Displaying rows in SQL Server where COUNT = 0

问题:

The following query only returns Region Names for regions where there have been orders.

   SELECT r.RegionName,
          COUNT (DISTINCT o.uid)
     FROM Orders AS o
LEFT JOIN Customers AS c ON o.CustomerID = c.uid
LEFT JOIN Regions AS r ON c.Region = r.uid
    WHERE (r.RegionName NOT LIKE 'NULL') 
      AND (r.RegionName <> '') 
      AND (r.RegionName NOT LIKE 'Region 1') 
      AND (o.DateOrdered LIKE '7%2011%')
 GROUP BY r.RegionName
 ORDER BY r.RegionName

How can I modify it so that all region names show up even when the "COUNT" is "0"?

回答1:

You need to either change your JOIN to Regions to be a RIGHT JOIN or make Regions the FROM table and then JOIN to the other tables from there.

I prefer the second method, since it seems more intuitive to me. You care about Regions here and you're trying to get information about Regions, so that should be in the FROM (IMO):

SELECT
    R.RegionName,
    COUNT(O.uid)
FROM
    Regions R
LEFT OUTER JOIN Customers C ON C.Region = R.uid  -- I really don't like this naming convention
LEFT OUTER JOIN Orders O ON
    O.CustomerID = C.uid AND
    O.DateOrdered LIKE '7%2011%'  -- Is your date really stored as a string? Ugh!
WHERE
    R.RegionName <> 'NULL' AND   -- This is VERY bad...
    R.RegionName <> '' AND
    R.RegionName <> 'Region 1'
GROUP BY
    R.RegionName
ORDER BY
    R.RegionName