Concatenate tables, but empty rows require 0
I don't know how to explain the scenario with words. So I write examples:
I have a table named tblType
:
type_id | type_name
---------------------
1 | abb
2 | cda
3 | edg
4 | hij
5 | klm
And I have another table named tblRequest
:
req_id | type_id | user_id | duration
-------------------------------------------
1 | 4 | 1002 | 20
2 | 1 | 1002 | 60
3 | 5 | 1008 | 60
....
So what I'm trying to do is select SUM () from duration
for each type
for a specific user.
This is what I tried:
SELECT
SUM(r.`duration`) AS `duration`,
t.`type_id`,
t.`type_name`
FROM `tblRequest` AS r
LEFT JOIN `tblType` AS t ON r.`type_id` = t.`type_id`
WHERE r.`user_id` = '1002'
GROUP BY r.`type_id`
It might return something like this:
type_id | type_name | duration
-------------------------------
1 | abb | 60
4 | hij | 20
It works. But the problem is I want to get 0
as a value for another types
that doesn't have a row in tblRequest
. What I mean is that the output should look like this:
type_id | type_name | duration
-------------------------------
1 | abb | 60
2 | cda | 0
3 | edg | 0
4 | hij | 20
5 | klm | 0
I mean it should receive strings of all types, but 0 as a value for those types that don't have a string in tblRequest
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You can do the aggregation on tblRequest
and only then join it, using left join
to handle missing rows and coalesce
convert null
to 0s:
SELECT t.type_id, type_name, COALESCE(sum_duration, 0) AS duration
FROM tblType t
LEFT JOIN (SELECT type_id, SUM(duration) AS sum_duration
FROM tblRequest
WHERE user_id = '1002'
GROUP BY type_id) r ON t.type_id = r.type_id
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