使用 SQL 连接单个表中的对数

Question

I have two tables of events in bigquery that look like as follows. The main idea is two count the number of events in each table (are always pairs of event_id and user_id) and join them in a single table that for each pair in any table it tells the number of events.

table 1:

| event_id | user id |
| -------- | ------- |
| 1        | 1       |
| 2        | 1       |
| 2        | 3       |
| 2        | 5       |
| 1        | 1       |
| 4        | 7       |

table 2:

| event_id | user id |
| -------- | ------- |
| 1        | 1       |
| 3        | 1       |
| 2        | 3       |

I would like to get a table which has the number of events of each table:

| event_id | user id | num_events_table1 | num_events_table2 |
| -------- | ------- | ----------------- | ----------------- |
| 1        | 1       | 2                 | 1                 |
| 2        | 1       | 1                 | 0                 |
| 2        | 3       | 1                 | 1                 |
| 2        | 5       | 1                 | 0                 |
| 4        | 7       | 1                 | 0                 |
| 3        | 1       | 0                 | 1                 |

Any idea of how to do this with sql? I have tried this:

SELECT i1, e1, num_viewed, num_displayed FROM
(SELECT id as i1, event as e1, count(*) as num_viewed
FROM table_1
group by id, event) a
full outer JOIN (SELECT id as i2, event as e2, count(*) as num_displayed
FROM table_2
group by id, event) b
on a.i1 = b.i2 and a.e1 = b.e2

This is not getting exactly what I want. I amb getting i1 which are null and e1 that are null.

Answer 1

Consider below

#standardSQL
with `project.dataset.table1` as (
  select 1 event_id, 1 user_id union all
  select 2, 1 union all
  select 2, 3 union all
  select 2, 5 union all
  select 1, 1 union all
  select 4, 7 
), `project.dataset.table2` as (
  select 1 event_id, 1 user_id union all
  select 3, 1 union all
  select 2, 3 
)    
select event_id, user_id,
  countif(source = 1) as num_events_table1,
  countif(source = 2) as num_events_table2
from (
  select 1 source, * from `project.dataset.table1`
  union all 
  select 2, * from `project.dataset.table2`
)
group by event_id, user_id  

  

if applied to sample data in your question - output is

Answer 2

If I understand correctly, the simplest method is to modify your query via a USING clause along with COALESCE() :

SELECT id, event, COALESCE(num_viewed, 0), COALESCE(num_displayed, 0)
FROM (SELECT id, event, count(*) as num_viewed
      FROM table_1
      GROUP BY id, event
     ) t1 FULL JOIN
     (SELECT id , event, COUNT(*) as num_displayed
      FROM table_2
      GROUP BY id, event
     ) t2
     USING (id, event);

Note: This requires that the two columns used for the JOIN have the same name. If this is not the case, then you might still need column aliases in the subqueries.

Answer 3

One way is aggregate the union

select event_id, user id, sum(cnt1) cnt1, sum(cnt2) cnt2
from (
    select event_id, user id, 1 cnt1, 0 cnt2
    from table_1
    union all
    select event_id, user id, 0 cnt1, 1 cnt2
    from table_2 ) t
group by event_id, user id