DB2 SQL使用group by读取2个表

Question

I need to read 2 tables selecting unique dates and count the number of entries from both tables by date.

Example:
table 1                table 2
date                   date
----------             --------
2018-03-20             2018-03-15
2018-03-20             2018-03-20
2018-03-25             

The final result set should be:

date         sum from table 1  sum from table 2    total count
2018-03-15    0                     1                      1
2018-03-20    2                     1                      3
2018-03-25    1                     0                      1

could someone help how should I write the code with final result as above

Answer 1

I would recommend doing this using union all and group by :

select date, sum(in_1) as sum_1, sum(in_2) as cnt_2,
       sum(in_1) + sum(in_2) as total_cnt
from ((select date, 1 as in_1, 0 as in_2 from table1) union all
      (select date, 0, 1 from table2)
     ) t
group by date
order by date;

Answer 2

Do union all from table1 and table2 then do a group by. Sum up the row count per table source.

select date,
   sum(case when tbl=1 then 1 else 0 end) as sum_tbl_1,
   sum(case when tbl=2 then 1 else 0 end) as sum_tbl_2,
   sum(1) as total_count
from (
  select date, 1 as tbl
  from myTable1
    union all
  select date, 2 as tbl
from myTable2) t
group by t.date
order by t.date