更新连接查询不更新表中的记录

Question

In my attendance table I want to update a record with S_ID and C_ID from StudentCourse bridge table but its not updating.

https://imgur.com/a/7ZBItur

Its working when I use it to select and display 2 columns from the StudentCourse table and 1 column from the Attendance table but it doesn't work when I use it to update the attendance table which for now is empty

    UPDATE Attendance
    SET S_ID = sc.S_ID,
    C_ID = sc.C_ID
    FROM Attendance a
    left outer join StudentCourse sc ON a.S_ID = sc.S_ID
    WHERE sc.S_ID=2 and sc.C_ID=2

There are two actually four tables:

• Student(S_ID(primary key))
• Course(C_ID(Primary key))
• StudentCourse((S_ID,C_ID(Foreign keys))Bridge table)
• and attendance table with (S_ID,C_ID(Foreign keys))

What I am doing is displaying data on datagridview by joining tables and selecting S_ID , S_Name , C_ID and Pre_Abs ( Attendance table column)columns from all these tables.

Now, I want to insert the info present in datagridview to the attendance table when I click on button.

I have done this already with simple insert query to attendance table by using datagrdview.rows[i].cell[2] property.

I want to know if there is any better idea to do this so that I can use JOIN instead of using datagridview property with for loop.

For now my attendance table is empty while Student , Course , and StudentCourse tables are filled with the data.

What I want is to display record( S_ID , C_ID ) from studentCourse table and ( Pre_Abs ) from Attendance table and when I submit the attendance; I want it to store Pre_abs record against each S_ID , C_ID in the attendance table.

I don't think I can explain it any further.

Answer 1

You have declared alias for the table Attendance. So you should use alias reference before the column name as the same column name also available in other table. Can you please try this-

UPDATE a
SET a.S_ID = sc.S_ID,
a.C_ID = sc.C_ID
FROM Attendance a
LEFT OUTER JOIN StudentCourse sc ON a.S_ID = sc.S_ID
WHERE sc.S_ID=2 and sc.C_ID=2

I have also doubt about you where condition as you are Update table Attendance but filtering records on table StudentCourse. use of wrong filter can resultant in update all rows in table Attendance. Please be careful.

Answer 2

Hmmm. I do not understand the reason why your Attendance table exists in the first place. Your StudentCourse table already seems to contain info about which students are having which courses. I'll just assume the Attendance table has to deal with registering student attendance for each single lecture/lesson within a course.

Anyway, as @Psi already commented, you seem to initially want an INSERT query to create a record in the Attendance table with data based on information in the StudentCourse table. I guess that you are thinking about it too hard. ;-) You may try something like this:

INSERT INTO Attendance (S_ID, C_ID, Pre_Abs)
SELECT S_ID, C_ID, Pre_Abs
FROM StudentCourse
WHERE S_ID = 2 AND C_ID = 2

However, it is currently unclear how field Pre_Abs should be filled... You try to get it from table Attendance , but that would seem to be invalid.

Also, when creating INSERT-queries, make sure that the source data values (in the SELECT or VALUES clause) are in the same order as the target fields (in the INSERT clause). If the field/value order doesn't match, the data gets mixed up (if the query does not fail due to data type errors)!

And last but not least: if both Attendance and StudentCourse tables share the same composite key (S_ID, C_ID) , you should use that full key when joining related records of those tables:

FROM
    Attendance AS A
    LEFT JOIN StudentCourse AS SC ON
        SC.S_ID = A.S_ID AND
        SC.C_ID = A.C_ID

Hope this helps a little...