c# - 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"];
 }

为什么点击按钮会得到这样的结果？

Answer 1

您的查询生成笛卡尔积，因为您未能定义记录之间的关联方式。你需要添加一个条件，

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

这意味着 person 表中的记录与 student 表中的 ID 相关。

要进一步了解有关联接的更多信息，请访问以下链接:

• Visual Representation of SQL Joins