SQL LIKE 表达式给出空结果

Question

似乎使用 Like 需要一些我不知道的精确格式。这是代码：

private void DoFilter()
{
    using (connection = new SqlConnection(connectionString))
    using (SqlDataAdapter adapter = new SqlDataAdapter("SELECT * FROM Customers WHERE CompanyName = @filter + '%'", connection))
    {
        adapter.SelectCommand.Parameters.AddWithValue("@filter", txtFilter.Text.Trim());

        DataTable TCustomers = new DataTable();
        adapter.Fill(TCustomers);

        lstCustomers.DisplayMember = "CompanyName";
        lstCustomers.ValueMember = "Id";
        lstCustomers.DataSource = TCustomers;

    }
}

代码没有引发任何错误。它只是将 ListBox (lstCustomers) 留空。（txtFilter是一个TextBox，过滤字符串在里面）

Answer 1

使用like operator代替=

SELECT * FROM Customers WHERE CompanyName like @filter + '%'"

Answer 2

您没有使用 LIKE 运算符，您使用的是 = 运算符。

应该是：

using (SqlDataAdapter adapter = new SqlDataAdapter(
    "SELECT * FROM Customers WHERE CompanyName LIKE @filter + '%'", connection))

按照您编写的方式，SQL 将匹配末尾带有 % 的精确字符串，因此您不会得到任何结果。

Answer 3

'Like' 应用作 (SELECT * FROM Customers WHERE CompanyName LIKE @filter + '%'):

private void DoFilter()
{
    using (connection = new SqlConnection(connectionString))
    using (SqlDataAdapter adapter = new SqlDataAdapter("SELECT * FROM Customers WHERE CompanyName LIKE @filter + '%'", connection))
    {
        adapter.SelectCommand.Parameters.AddWithValue("@filter", txtFilter.Text.Trim());

        DataTable TCustomers = new DataTable();
        adapter.Fill(TCustomers);

        lstCustomers.DisplayMember = "CompanyName";
        lstCustomers.ValueMember = "Id";
        lstCustomers.DataSource = TCustomers;

    }
}

SQL LIKE 表达式给出空结果

SQL LIKE expression gives empty result

c#

sql

sql-server

sql-like