如何在多个表的COUNT个汇总中计算零结果？

Question

I have 3 example tables: 1. Person

PID         PNAME                         
----------- ------------------------------
111111111.  James Brock
222222222.  Dean Sampson
333333333.  Adam Klein
444444444.  Sam Toggle

2. Company

   CID CNAME

   ---

   123456789. Company A
   123456790. Company B
   123456791. Company C
   123456792. Company D

3. Working

   PID CNAME

   ---

   111111111. Company B
   111111112. Company A
   111111113. Company A
   111111114. Company C

I want to show the # of employees at each company, including those that don't have any employees (ie: 0 of them)

So I have this so far:

SELECT CNAME, COUNT(*) as EMPLOYEES
    FROM PERSON P, WORKING W, COMPANY C
    WHERE P.PID = W.PID
        AND W.CNAME = C.CNAME
    GROUP BY CNAME;

Which returns:

CNAME       EMPLOYEES                         
----------- ------------------------------
Company A   2
Company B   1
Company C   1

Notice that Company D is missing

I want this:

CNAME       EMPLOYEES                         
----------- ------------------------------
Company A   2
Company B   1
Company C   1
Company D   0

Is there an approach I can take to get the result I want? All other examples I found online only use 2 tables to match.

Answer 1

Never use commas in the FROM clause. Always use proper, explicit JOIN syntax. Then, the LEFT JOIN would be obvious.

SELECT C.CNAME, COUNT(W.PID) as EMPLOYEES
FROM COMPANY C LEFT JOIN
     WORKING W
     ON W.CNAME = C.CNAME
GROUP BY C.CNAME;

Note that the JOIN to PERSON is unnecessary. You are only counting the people, not looking up their names.