如何根據用戶選擇構建動態字符串？

Question

我允許用戶選擇將用於SQL查詢Where子句的過濾器。他挑的複選框，例如：

CreateDate 
User 
Nummer 
Id 

我保持變量爲他們每個人：

Dim CreateDate as String = "CreateDate >= 2017-08-01" 
Dim User as String = "John" 
Dim Nummer as String = "789" 
Dim Id as String = "1" 

比方說，用戶選擇用戶，Nummer和ID。這應該構造以下字符串輸出：

Dim finalWhereString as String = "CreateDate >= 2017-08-01 And User = John And Id = 1" 

對我來說，棘手的部分是正確連接字符串。如何在字符串之間的正確位置添加「And」子句？

Answer 1

將WHERE 1 = 1添加到基本查詢，然後將AND添加到每個選定的條件。
肯定使用SqlParameter將值傳遞給sql查詢。

public class Condition 
{ 
    public Func<bool> IsSelected { get; set; } 
    public string Text { get; set; } 
    public SqlParameter Value { get; set; } 
} 

var conditions = new[] 
{ 
    new Condition 
    { 
     IsSelected =() => checkBoxDate.Checked, 
     Text = "CreateDate >= @CreatedDate", 
     Value = new SqlParameter("@CreatedDate", new DateTime(2017, 8, 1)) 
    }, 
    new Condition 
    { 
     IsSelected =() => checkBoxUser.Checked, 
     Text = "User = @User", 
     Value = new SqlParameter("@User", "John") 
    }, 
    new Condition 
    { 
     IsSelected =() => checkBoxNumber.Checked, 
     Text = "Number = @Number", 
     Value = new SqlParameter("@Number", 789) 
    }, 
    new Condition 
    { 
     IsSelected =() => checkBoxId.Checked, 
     Text = "Id = @Id", 
     Value = new SqlParameter("@Id", 12) 
    } 
} 

var selectedConditions = conditions.Where(condition => condition.IsSelected()).ToList(); 


var baseQuery = "SELECT Id, Number, User, CreatedDate FROM MyTable WHERE 1 = 1"; 
var parameters = selectedConditions.Select(condition => condition.Value); 
var commandText = 
    selectedConditions.Aggregate(new StringBuilder(), 
           (text, condition) => 
           { 
            text.Append(" AND "); 
            text.Append(condition.Text); 
            return text; 
           }, 
           (text) => 
           { 
            text.Insert(0, baseQuery); 
            return text.ToString(); 
           }); 

using (var connnection = new SqlConnection(connectionString)) 
using (var command = new SqlCommand(commandText, connection)) 
{ 
    command.Parameters.AddRange(parameters); 
    connection.Open(); 

    // execute command   
} 

由於@jmoreno建議您不要將WHERE 1 = 1添加到查詢中。然後SQL建設將看起來像下面的代碼 - 選擇你認爲更適合你的需求的方法。

var baseQuery = "SELECT Id, Number, User, CreatedDate FROM MyTable"; 
var selectedText = selectedConditions.Select(condition => condition.Text); 
var commandText = 
    selectedConditions.Any() 
     ? $"{baseQuery} WHERE {string.Join(" AND ", selectedText)}" 
     : baseQuery;