c# - MySQL 查询在 C# 中不起作用，但直接在数据库上执行时有效

Question

在我的 C# 项目中，我尝试通过使用逗号分隔的字符串列表动态填充 WHERE 子句来查询数据库表:

List<string> productNames = new List<string>() { "A", "B", "C" };

// Construct SQL query
string names = String.Join(",", productNames.Select(n => "'" + n.ToLower() + "'").ToArray());
// names = 'a', 'b', 'c'

string query = "SELECT * FROM products WHERE LOWER(name) IN (@names);";

MySqlCommand cmd = new MySqlCommand(query, dbConn);
cmd.Parameters.AddWithValue("@names", names);

MySqlDataReader row = cmd.ExecuteReader();
while (row.Read())
{
    // Zero rows returned
}

当我运行上述命令时，没有返回任何行。但是，当我通过 MySQL Workbench 直接在数据库上运行 SQL 查询时，会找到以下行:

SELECT * FROM products WHERE LOWER(name) IN ('a', 'b', 'c');
// 3 rows found

为什么这在我的 C 代码中不起作用？

Answer 1

Brain 在他的博客文章 here 中对此进行了很好的解释。 。以下答案是这篇文章的摘录:您需要一次向数组中添加一个值。

List<string> productNames = new List<string>() { "A", "B", "C" };
var parameters = new string[productNames.Count];
var cmd = new SqlCommand();
for (int i = 0; i < productNames.Count; i++)
{
    parameters[i] = string.Format("@name{0}", i);
    cmd.Parameters.AddWithValue(parameters[i], productNames[i]);
}

cmd.CommandText = string.Format("SELECT * FROM products WHERE LOWER(name) IN ({0})", string.Join(", ", parameters));
cmd.Connection = new SqlConnection(connStr);

这是一个扩展且可重用的解决方案

public static class SqlCommandExt
{
    /// <summary>
    /// This will add an array of parameters to a SqlCommand. This is used for an IN statement.
    /// Use the returned value for the IN part of your SQL call. (i.e. SELECT * FROM table WHERE field IN ({paramNameRoot}))
    /// </summary>
    /// <param name="cmd">The SqlCommand object to add parameters to.</param>
    /// <param name="values">The array of strings that need to be added as parameters.</param>
    /// <param name="paramNameRoot">What the parameter should be named followed by a unique value for each value. This value surrounded by {} in the CommandText will be replaced.</param>
    /// <param name="start">The beginning number to append to the end of paramNameRoot for each value.</param>
    /// <param name="separator">The string that separates the parameter names in the sql command.</param>
    public static SqlParameter[] AddArrayParameters<T>(this SqlCommand cmd, IEnumerable<T> values, string paramNameRoot, int start = 1, string separator = ", ")
    {
        /* An array cannot be simply added as a parameter to a SqlCommand so we need to loop through things and add it manually. 
         * Each item in the array will end up being it's own SqlParameter so the return value for this must be used as part of the
         * IN statement in the CommandText.
         */
        var parameters = new List<SqlParameter>();
        var parameterNames = new List<string>();
        var paramNbr = start;
        foreach(var value in values)
        {
            var paramName = string.Format("@{0}{1}", paramNameRoot, paramNbr++);
            parameterNames.Add(paramName);
            parameters.Add(cmd.Parameters.AddWithValue(paramName, value));
        }

        cmd.CommandText = cmd.CommandText.Replace("{" + paramNameRoot + "}", string.Join(separator, parameterNames.ToArray()));

        return parameters.ToArray();
    }
}

你可以在你的方法中调用它，例如

var cmd = new SqlCommand("SELECT * FROM products WHERE LOWER(name) IN ({name})");
cmd.AddArrayParameters(new int[] {"A", "B", "C" }, "name");

请注意，sql 语句中的 "{name}" 与我们发送到 AddArrayParameters 的参数名称相同。 AddArrayParameters 将用正确的参数替换该值。