Question

I have a query,

SELECT E.staffid AS staff
SUM(E.Frequency='Less') AS lessCount, 
SUM(E.Frequency='More') AS moreCount,  
FROM effort_frequency E 
INNER JOIN ost_staff S ON S.staff_id = E.staffid 
WHERE E.log_date BETWEEN '2012-05-01' AND '2012-05-23' 
GROUP BY E.staffid

I get the below table,

+-------+-----------+-----------+
| Staff | lessCount | moreCount |
+-------+-----------+-----------+
|  1    |    2      |     3     |
|  3    |    1      |     4     |
|  5    |    2      |     3     |
+-------+-----------+-----------+

You can see that the 2 & 4 is missing in the result, There will be no rows with staff_id 2 & 4 in the effort_frequency table for the given date, but I’m trying to join the ost_staff table, so that I can get something like the below table, The ost_staff will contain all the staff id, lets say 1-5.

+-------+-----------+-----------+
| Staff | lessCount | moreCount |
+-------+-----------+-----------+
|  1    |    2      |     3     |
|  2    |    0      |     0     |
|  3    |    1      |     4     |
|  4    |    0      |     0     |
|  5    |    2      |     3     |
+-------+-----------+-----------+

I tried outer joins but its not working, I think I’m making mistake somewhere.

Answer 1

Move the WHERE clause to ON clause, and change the INNER JOIN to RIGHT JOIN. And change the GROUP BY E.staffid to GROUP BY S.staff_id; likewise with SELECT E.staffid, change it to SELECT s.staff_id:

SELECT S.staff_id AS staff

    COALESCE(SUM(E.Frequency='Less'),0) AS lessCount, 
    COALESCE(SUM(E.Frequency='More'),0) AS moreCount,  

FROM effort_frequency E 

RIGHT JOIN ost_staff S 
ON S.staff_id = E.staffid 
AND E.log_date BETWEEN '2012-05-01' AND '2012-05-23' 

GROUP BY S.staff_id

If you want to use LEFT JOIN, switch the tables ost_staff and effort_frequency:

SELECT S.staff_id AS staff

    COALESCE(SUM(E.Frequency='Less'),0) AS lessCount, 
    COALESCE(SUM(E.Frequency='More'),0) AS moreCount,  

FROM ost_staff S 

LEFT JOIN effort_frequency E 
ON S.staff_id = E.staffid 
AND E.log_date BETWEEN '2012-05-01' AND '2012-05-23' 

GROUP BY s.staff_id

You can also use IFNULL in lieu of COALESCE. IFNULL works only on MySQL though