为什么我的SELECT与dataGrid重复？

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; 
} 

和按钮：

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

为什么会变成这样给我这样的按钮，结果点击？因为你没有定义的记录如何相互关联

Answer 1

你查询产生笛卡尔乘积。您需要添加一个条件，

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

这意味着，从表person记录与ID表student有关。

为了进一步获得更多的知识有关加入，请访问以下链接：

• Visual Representation of SQL Joins