形成查询以返回有关两个不同表的信息

Question

I have one table shop_inventory and another shops . I want to count the number of DISTINCT zbid from shop_inventory AND the number of rows in shops where cid=1 AND zbid!=0 . I tried it like so:

SELECT COUNT(a.cid) shops,COUNT(DISTINCT b.zbid) buyers 
FROM shops a 
JOIN shop_inventory b ON b.cid=a.cid 
WHERE a.zbid!=0 AND a.cid=1

However, this returned 100 shops instead of 2, which is the correct answer. I guess I'm not understanding how JOIN works correctly. Can someone offer a fix for this query?

Answer 1

The JOIN on many related rows was throwing off your count of shops.

Try this solution:

SELECT COUNT(DISTINCT a.cid, a.zbid) shops, 
       COUNT(DISTINCT b.zbid) buyers
FROM   shops a
JOIN   shop_inventory b ON a.cid = b.cid
WHERE  a.cid = 1 AND a.zbid <> 0

Answer 2

If this gets you the count of shops you want:

SELECT COUNT(1) AS shops
  FROM shops a
 WHERE a.zbid != 0
   AND a.cid = 1

Then one way to include a count from another table is to use a correlated subquery. (There are some performance issues with this approach, this works well if the outer query returns a limited number of rows.)

SELECT COUNT(1) AS shops
     , ( SELECT COUNT(DISTINCT b.zbid)
           FROM shop_inventory b
          WHERE b.cid = a.cid
       ) AS buyers
  FROM shops a
 WHERE a.zbid != 0
   AND a.cid = 1

Another approach is to use a join of the subqueries (inline views)...

SELECT COUNT(1) AS shops
     , c.buyers AS buyers
  FROM shops a
  JOIN ( SELECT b.cid, COUNT(DISTINCT b.zbid) AS buyers
           FROM shop_inventory b
          WHERE b.cid = 1
          GROUP BY b.cid
       ) c
 WHERE a.zbid != 0
   AND a.cid = 1
   AND a.cid = c.cid

If those don't return the result set you are looking for, then I have probably misunderstood the specification.