Adding new rows into query from nonexistent data in the database table - sql

Question

I have the following sample table:
+----------+------+-------+
| DATE | NAME | HOURS |
+----------+------+-------+
| 2018-5-3 | JOHN | 8 |
+----------+------+-------+
| 2018-5-9 | JOHN | 5 |
+----------+------+-------+
How can I generate a query that fills new rows to the existent data, e.g, sample query result:
+-----------+------+-------+
| DATE | NAME | HOURS |
+-----------+------+-------+
| 2018-5-1 | JOHN | 0 |
+-----------+------+-------+
| 2018-5-2 | JOHN | 0 |
+-----------+------+-------+
| 2018-5-3 | JOHN | 8 |
+-----------+------+-------+
| 2018-5-4 | JOHN | 0 |
+-----------+------+-------+
| 2018-5-5 | JOHN | 0 |
+-----------+------+-------+
| 2018-5-6 | JOHN | 0 |
+-----------+------+-------+
| 2018-5-7 | JOHN | 0 |
+-----------+------+-------+
| 2018-5-8 | JOHN | 0 |
+-----------+------+-------+
| 2018-5-9 | JOHN | 5 |
+-----------+------+-------+
| 2018-5-10 | JOHN | 0 |
+-----------+------+-------+
Check that I've added 0 into HOURS column because JOHN doesn't appear with hours in the specified date (only in 2018-5-3 and 2018-5-8). I am currently trying to get this result. This is only the begin of a big table I need to process, so I'll need to generate this fixed values per user. I was trying using left/right join with previously generated dates but it didn't work.
Can you advice me the best way to accomplish it? Thanks.

Answer 1

Use generate_series() and left join:
select g.dte, t.name, coalesce(t.hours, 0) as hours
from generate_series('2018-05-01'::date, '2018-05-10'::date, interval '1 day') g(dte) left join
t
on g.dte = t.date;
For multiple users, you need to generate all the rows for all the users and then left join:
select g.dte, n.name, coalesce(t.hours, 0) as hours
from generate_series('2018-05-01'::date, '2018-05-10'::date, interval '1 day'
) g(dte) cross join
(select distinct name from t) n left join
t
on g.dte = t.date and n.name = t.name;