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Dynamic SQL in Stored Procedure

Suggested Videos
Part 139 - Implement search web page using ASP.NET and Stored Procedure
Part 140 - Implement search web page using ASP.NET and Dynamic SQL
Part 141 - Prevent sql injection with dynamic sql

In this video we will discuss, using dynamic sql in a stored procedure and it's implications from sql injection perspective. We will discuss performance implications of using dynamic sql in a stored procedure in a later video.



Consider the following stored procedure "spSearchEmployees". We implemented this procedure in Part 139 of SQL Server tutorial. This stored procedure does not have any dynamic sql in it. It is all static sql and is immune to sql injection.

Create Procedure spSearchEmployees
@FirstName nvarchar(100) = NULL,
@LastName nvarchar(100) = NULL,
@Gender nvarchar(50) = NULL,
@Salary int = NULL
As
Begin

     Select * from Employees where
     (FirstName = @FirstName OR @FirstName IS NULL) AND
     (LastName  = @LastName  OR @LastName  IS NULL) AND
     (Gender      = @Gender    OR @Gender    IS NULL) AND
     (Salary      = @Salary    OR @Salary    IS NULL)
End
Go



Whether you are creating your dynamic sql queries in a client application like ASP.NET web application or in a stored procedure, you should never ever concatenate user input values. Instead you should be using parameters.

Notice in the following example, we are creating dynamic sql queries by concatenating parameter values, instead of using parameterized queries. This stored procedure is prone to SQL injection. Let's prove this by creating a "Search Page" that calls this procedure.

Create Procedure spSearchEmployeesBadDynamicSQL
@FirstName nvarchar(100) = NULL,
@LastName nvarchar(100) = NULL,
@Gender nvarchar(50) = NULL,
@Salary int = NULL
As
Begin

     Declare @sql nvarchar(max)

     Set @sql = 'Select * from Employees where 1 = 1'
         
     if(@FirstName is not null)
          Set @sql = @sql + ' and FirstName=''' + @FirstName + ''''
     if(@LastName is not null)
          Set @sql = @sql + ' and LastName=''' + @LastName + ''''
     if(@Gender is not null)
          Set @sql = @sql + ' and Gender=''' + @Gender + ''''
     if(@Salary is not null)
          Set @sql = @sql + ' and Salary=''' + @Salary + ''''

     Execute sp_executesql @sql
End
Go

Add a Web Page to the project that we have been working with in our previous video. Name it "DynamicSQLInStoredProcedure.aspx". Copy and paste the following HTML on the page.

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Employee Search</title>
    <link rel="stylesheet"
        href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css"
        type="text/css" />
</head>
<body style="padding-top: 10px">
    <div class="col-xs-8 col-xs-offset-2">
        <form id="form1" runat="server" class="form-horizontal">
            <div class="panel panel-primary">
                <div class="panel-heading">
                    <h3>Employee Search Form</h3>
                </div>
                <div class="panel-body">
                    <div class="form-group">
                        <label for="inputFirstname" class="control-label col-xs-2">
                            Firstname
                        </label>
                        <div class="col-xs-10">
                            <input type="text" runat="server" class="form-control"
                                id="inputFirstname" placeholder="Firstname" />
                        </div>
                    </div>

                    <div class="form-group">
                        <label for="inputLastname" class="control-label col-xs-2">
                            Lastname
                        </label>
                        <div class="col-xs-10">
                            <input type="text" runat="server" class="form-control"
                                id="inputLastname" placeholder="Lastname" />
                        </div>
                    </div>

                    <div class="form-group">
                        <label for="inputGender" class="control-label col-xs-2">
                            Gender
                        </label>
                        <div class="col-xs-10">
                            <input type="text" runat="server" class="form-control"
                                id="inputGender" placeholder="Gender" />
                        </div>
                    </div>

                    <div class="form-group">
                        <label for="inputSalary" class="control-label col-xs-2">
                            Salary
                        </label>
                        <div class="col-xs-10">
                            <input type="number" runat="server" class="form-control"
                                id="inputSalary" placeholder="Salary" />
                        </div>
                    </div>
                    <div class="form-group">
                        <div class="col-xs-10 col-xs-offset-2">
                            <asp:Button ID="btnSearch" runat="server" Text="Search"
                                CssClass="btn btn-primary" OnClick="btnSearch_Click" />
                        </div>
                    </div>
                </div>
            </div>

            <div class="panel panel-primary">
                <div class="panel-heading">
                    <h3>Search Results</h3>
                </div>
                <div class="panel-body">
                    <div class="col-xs-10">
                        <asp:GridView CssClass="table table-bordered"
                            ID="gvSearchResults" runat="server">
                        </asp:GridView>
                    </div>
                </div>
            </div>
        </form>
    </div>
</body>
</html>

Copy and paste the following code in the code-behind page. 

using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

namespace DynamicSQLDemo
{
    public partial class DynamicSQLInStoredProcedure : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {}

        protected void btnSearch_Click(object sender, EventArgs e)
        {
            string connectionStr = ConfigurationManager
                .ConnectionStrings["connectionStr"].ConnectionString;
            using (SqlConnection con = new SqlConnection(connectionStr))
            {
                SqlCommand cmd = new SqlCommand();
                cmd.Connection = con;
                cmd.CommandText = "spSearchEmployeesGoodDynamicSQL";
                cmd.CommandType = CommandType.StoredProcedure;

                if (inputFirstname.Value.Trim() != "")
                {
                    SqlParameter param = new SqlParameter("@FirstName",
                        inputFirstname.Value);
                    cmd.Parameters.Add(param);
                }

                if (inputLastname.Value.Trim() != "")
                {
                    SqlParameter param = new SqlParameter("@LastName",
                        inputLastname.Value);
                    cmd.Parameters.Add(param);
                }

                if (inputGender.Value.Trim() != "")
                {
                    SqlParameter param = new SqlParameter("@Gender",
                        inputGender.Value);
                    cmd.Parameters.Add(param);
                }

                if (inputSalary.Value.Trim() != "")
                {
                    SqlParameter param = new SqlParameter("@Salary",
                        inputSalary.Value);
                    cmd.Parameters.Add(param);
                }

                con.Open();
                SqlDataReader rdr = cmd.ExecuteReader();
                gvSearchResults.DataSource = rdr;
                gvSearchResults.DataBind();
            }
        }
    }
}

At this point, run the application and type the following text in the "Firsname" text and click "Search" button. Notice "SalesDB" database is dropped. Our application is prone to SQL injection as we have implemented dynamic sql in our stored procedure by concatenating strings instead of using parameters.
' Drop database SalesDB --

In the following stored procedure we have implemented dynamic sql by using parameters, so this is not prone to sql injecttion. This is an example for good dynamic sql implementation.

Create Procedure spSearchEmployeesGoodDynamicSQL
@FirstName nvarchar(100) = NULL,
@LastName nvarchar(100) = NULL,
@Gender nvarchar(50) = NULL,
@Salary int = NULL
As
Begin

     Declare @sql nvarchar(max)
     Declare @sqlParams nvarchar(max)

     Set @sql = 'Select * from Employees where 1 = 1'
         
     if(@FirstName is not null)
          Set @sql = @sql + ' and FirstName=@FN'
     if(@LastName is not null)
          Set @sql = @sql + ' and LastName=@LN'
     if(@Gender is not null)
          Set @sql = @sql + ' and Gender=@Gen'
     if(@Salary is not null)
          Set @sql = @sql + ' and Salary=@Sal'

     Execute sp_executesql @sql,
     N'@FN nvarchar(50), @LN nvarchar(50), @Gen nvarchar(50), @sal int',
     @FN=@FirstName, @LN=@LastName, @Gen=@Gender, @Sal=@Salary
End
Go

On the code-behind page, use stored procedure spSearchEmployeesGoodDynamicSQL instead of spSearchEmployeesBadDynamicSQL. We do not have to change any other code. At this point run the application one more time and type the following text in the "Firstname" textbox and click the "Search" button.
' Drop database SalesDB --

Notice "SalesDB" database is not dropped, So in this case our application is not susceptible to SQL injection attack.

Summary : Whether you are creating dynamic sql in a client application (like a web application) or in a stored procedure always use parameters instead of concatenating strings. Using parameters to create dynamic sql statements prevents sql injection.

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