Summarize count of multi table in single SQL query - sql

Question

I have three table with below details:
Table 1: worklog
+-----------+------------+-------------+
| worklogid | technician | description |
+-----------+------------+-------------+
| 1 | john | some text |
+-----------+------------+-------------+
| 2 | jack | some text |
+-----------+------------+-------------+
| 3 | john | some text |
+-----------+------------+-------------+
| 4 | jenifer | some text |
+-----------+------------+-------------+
Table 2: task
+--------+-------+-------------+
| taskid | owner | description |
+--------+-------+-------------+
| 1 | john | some text |
+--------+-------+-------------+
| 2 | john | some text |
+--------+-------+-------------+
| 3 | john | some text |
+--------+-------+-------------+
| 4 | jack | some text |
+--------+-------+-------------+
Table 3: request
+-----------+------------+-----------+-------------+
| requestid | technician | title | description |
+-----------+------------+-----------+-------------+
| 1 | john | some text | ... |
+-----------+------------+-----------+-------------+
| 2 | sara | some text | ... |
+-----------+------------+-----------+-------------+
| 3 | john | some text | ... |
+-----------+------------+-----------+-------------+
| 4 | jack | some text | ... |
+-----------+------------+-----------+-------------+
Now I need to SQL query for this result:
+------------+------------------+---------------+------------------+
| technician | count(worklogid) | count(taskid) | count(requestid) |
+------------+------------------+---------------+------------------+
| john | 2 | 3 | 2 |
+------------+------------------+---------------+------------------+
| jack | 1 | 1 | 1 |
+------------+------------------+---------------+------------------+
| jenifer | 1 | 0 | 0 |
+------------+------------------+---------------+------------------+
| sara | 0 | 0 | 1 |
+------------+------------------+---------------+------------------+
What should I do?

Answer 1

One method is to just use union all and aggregation:
select techician, sum(is_workid), sum(is_taskid), sum(is_requestid)
from ((select technician, 1 as is_workid, 0 as is_taskid, 0 as is_requestid
from worklog
) union all
(select owner, 0, 1, 0
from task
) union all
(select technician, 0, 0, 1
from request
)
) t
group by technician;
In Postgres, you can also aggregate before joining:
select *
from (select technician, count(*) as num_workid
from worklog
group by technician
) w full join
(select owner as technician, count(*) as num_task
from task
group by owner
) t
using (technician) full join
(select technician, count(*) as num_request
from request
group by technician
) w
using (technician);
With a full join, I find that using is simpler than on clauses. But the name needs to be the same in all the tables.