MYSQL仅当另一个表中不存在两列的唯一键对时，才如何更新表？

Question

I am setting up a voting program where I want to limit users to one vote up. I have two tables: (a) subject table and (b) vote table.

**Subject_Table**              

SID   Subject   Total_Votes          
1     Cows      5     
2     Chickens  3     


**Vote_Table**

VID   Subject   User    Voteup           
1     Cows      John    1     

Total votes equal votes from all users. Voteup can only equal 1 if an subject is voted on. I have made a unique value pair with Subject and User with the following statement without any problems:

ALTER TABLE Vote_Table ADD UNIQUE limitvote(Subject,User);

When a subject is voted on for the first time the following queries execute:

$sql="INSERT INTO Vote_Table (Subject, User,Voteup) VALUES ('$Subject', '$User', '$Voteup') ON DUPLICATE KEY UPDATE up=1";
$q = "UPDATE Subject_Table SET Total_Votes = $votes_up= //current votes plus 1;

Even if John votes twice the vote will always equal 1 in the Vote_Table due to ON DUPLICATE KEY UPDATE up=1. But this is not so in the Subject_Table. Without a constraint, John could vote infinitely. The constraint must be the subject-user pair. Constraints cannot be only Subject or User since John can vote for other subjects and other users can vote for Cows.

How can I check Vote_Table to see if the unique Subject-User pair (Cows and John)) exists before I update Subject_Table?

Answer 1

You can incorporate it into the WHERE clause:

UPDATE Subject_Table
   SET Total_Votes = Total_Votes + 1
 WHERE Subject = ...
   AND NOT EXISTS
        ( SELECT 1
            FROM Vote_Table
           WHERE Subject = ...
             AND User = ...
        )
;

That said, I'm not sure that you really need Subject_Table at all; you can create a view on the Vote_Table that will give you the same information, without having to create separate table. (See the the Wikipedia article on Database normalization .) That could look like this:

CREATE VIEW Subject_View AS
SELECT Subject,
       COUNT(1) AS Total_Votes
  FROM Vote_Table
 GROUP
    BY Subject
;