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Use a derived table for each query, and join
them on their common grouping column. For example, if the grouping column
is userid, you would use this:
select dt1.userid
, dt1.sumcol1 as table1_sumcol1
, dt2.sumcol1 as table2_sumcol1
, dt3.sumcol1 as table3_sumcol1
from (
select userid
, sum(col1) as sumcol1
from table1
group
by userid
) as dt1
inner
join (
select userid
, sum(col1) as sumcol1
from table2
group
by userid
) as dt2
on dt1.userid = dt2.userid
inner
join (
select userid
, sum(col1) as sumcol1
from table3
group
by userid
) as dt3
on dt1.userid = dt3.userid
Notice how the derived tables each have an alias.
Also, in the SELECT list of the outer query, notice that
column aliases are needed to distinguish the columns
from each derived table.
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