Using Parameters for SQL Server Queries and Stored Procedures

Problem

One of the benefits of SQL is the ability to write a query and use parameters to dynamically act upon the data. Depending on the situation, there can be benefits to using a parameter query, but it is not always clear when or how to do this. In this tip, we look at different ways to pass in values as SQL parameters to queries and the advantages and disadvantages.

Solution

Properly parameterizing queries can bring advantages such as:

Encouraging execution plan reuse for complex queries.
Providing some protection against SQL Injection Attacks under some circumstances

Generally, when creating a condition in a query where you might use one of several values, it makes sense to parameterize. But, as will be discussed later in this tip, there are cases where the query cannot be fully parameterized.

Parameterizing a Query By Making It a Stored Procedure

If you want to find sales data for Jack, you could use a non-parameterized query that just pulls that data:

select SalesPerson, Mon, amount
from SalesData
where SalesPerson = 'Jack';

Under some circumstances, SQL Server can attempt to parameterize this behind the scenes to facilitate execution plan reuse. But its ability to do that can be limited. If you want to use this query repeatedly to get the data for different salespeople, you could instead parameterize the query and turn it into a stored procedure:

create procedure getSalesperson
@sp varchar(25)
as
select SalesPerson, Mon, amount
from SalesData
where SalesPerson = @sp;
go

And it could be called:

declare @sp varchar(25)
set @sp = 'Jack'
exec getSalesperson @sp

Doing it this way explicitly tells SQL Server what the parameters are, which makes query execution plan reuse more likely. It also ensures that the salesperson value is handled in a way that is normally safer and makes SQL Injection Attacks through this procedure more difficult.

This is substantially different from a stored procedure that builds the query through concatenation:

create procedure getSalesperson_bad
@sp varchar(25)
as
declare @sql varchar(max)
set @sql = '
select SalesPerson, Mon, amount
from SalesData
where SalesPerson = ''' + @sp + ''';'
exec (@sql)
go

This second version builds a non-parameterized query using dynamic SQL. It is simple to exploit a procedure like this in a SQL Injection Attack. It also does not explicitly tell SQL Server where the parameters are.

Parameterizing in T-SQL with sp_executesql

Another direct way to parameterize a query in T-SQL is to use sp_executesql and explicitly add your parameters. It looks like:

declare @sql nvarchar(4000)
declare @monthNo int
declare @minAmount decimal
set @sql = N'
select 
 SalesPerson 
from 
 dbo.SalesData
where 
 mon = @MonthNo
 and amount > @minAmount'
 
set @monthNo = 2
set @minAmount = 100
 
exec sp_executesql @sql, N'@monthNo int, @minAmount decimal', @monthNo, @minAmount

With sp_executesql, the first parameter executed is the SQL code. The second lists the parameters that will be supplied and indicates whether they are output variables. Then, the actual parameters are passed into the procedure. Both the SQL statement and the list of parameters must be presented in unicode (nvarchar, nchar, or a string prefixed by N like the parameter list in the example).

Parameter Query in SQL from other languages

Languages that interact with SQL tend to make it simple to parameterize. To parameterize a SqlCommand, put the names of the parameters in the CommandText and then use Parameters.Add to add parameters that match the name to the command before executing. It looks like:

using System;
using System.Text;
using System.Data.Sql;
using System.Data.SqlClient;
namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            using (SqlConnection conn = new SqlConnection("YourConnectionString"))
            {
                conn.Open();
                string sqlCom = "select SalesPerson from dbo.SalesData where mon = @monthNo and amount > @minAmount";
                SqlCommand cmd = new SqlCommand(sqlCom, conn);
                cmd.Parameters.Add("@monthNo", System.Data.SqlDbType.Int);
                cmd.Parameters["@monthNo"].Value = 2;
                cmd.Parameters.Add("@minAmount", System.Data.SqlDbType.Decimal);
                cmd.Parameters["@minAmount"].Value = 100.00;
                SqlDataReader reader = cmd.ExecuteReader();
                while (reader.Read())
                {
                    Console.WriteLine("SalesPerson = {0}", reader[0]);
                }
                reader.Close();
            }
        }
    }
}

A Python example using pyodbc would look like:

import pyodbc 
connString = (r"YourConnString")
conn = pyodbc.connect(connString)
curs = conn.cursor()
sql = """select SalesPerson 
        from dbo.SalesData 
        where mon = ? and amount > ?"""
curs.execute(sql, (2, 100.00))
for result in curs:
    print result

When the query cannot be (fully) parameterized

Parameterization brings several benefits, including some protection against SQL injection attacks under some circumstances. Certain types of dynamic queries cannot be fully parameterized. For instance, SQL Server will not accept a table name or column name as a parameter. If you tried to do it with sp_executesql:

declare @sql nvarchar(4000)
declare @colName nvarchar(256)
set @sql = N'
select @colName 
from dbo.SalesData
where mon = 2 and amount > 100.0'
set @colName = 'SalesPerson'
exec sp_executesql @sql, N'@colName nvarchar(256)', @colName

The server merely returns a result set of “SalesPerson”. Using a parameter for a table name in a query causes the server to try to interpret the parameter as a table variable. It gives an error like: “Msg 1087, Level 16, State 1, Line 3 Must declare the table variable “@tableName”.”

So, a procedure meant to run against an arbitrary table would need to build the SQL command by constructing the string. Other parts of that query could still be parameterized of course. A simplified example could look like:

set @tableName = 'SalesData'
set @monthNo = 2
set @sql = N'
 select SalesPerson 
 from ' + @tableName + '
 where mon = @monthNo'
exec sp_executesql @sql, N'@monthNo int', @monthNo

However, building the string that way can make SQL Injection attacks simpler, especially if the user is directly prompted to supply the table or column names. Depending on the expected use cases, it may be wise to perform some string validation before execution. Ensuring the application runs with the minimal necessary access to SQL Server can help mitigate risk to a degree.

Summary

In general, properly applied parameterization can assist in SQL Server security and can have performance implications. But, some queries cannot be fully parameterized, such as when the column names, table names, or other clauses need to be added or modified dynamically. When non-parameterized dynamic SQL is used, the performance and security implications should be kept in mind.

Next Steps

To find more on parameterizing in the context of dynamic SQL, look at Erland Sommarskog’s The Curse and Blessings of Dynamic SQL
SQL Injection: Defense in Depth has more on using parameterization as one layer of defense against some SQL injection attacks.
SQL Server Simple and Forced Parameterization has more information on how to encourage SQL Server to reuse query execution plans without explicitly parameterizing the queries yourself.

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