Question

我想使用下拉列表中的值从SQL Server数据库中获取数据。

我的查询是

select  Age,City,State,Caste,IncomeMin,IncomeMax from Ruser
where (Age between '" + drplistagemin.SelectedItem + "' and '" + drplistagemax.SelectedItem + "') 
and (Religion= '" + drplistreligion.SelectedItem + "')  ");

我需要了解的是，如果Religion下拉列表的值是可选的而不是强制的，那么如何构建此查询？

Answer 1

在@Pratik的符号中：

SELECT Age,City,State,Caste,IncomeMin,IncomeMax
FROM Ruser
WHERE age BETWEEN @minAge AND @maxAge
AND religion = coalesce(@religion, religion);

Answer 2

我从不建议使用直接命令方法，但您可以尝试以下查询：

SELECT Age,City,State,Caste,IncomeMin,IncomeMax
FROM Ruser
WHERE age BETWEEN @minAge AND @maxAge
AND religion LIKE CASE WHEN @religonVal IS NULL THEN '%' ELSE @religonVal END;

请注意：您可以对存储过程中的变量使用适当的值。如果它能按您的意愿运作，请告诉我。

Answer 3

这样做的一种方法是为宗教使用NULL值，并将其转换为%，以便在SQL Server中进行LIKE比较。

此外 - 我总是将UI代码（事件处理程序等）与实际的数据库访问代码分开 - 所以在单独的DataAccess类中执行类似的操作（并且不要将其直接粘贴到页面代码中 - 背后）：

public List<RuserResults> GetRuserResults(int minAge, int maxAge, string religion)
{
    string selectStmt = "SELECT Age, City, State, Caste, IncomeMin, IncomeMax FROM Ruser " +
                        "WHERE Age BETWEEN @MinAge AND @MaxAge " + 
                        "AND Religion LIKE @religion";

    // set up your connection and command objects
    using(SqlConnection conn = new SqlConnection("--your-connection-string-here--"))
    using(SqlCommand cmd = new SqlCommand(selectStmt, conn))
    {
        // define the parameters
        cmd.Parameters.Add("@MinAge", SqlDbType.Int).Value = minAge;
        cmd.Parameters.Add("@MaxAge", SqlDbType.Int).Value = maxAge;
        cmd.Parameters.Add("@Religion", SqlDbType.VarChar, 100);

        // if you passed a value for the method parameter - use that value
        if(!string.IsNullOrEmpty(religion))
        {
           cmd.Parameters["@Religion"].Value = religion + "%";
        }
        else  // if no value was passed - just search for all religions
        {
           cmd.Parameters["@Religion"].Value = "%";
        }

        List<RuserResult> results = new List<RuserResult>();

        // open connection, run query, close connection
        conn.Open();

        using(SqlDataReader reader = cmd.ExecuteReader())
        {
           while(reader.Read())
           {
               // read the values, convert to a "RuserResults", and pass it back
               results.Add(ConvertReaderToRuserResult(reader));
           }
        }

        conn.Close();

        // return the results
        return results;
    }
}

然后从您的ASP.NET页面，您可以调用此

int minAge = Convert.ToInt32(drplistagemin.SelectedItem);
int maxAge = Convert.ToInt32(drplistagemax.SelectedItem);
string religion = drplistreligion.SelectedItem;

List<RuserResult> results = GetRuserResults(minAge, maxAge, religion);

// do something with the results returned here....

SQL Server中的可选查询参数

3 个答案: