排除存在于另一个表

Question

我有2台这样的记录：

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 

现在我已经添加了两个更多的表像这样：

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，以便它排除memberID这是blocked.blocked_memberID也是排除emailID哪些在markedAsRead.emailID

我该怎么做？

Answer 1

您可以使用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) 

你也可以查找的LEFT JOINS或NOT IN排除不存在于一个特定的表中的记录。

编辑：通常EXISTS()和LEFTJOIN有类似performaces，有时它甚至可以执行比加入更好。

LEFT JOIN sulotion：中

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