How can I update a field in one table with a field

问题:

Two tables:

COURSE_ROSTER - contains

• COURSE_ID as foreign key to COURSES
• USER_ID as field I need to insert into COURSES

COURSES - contains

• COURSE_ID as primary key
• INSTRUCTOR_ID as field that needs to be updated with USER_ID field from COURSE_ROSTER

What would the UPDATE sql syntax be? I am trying this, but no good... I'm missing something and I can't find it online.

UPDATE COURSES 
SET COURSES.INSTRUCTOR_ID = COURSE_ROSTER.USER_ID 
WHERE COURSE_ROSTER.COURSE_ID = COURSES.COURSE_ID

回答1:

Not all database vendors (SQL Server, Oracle, etc.) Implement Update syntax in the same way... You can use a join in SQL Server, but Oracle will not like that. I believe just about all will accept a correclated subquery however

  Update Courses C  
   SET Instructor_ID = 
          (Select User_ID from Course_Roster
           Where CourseID = C.Course_ID)

NOTE: The column User_ID in Course_Roster would probably be better named as InstructorId (or Instructor_Id) to avoid confusion

回答2:

    Update Courses
    SET Courses.Instructor_ID = Course_Roster.User_ID
    from Courses Inner Join Course_Roster 
    On Course_Roster.CourseID = Courses.Course_ID

This is assuming that your DBMS allows for joins on your update queries. SQL Server definitely allows this. If you cannot do something like this you need to look towards using a subquery.

回答3:

UPDATE COURSES 
SET COURSES.INSTRUCTOR_ID = COURSE_ROSTER.USER_ID 
FROM COURSES
INNER JOIN COURSE_ROSTER
    ON COURSE_ROSTER.COURSE_ID = COURSES.COURSE_ID

回答4:

Why do you need the column course.instructor_id if you fill it with COURSE_ROSTER.user_id? Isn't it redundant storage?

回答5:

UPDATE COURSES 
SET INSTRUCTOR_ID = CR.USER_ID 
FROM COURSES C
INNER JOIN COURSE_ROSTER CR   
   ON CR.COURSE_ID = C.COURSE_ID