MYSQL，如果存在

Question

我有 3 个 table。用户、警报和首选项。用户显然是我的用户信息table。 Alert 是我的 table 存储用户 alert/notification 设置的地方，pref 是我存储用户首选项的地方。

并非所有用户都有首选项和警报条目。当我尝试拨打警报时，我运行因我的 SQL 知识而陷入困境。

我正在尝试对 user、alert 和 pref 进行 3 种方式加入，其中 uid 相同。我想 Select 所有 user.id 并忽略所有具有 alert.deleted 和 pref.deleted = 1

的 uid

    user    |    alert    |    pref
id           uid           uid
             alert_id      pref_id
             deleted       deleted

Answer 1

一种方法是

SELECT id
FROM   user
WHERE  user.id not in (SELECT uid FROM alert WHERE deleted = 1)
AND    user.id not in (SELECT uid FROM pref WHERE deleted = 1);

但是如果你想说你想要所有真正有警报和偏好的用户，而不是那些被删除的用户，你可以使用

SELECT     id
FROM       user
INNER JOIN pref
ON         user.id = pref.uid
AND        pref.deleted = 0
INNER JOIN alert
ON         user.id = alert.uid
AND        alert.deleted = 0;

在您的问题中，您说的是获取警报，因此可能没有特定用户的任何偏好，但您确实想要所有尚未删除的警报，然后使用

SELECT          id
FROM            user
LEFT OUTER JOIN pref
ON              user.id = pref.uid
AND             pref.deleted = 0
INNER JOIN      alert
ON              user.id = alert.uid
AND             alert.deleted = 0;

任你选。

Answer 2

SELECT id
FROM user u
WHERE NOT EXISTS (SELECT 1 FROM alert WHERE uid = u.id AND deleted = 1)
  AND NOT EXISTS (SELECT 1 FROM pref WHERE uid = u.id AND deleted = 1)
;

MYSQL，如果存在

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