爲什麼我的SELECT與dataGrid重複？

Question

public static DataSet selectStudent() 
{ 
     MySqlConnection conn = connection(); 
     conn.Open(); 
     MySqlCommand cmd = new MySqlCommand(); 
     cmd.Connection = conn; 
     MySqlDataAdapter adap = new MySqlDataAdapter(@"SELECT person.*, student.gradePointAverage, student.majorField FROM person JOIN student", conn); 
     MySqlCommandBuilder sqlCmd = new MySqlCommandBuilder(adap); 
     DataSet sqlSet = new DataSet(); 
     adap.Fill (sqlSet, "studentInfo"); 
     conn.Close(); 
     return sqlSet; 
} 

和按鈕：

private void btnAdminStudentView_Click(object sender, EventArgs e) 
{ 
    DataSet ds = studentHelperClass.selectStudent(); 
    dataGridStudent.DataSource = ds.Tables["studentInfo"]; 
} 

爲什麼會變成這樣給我這樣的按鈕，結果點擊？因爲你沒有定義的記錄如何相互關聯

Answer 1

你查詢產生笛卡爾乘積。您需要添加一個條件，

SELECT person.*, student.gradePointAverage, student.majorField 
FROM person JOIN student 
    ON person.ID = student.ID // example only 

這意味着，從表person記錄與ID表student有關。

爲了進一步獲得更多的知識有關加入，請訪問以下鏈接：

• Visual Representation of SQL Joins