SQL Server：相同的列但不同的值

Question

I have two tables: Users and Messages .

I try to write a proper select sentence, it does look like this for now:

SELECT
  Messages.Message_id,
  Messages.From_id,
  Users.Username,
  Messages.To_id,
  Users.Username,
  Messages.Date,
  Messages.Subject,
  Messages.Text,
  Messages.Picture
FROM
  Messages
INNER JOIN
  Users
ON Messages.From_id = Users.User_id
AND Messages.To_id = Users.User_id

I worry about Users.Username that repeats two times and I am not sure that this will show me a proper result.

Please help me to create a proper select sentence.

Answer 1

It is not different values. There is no join defined so it will fall down to ONE join condition, outputting the values two times. Done. If anything, this is something a code review would flag as obviously wrong - it is possible you want to show users from and to, in which case you need:

• Join TWO times (ie TWO inner join statements)
• Both joins having other name aliases (ie AS Users_To and As Users_From AND
• obviously getting the name from both aliases and not the table name.

Answer 2

Try the below query,

SELECT
Messages.Message_id,
Messages.From_id,
FromUser.Username,
Messages.To_id,
ToUser.Username,
Messages.Date,
Messages.Subject,
Messages.Text,
Messages.Picture
FROM
Messages
INNER JOIN
Users FromUser on Messages.From_id = FromUser.User_Id
INNER JOIN
Users ToUser on Messages.To_id = ToUser.User_Id