.*, sum_f1, sum_f2
FROM tableT t
LEFT JOIN (SELECT a.id, SUM(f1) AS sum_f1
 FROM tableA a
 GROUP BY a.id) AS a_agg
ON t.id=a_agg.id
LEFT JOIN (SELECT b.id, SUM(f2) AS sum_f2
 FROM tableB b
 GROUP BY b.id) as b_agg
ON t.id=b_agg.id
GROUP BY t.id

Will be much more efficient. (this also assumes that ONLY_FULL_GROUP_BY is disabled - otherwise you'll need to replace 't.*' in the SELECT clause and 't.id' in the group by clause with each of the attributes you need from the table).

Note that in practice you're rarely going to be looking at all your data in a single query. Since MySQL doesn't handle push predicates very well, simply adding a filter in the outer SELECT probably won't be the optimal solution.

Once you've structured your query as above, it's trivial to add the sums:

SELECT t.*, sum_f1, sum_f2, 
 IFNULL(sum_f1,0)+INFULL(sum_f2,0) AS sum3
FROM tableT t
LEFT JOIN (SELECT a.id, SUM(f1) AS sum_f1
 FROM tableA a
 GROUP BY a.id) AS a_agg
ON t.id=a_agg.id
LEFT JOIN (SELECT b.id, SUM(f2) AS sum_f2
 FROM tableB b
 GROUP BY b.id) as b_agg
ON t.id=b_agg.id
GROUP BY t.id

(but note that I've made a lot of assumptions about your data and schema)