MySQL的：如何只選擇列（接受）= 1的地方？

Question

這是我的代碼，除非我僅想限制它以僅過濾來自好友表的“已接受”，否則它會正確顯示，即= 1。

SELECT users.id, users.username, users.avatar, COUNT(friends.accepted) AS friends 
FROM users
  LEFT JOIN friends ON friends.user1=users.username || friends.user2=users.username
GROUP BY users.username
ORDER BY friends DESC LIMIT 20

Answer 1

如果只想讓一個朋友接受一個人，請使用HAVING （為簡單HAVING ， HAVING等效於WHERE來過濾聚合結果）：

SELECT users.id, users.username, users.avatar, COUNT(*) AS friends 
FROM users
LEFT JOIN friends ON friends.user1=users.username || friends.user2=users.username
GROUP BY users.username
HAVING COUNT(friends.accepted) = 1
ORDER BY friends DESC LIMIT 20

如果您要計算接受的朋友數= 1，則應該使用簡單的WHERE ：

SELECT users.id, users.username, users.avatar, COUNT(*) AS friends 
FROM users
LEFT JOIN friends ON friends.user1=users.username || friends.user2=users.username
WHERE friends.accepted = 1
GROUP BY users.username
ORDER BY friends DESC LIMIT 20

或者，如果您還想列出沒有朋友的人，那么他們被接受= 1：

SELECT users.id, users.username, users.avatar, 
    SUM(CASE friends.accepted = 1 THEN 1 ELSE 0 END) AS friends 
FROM users
LEFT JOIN friends ON friends.user1=users.username || friends.user2=users.username
GROUP BY users.username
ORDER BY friends DESC LIMIT 20

Answer 2

使用具有規則的檢查條件來確認接受的計數是否為1

SELECT users.id, users.username, users.avatar, COUNT(friends.accepted) AS friends 
  FROM users
  LEFT JOIN friends ON friends.user1=users.username || friends.user2=users.username
  GROUP BY users.username
having count(friends.accepted)=1
  ORDER BY friends DESC LIMIT 20

Answer 3

COUNT(columnname)計算columnname不為null的所有記錄。 因此，您不想使用COUNT(friends.accepted) ，因為0和1都不為空，因此您要計算所有朋友是否接受。

請改用SUM來計算所有接受的= 1。 如果用戶根本沒有朋友，那么您將獲得NULL。 在這種情況下，請使用COALESCE將NULL設置為零，如果這會打擾您的話。

SELECT 
  users.id, 
  users.username, 
  users.avatar, 
  COALESCE(SUM(friends.accepted),0) AS friends 
FROM users
LEFT JOIN friends ON users.username IN (friends.user1, friends.user2)
GROUP BY users.id
ORDER BY friends DESC LIMIT 20;

（在您只對接受的朋友感興趣的簡單查詢中，您當然可以在ON子句中簡單地添加AND accepted = 1 ，以便僅獲取接受的朋友並使用COUNT 。）

另一種方法是在子查詢中計數：

SELECT id, username, avatar, 
  (
    select count(*) 
    from friends 
    where users.username in (friends.user1, friends.user2)
    and friends.accepted = 1
  ) as friends
FROM users
ORDER BY friends DESC LIMIT 20;