SQL 表加入另一个表

Question

I have 2 Tables (maintable and categorytable)

maintable records:

categoryCode  field2   field3

1             XXXXXX1  ACTIVE
3             XXXXXX2  ACTIVE
1             XXXXXX3  ACTIVE
1             XXXXXX4  ACTIVE
3             XXXXXX5  ACTIVE
3             XXXXXX5  NOT ACTIVE

categorytable records:

categoryCode        categoryname

1                   categoryname1
2                   categoryname2
3                   categoryname3

So far I have this query

  SELECT COUNT(*) AS recordcount,
         categoryCode AS catCode,
         categorytable.categoryname
    FROM maintable, 
         categorytable
   WHERE categorytable.categoryCode = maintable.categoryCode 
     AND maintable.field3 = 'ACTIVE'
GROUP BY maintable.categoryCode 
ORDER BY categorytable.categoryCode

with the following output

recordcount catCode categoryname
----------------------------------
3           1       categoryname1
2           3       categoryname3

But I need an output something like this (Categories with 0 records included showing a recordcount of 0):

recordcount catCode categoryname
3           1       categoryname1
0           2       categoryname2
2           3       categoryname3

Answer 1

SELECT 
    coalesce(COUNT(maintable.categoryCode),0) AS recordcount, 
    categorytable.categoryCode AS catCode, 
    categorytable.categoryname
FROM 
    maintable 
RIGHT JOIN categorytable ON categorytable.categoryCode = maintable.categoryCode 
GROUP BY 
    categorytable.categoryCode, 
    categorytable.categoryname 
ORDER BY 
    categorytable.categoryCode

or, with a left join

SELECT 
    coalesce(COUNT(maintable.categoryCode),0) AS recordcount, 
    categorytable.categoryCode AS catCode, 
    categorytable.categoryname
FROM 
    categorytable
LEFT JOIN maintable ON categorytable.categoryCode = maintable.categoryCode 
GROUP BY 
    categorytable.categoryCode, 
    categorytable.categoryname 
ORDER BY 
    categorytable.categoryCode

Answer 2

You need to use a left join:

SELECT COUNT(*) AS recordcount, categoryCode AS catCode, categorytable.categoryname
FROM categorytable
LEFT JOIN maintable ON categorytable.categoryCode = maintable.categoryCode 
GROUP BY maintable.categoryCode 
ORDER BY categorytable.categoryCode

If you don't want to reverse the table names see AlexanderMP's answer.