c# - 根据组合框所选项目填充列表框时出现 SQL 语法错误

Question

我正在开发一个应用程序，其中有四个组合框，即(用于类(class)的组合框1，用于组的组合框2，用于部分的组合框3，用于月份的组合框4)和一个列表框。我已从数据库中获取数据到“OnLoad Function”中的所有这些组合框。现在我想根据组合框在列表框中显示学生姓名。即当我从类(comboBox1)中选择“第 7 类”时。它应该显示所选类(class)(第 7 类)、所选组(在组合框 2 中选择)和所选部分(在组合框 3 中选择)的学生。这是我的代码，但它给了我 SQL 语法异常和 indexOutOfRange 异常。附上的数据库是mysql。 “dbOperation”是“myDatabse”类的对象，我在其中连接了数据库并实现了所有查询。

   public partial class FeeVoucherPrint : Form
    {

        myDatabase dbOperation = new myDatabase();
        DataTable table = new DataTable();
        public FeeVoucherPrint()
        {
            InitializeComponent();
            this.MaximizeBox = false;
            this.MinimizeBox = false;

        }

        private void FeeVoucherPrint_Load(object sender, EventArgs e)
        {
            table = dbOperation.select("* from class");
            comboBox1.DataSource = table;
            comboBox1.DisplayMember = "name";
            comboBox1.ValueMember = "id";

            table = dbOperation.select("* from `group`");
            comboBox2.DataSource = table;
            comboBox2.DisplayMember = "name";
            comboBox2.ValueMember = "id";

            table = dbOperation.select("* from section");
            comboBox3.DataSource = table;
            comboBox3.DisplayMember = "name";
            comboBox3.ValueMember = "id";

            table = dbOperation.select("* from months");
            comboBox4.DataSource = table;
            comboBox4.DisplayMember = "month";
            comboBox4.ValueMember = "id";
        }

        private void fill()
        {
            try
            {
                table = dbOperation.select("studentid from studentinclass where classid = " + comboBox1.SelectedValue + " and groupid = " + comboBox2.SelectedValue);
                table = dbOperation.select("* from student where studid = " + table.Rows[0]["studentid"]);
                listBox1.DataSource = table;
                listBox1.DisplayMember = "name";
                listBox1.ValueMember = "studid";
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }
        }
        private void button1_Click(object sender, EventArgs e)
        {

        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            fill();
        }

        private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
            fill();
        }

        private void comboBox3_SelectedIndexChanged(object sender, EventArgs e)
        {
            fill()
        }


    }

Answer 1

看起来您正在使用 DataTable.Select方法(在本例中是dbOperation.select)以错误的方式。

Gets an array of all DataRow objects that match the filter criteria.

这就是为什么您的 studentid from Studentinclass where 部分位于第一个 .Select 中，而 * from Student where 部分位于第二个 .Select 是不必要的。它们不是过滤器。

阅读 DataColumn.Expression property 的备注部分文档。