SQL 聚合来自多个表的数据并对指定的列求和

Question

I have three tables as below:
Table 1 :

user_id   user_label   code1   count1 
-------   ----------   ------  ------
1         x            a       1 
1         x            c       1
1         y            a       1 
2         x            a       1

Table 2 :

user_id   user_label   code2   count2 
-------   ----------   ------  ------
1         x            b       1 
1         x            d       2
1         y            b       1 
2         x            b       1

Table 3 :

user_id   user_label   code3   count3 
-------   ----------   ------  ------
1         x            c       1 
1         x            e       1
1         y            c       1 
2         x            c       1

And I would like to sum the count from these three tables for the same user_id + user_label + code , and keep the rest records, the desired result would look like below :

user_id   user_label   code   total_count 
-------   ----------   ------  ------
1         x            a       1 
1         x            c       2
1         x            b       1 
2         x            d       2
1         x            e       1
1         y            a       1
1         y            b       1
1         y            c       1
2         x            a       1
2         x            b       1
2         x            c       1

The records (1,x,c) can be found both in Table 1 and Table 3 and thus their count should be summed, and the rest stays the same in the result table.
Right now what I have in mind is using UNION operation like below :

SELECT * FROM tb1   UNION
SELECT * FROM tb2  UNION
SELECT * FROM tb3 

This will give me all distinct rows from those three tables but I am not sure how to do the summation on counts on top of that, any help or suggestion would be appreciated.

Answer 1

As you noted, union will remove duplicates, so you should use union all . Once you do that, you can wrap that query with an aggregate query to get the sum of the counts:

SELECT   user_id, user_label, code, SUM(cnt) AS total_count
FROM     (SELECT user_id, user_label, code1 as code, count1
          FROM   table1
          UNION ALL
          SELECT user_id, user_label, code2, count2
          FROM   table2
          UNION ALL
          SELECT user_id, user_label, code3, count3
          FROM   table3) t
GROUP BY user_id, user_label, code

Answer 2

I would be more explicit in the select statements then wrap the union into a subquery.

SELECT user_id, user_label,code, SUM(x_count) as total_count FROM
    SELECT user_id, user_label, code1 as code, count1 as x_count
    FROM tb1 
    UNION
    SELECT user_id, user_label, code2 as code, count2 as x_count
    FROM tb2 
    UNION
    SELECT user_id, user_label, code3 as code, count3 as x_count
    FROM tb3)
GROUP BY user_id, user_label, code

Answer 3

You don't have duplicates within tables, so you can also use full join :

select user_id, user_label, code1,
       (coalesce(t1.count1, 0) + coalesce(t2.count1, 0) + coalesce(t3.count1, 0)
       ) as total_count
from table1 t1 full join
     table2 t2
     using (user_id, user_label, code1) full join
     table3 t3
     using (user_id, user_label, code1) ;