排除存在于另一个表中的记录

Question

我有两张这样的桌子：

emails:
emailID int(10) auto_increment, memberID int(10), emailData text, and so on

members:
memberID int(10) auto_increment, user_name char(40), password char(50), and so on

我的查询是这样的：

select 
emails.emailID, emails.emailData, 
members.memberID, members.user_name 
from emails, members where
emails.memberID = members.memberID

现在我又添加了两个类似这样的表格：

blocked:
id int(10) auto_increment, memberID int(10), blocked_memberID int(10)

markedAsRead:
id int(10) auto_increment, memberID int(10), emailID int(10)

我想修改我的原始查询，使其排除 memberID 中的 blocked.blocked_memberID 并排除 emailID 中的 markedAsRead.emailID

我该怎么做？

Answer 1

你可以使用NOT EXISTS：

SELECT ....
FROM   .... 
WHERE  ..... // Replace the dots with Your Query
  AND NOT EXISTS(SELECT 1 FROM blocked
                 WHERE emails.memberID = blocked.memberID)
  AND NOT EXISTS(SELECT 1 FROM markedAsRead
                 WHERE emails.emailID = markedAsRead.emailID)

您还可以查找 LEFT JOINS 或 NOT IN 以排除特定表中不存在的记录。

编辑： 通常EXISTS() 和LEFTJOIN 具有相似的性能，有时甚至可以比连接性能更好。

LEFT JOINsolotion：

SELECT ...
FROM ...
LEFT JOIN blocked 
 ON(WHERE emails.memberID = blocked.memberID)
LEFT JOIN markedAsRead
 ON(emails.emailID = markedAsRead.emailID)
WHERE ...
 AND blocked.memberID IS NULL
 AND markedAsRead.emailID IS NULL