如何在SQL中設置數據之間的關系限制

Question

因此，我有一個Player表，其中包含PlayerId，firstname，lastname等字段。還有一個Teams表，其中包含TeamId，teamname等字段。我還有一個查找表，通過顯示將兩個表鏈接在一起球員在哪支球隊上。 我的問題是，如果我想向用戶顯示尚未超過20人限制的團隊，但團隊中仍有空間，我該怎么做？

Answer 1

顯示球員人數不超過20的隊伍：

SELECT t.teamname
FROM team t
LEFT JOIN player_team pt ON
  t.teamid = pt.teamid
GROUP BY t.teamname
HAVING COUNT(pt.playerid) <= 20 -- you probably mean < 20

添加有關插槽可用性的信息：

SELECT t.teamname, CONCAT(COUNT(pt.playerid), '/20') AS slots_taken -- if 20 is max
FROM team t
LEFT JOIN player_team pt ON
  t.teamid = pt.teamid
GROUP BY t.teamname
HAVING COUNT(pt.playerid) <= 20 -- you probably mean < 20

Answer 2

將以下內容視為表格和列，

player_table
------------
playerid
firstname
lastname

.
teams
-----
teamid
teamname

.
player_team_association
------------------------
teamid
playerid
.

您的查詢應如下所示

select t.teamname, count(pta.playerid) as PlayerLimit,
(20 - PlayerLimit) as AvailableSlots
from teams t,player_team_association  pta
where t.teamid = pta.teamid
group by pta.teamid having PlayerLimit <=20