Question

Here i am again. Thank you so much for help.

Here is my Old question on how i can select and display the grade of student in a junction table.

Select and display all rows belonging to a specific ID

I did it successfully but now i am having trouble updating the grade in the junction table.

Here is my Example table again.

    "student"
-----------------------
|studentID | FullName |
-----------------------
|1234      | John    |
|1235      | Michael |
|1236      | Bryce   |

        "subject_bsit"
-----------------------------------
|subject_id| subject_name  |grade |
-----------------------------------
|    1     | Programming   |  3   |
|    2     | Networking    |  2.5 |
|    3     | Algorithm     |  1.75|
|    4     | Physical Educ |  2   |

This is the Junction table to connect the 
two now.

       "student_subject"
----------------------------
| student_id | subject_id |
----------------------------
|   1235     |      1     |
|   1235     |      2     |
|   1235     |      3     |
|   1234     |      1     |

And here is the query that i did. This is a example im trying to update the grade of studentID 1235 that have subject_id = 1

UPDATE 3:

sql = "UPDATE student_subject " & _
    " INNER JOIN subject_bsit " & _
    " ON subject_bsit.subject_id = student_subject.sub_id " & _
    " SET grade = 1 " & _
    " where student_subject.student_id='" & Txtbox.Text & "' AND student_subject.sub_id = 1"

The & Txtbox.Text & is where the user input the studentid don't mind it. Thank you so much im using visual studion vb.net.

And this is the exact error that i got.

You have an error in SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'FROM student_subject INNER JOIN subject_bsit ON subject_bsit.subject_id = studen' at line 1

Answer 1

您的查询不稳定，应如下所示

UPDATE student_subject 
INNER JOIN subject_bsit 
ON subject_bsit.subject_id = student_subject.sub_id 
SET grade=? 
where student_subject.student_id='" & Txtbox.Text & "' AND student_subject.sub_id = 1

Answer 2

首先，您不需要更新student_subject。由于subject_bsit表中没有Student_id列，因此无法将成绩与学生联系起来。

更新subject_bsit 设定等级= @等级其中subject_ID = @subject_ID

这将完成相同的事情。

这是我建议的模式。

学生

+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| studentID   | int(11)      | NO   | PRI | NULL    | auto_increment |
| studentName | varchar(100) | NO   |     | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+

成绩

+-----------+--------------+------+-----+---------+-------+
| Field     | Type         | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| studentID | int(11)      | NO   | PRI | NULL    |       |
| subjectID | int(11)      | NO   | PRI | NULL    |       |
| grade     | decimal(3,2) | YES  |     | NULL    |       |
+-----------+--------------+------+-----+---------+-------+

两个外键

grades.studentID（FK）<-students.studentID（PK）
grades.subjectID（FK）<-subject.subjectID（PK）

请注意，成绩表具有由2个字段组成的复合键。

主题

+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| subjectID   | int(11)      | NO   | PRI | NULL    | auto_increment |
| subjectName | varchar(100) | NO   |     | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+

与数据库通信时，请始终使用参数。这将有助于保护您的数据库免受Sql Injection的侵害。

即使出现错误，“使用”块也会关闭并处置数据库对象。

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim query As String = "Update grades 
                           Set grade = @grade
                           Where studentId = @studentId And subjectID = @subjectID"
    Using cn As New MySqlConnection(My.Settings.StudentConnection)
        Using cmd As New MySqlCommand(query, cn)
            cmd.Parameters.Add("@grade", MySqlDbType.Decimal).Value = 1
            cmd.Parameters.Add("@studentID", MySqlDbType.Int32).Value = CInt(TextBox1.Text)
            cmd.Parameters.Add("@subjectID", MySqlDbType.Int32).Value = 1
            cn.Open()
            cmd.ExecuteNonQuery()
        End Using
    End Using
End Sub

要显示学生的成绩，请将DataGridView添加到您的表单中。

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
    Dim dt As New DataTable
    Dim query As String = "Select st.studentName, sub.subjectName, g.grade
                            From grades g
                            Inner Join  students st  on g.studentID = st.studentID
                            Inner Join subjects sub on g.subjectID = sub.subjectID
                            Where st.studentID = @studentID;"
    Using cn As New MySqlConnection(My.Settings.StudentConnection)
        Using cmd As New MySqlCommand(query, cn)
            cmd.Parameters.Add("@studentID", MySqlDbType.Int32).Value = CInt(TextBox1.Text)
            cn.Open()
            dt.Load(cmd.ExecuteReader)
        End Using
    End Using
    DataGridView1.DataSource = dt
End Sub

How to update row in junction table for student grade

2 个答案: