MySQL SUM 0在JOIN上没有结果

Question

I have two tables, 'authorization' and 'transaction'.

authorization table

Auth_ID | User_ID | Auth_Hours
-------------------------------
5       | 1       | 60
6       | 2       | 40
7       | 3       | 50

transaction table

Auth_ID | User_ID | Used_Hours
-------------------------------
5       | 1       | 5
6       | 2       | 2
5       | 1       | 20
6       | 2       | 17
5       | 1       | 11
6       | 2       | 9

I want my query to sum Used_Hours and group by Auth_ID and User_ID from the transaction table. Here's the catch - I also want the query result to show users who have not used any of their Auth_Hours with a 0. See below:

QUERY

SELECT a.Auth_ID, a.USER_ID, a.Auth_Hours, SUM(t.Used_Hours)
FROM AUTHORIZATION a
JOIN TRANSACTION t
on a.Auth_ID = t.Auth_ID
and a.User_ID = t.User_ID
GROUP BY a.Auth_ID, a.USER_ID, a.Auth_Hours

Actual Result

Auth_ID | User_ID | Auth_Hours | Total Hours Used
-------------------------------------------------
5       | 1       | 60         | 36
6       | 2       | 40         | 28

Wanted Result

Auth_ID | User_ID | Auth_Hours | Total Hours Used
-------------------------------------------------
5       | 1       | 60         | 36
6       | 2       | 40         | 28
7       | 3       | 50         | 0

I would imagine the query to be relatively simple.

Answer 1

The JOIN statement is a shortcut for INNER JOIN which returns records that have matching values in both tables. If you want to return all records from one table and the matching records from the other table, then you should use an outer join ( LEFT [OUTER] JOIN or RIGHT [OUTER] JOIN ). Then you can use the IFNULL() or COALESCE() functions to convert NULL s to zeros:

SELECT a.Auth_ID, a.USER_ID, a.Auth_Hours, IFNULL(SUM(t.Used_Hours), 0) AS 'Total Hours Used'
FROM authorization a
LEFT JOIN transaction t ON a.Auth_ID = t.Auth_ID AND a.User_ID = t.User_ID
GROUP BY a.Auth_ID, a.USER_ID, a.Auth_Hours

Notice that I used single quotes to assign a string with spaces as a name to the total field (as you used in your examples). This will work in all databases. In MySQL you can also use back ticks, but that only works in MySQL.

Here is a good illustration about the different types of joining tables .

Answer 2

this query work in any mysql Engine or Version. use this query:

SELECT a.Auth_ID, a.USER_ID, a.Auth_Hours, CASE COUNT(t.Used_Hours) WHEN 0 THEN 0 ELSE SUM(t.Used_Hours) END AS 'Total Hours Used'
FROM authorization a
LEFT JOIN TRANSACTION t ON a.Auth_ID = t.Auth_ID AND a.User_ID = t.User_ID
GROUP BY a.Auth_ID, a.USER_ID, a.Auth_Hours