SQL内部连接有3个表

Question

So I believe I need a Inner Join for this query, but am not 100% sure.

First of all see my database diagram:

Click here for database diagram

What I'm trying to achieve:

• I'm trying to get all messages (so user_username, text, posted_at FROM Messages) where the user_username matches with the followed_username, and where the followed_username has a follower_username that = ?.
• So essentially, everyone followed by ?, I want to get all their messages.

Where ? = an inputed username

What I've tried so far

I've tried a number of sql statements and have thus far been unable to get be successful. These are some I have tried.

$sql = "SELECT user_username, text, posted_at FROM Messages, Users, User_Follows WHERE user_username = (SELECT username FROM Users WHERE username = (SELECT followed_username FROM User_Follows WHERE follower_username = ?)) ORDER BY posted_at DESC;";
$sql = "SELECT user_username, text, posted_at FROM Messages, User_Follows WHERE follower_username = ? AND followed_username = user_username;";
$sql = "SELECT user_username, text, posted_at FROM Messages JOIN User_Follows ON user_username = followed_username WHERE follower_username = followed_username;";

I now think I need to use an inner join to achieve what I want, but am not too sure whether this is correct, or how to go about it.

Thanks in advance.

Answer 1

Try something like:

SELECT M.user_username, M.text, M.posted_at 
FROM Messages M  
INNER JOIN Users U on  M.user_username= u.username
INNER JOIN User_Follows UF  on UF.followed_username = u.username
WHERE UFfollower_username = ? 
ORDER BY posted_at DESC;

Note for future designs that using a long varchar or nvarchar field is a poor choice for a field you will be joining on. Further, if the field is a character type field, it is often subject to being changed over time which is also a bad choice for a key field.

Integer joins are much faster generally. It might be ok if it can save you from having to to do some joins but in general it is a poor idea.

If someone changed his username (which does happen in every system I have ever worked in) you would then have to update all the child tables which could be quite a lot of records to update. If you used a surrogate key, you would only need to update the parent table.

Additionally the word text is a reserved word for many databases and it is best to avoid those in naming fields.

Answer 2

a simple sub query should do it:

select * from messages where user_username in
(
  select followed_username from user_follows
  where follower_username = 'your_input_username'
);