[sql] Why do we always prefer using parameters in SQL statements?

I am very new to working with databases. Now I can write SELECT, UPDATE, DELETE, and INSERT commands. But I have seen many forums where we prefer to write:

SELECT empSalary from employee where salary = @salary

...instead of:

SELECT empSalary from employee where salary = txtSalary.Text

Why do we always prefer to use parameters and how would I use them?

I wanted to know the use and benefits of the first method. I have even heard of SQL injection but I don't fully understand it. I don't even know if SQL injection is related to my question.

Old post but wanted to ensure newcomers are aware of Stored procedures.

My 10¢ worth here is that if you are able to write your SQL statement as a stored procedure, that in my view is the optimum approach. I ALWAYS use stored procs and never loop through records in my main code. For Example: SQL Table > SQL Stored Procedures > IIS/Dot.NET > Class.

When you use stored procedures, you can restrict the user to EXECUTE permission only, thus reducing security risks.

Your stored procedure is inherently paramerised, and you can specify input and output parameters.

The stored procedure (if it returns data via SELECT statement) can be accessed and read in the exact same way as you would a regular SELECT statement in your code.

It also runs faster as it is compiled on the SQL Server.

Did I also mention you can do multiple steps, e.g. update a table, check values on another DB server, and then once finally finished, return data to the client, all on the same server, and no interaction with the client. So this is MUCH faster than coding this logic in your code.

Two years after my first go, I'm recidivating...

Why do we prefer parameters? SQL injection is obviously a big reason, but could it be that we're secretly longing to get back to SQL as a language. SQL in string literals is already a weird cultural practice, but at least you can copy and paste your request into management studio. SQL dynamically constructed with host language conditionals and control structures, when SQL has conditionals and control structures, is just level 0 barbarism. You have to run your app in debug, or with a trace, to see what SQL it generates.

Don't stop with just parameters. Go all the way and use QueryFirst (disclaimer: which I wrote). Your SQL lives in a .sql file. You edit it in the fabulous TSQL editor window, with syntax validation and Intellisense for your tables and columns. You can assign test data in the special comments section and click "play" to run your query right there in the window. Creating a parameter is as easy as putting "@myParam" in your SQL. Then, each time you save, QueryFirst generates the C# wrapper for your query. Your parameters pop up, strongly typed, as arguments to the Execute() methods. Your results are returned in an IEnumerable or List of strongly typed POCOs, the types generated from the actual schema returned by your query. If your query doesn't run, your app won't compile. If your db schema changes and your query runs but some columns disappear, the compile error points to the line in your code that tries to access the missing data. And there are numerous other advantages. Why would you want to access data any other way?

In addition to other answers need to add that parameters not only helps prevent sql injection but can improve performance of queries. Sql server caching parameterized query plans and reuse them on repeated queries execution. If you not parameterized your query then sql server would compile new plan on each query(with some exclusion) execution if text of query would differ.

More information about query plan caching

In Sql when any word contain @ sign it means it is variable and we use this variable to set value in it and use it on number area on the same sql script because it is only restricted on the single script while you can declare lot of variables of same type and name on many script. We use this variable in stored procedure lot because stored procedure are pre-compiled queries and we can pass values in these variable from script, desktop and websites for further information read Declare Local Variable, Sql Stored Procedure and sql injections.

Also read Protect from sql injection it will guide how you can protect your database.

Hope it help you to understand also any question comment me.

Other answers cover why parameters are important, but there is a downside! In .net, there are several methods for creating parameters (Add, AddWithValue), but they all require you to worry, needlessly, about the parameter name, and they all reduce the readability of the SQL in the code. Right when you're trying to meditate on the SQL, you need to hunt around above or below to see what value has been used in the parameter.

I humbly claim my little SqlBuilder class is the most elegant way to write parameterized queries. Your code will look like this...

C#

var bldr = new SqlBuilder( myCommand );
bldr.Append("SELECT * FROM CUSTOMERS WHERE ID = ").Value(myId);
//or
bldr.Append("SELECT * FROM CUSTOMERS WHERE NAME LIKE ").FuzzyValue(myName);
myCommand.CommandText = bldr.ToString();

Your code will be shorter and much more readable. You don't even need extra lines, and, when you're reading back, you don't need to hunt around for the value of parameters. The class you need is here...

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.SqlClient;

public class SqlBuilder
{
private StringBuilder _rq;
private SqlCommand _cmd;
private int _seq;
public SqlBuilder(SqlCommand cmd)
{
    _rq = new StringBuilder();
    _cmd = cmd;
    _seq = 0;
}
public SqlBuilder Append(String str)
{
    _rq.Append(str);
    return this;
}
public SqlBuilder Value(Object value)
{
    string paramName = "@SqlBuilderParam" + _seq++;
    _rq.Append(paramName);
    _cmd.Parameters.AddWithValue(paramName, value);
    return this;
}
public SqlBuilder FuzzyValue(Object value)
{
    string paramName = "@SqlBuilderParam" + _seq++;
    _rq.Append("'%' + " + paramName + " + '%'");
    _cmd.Parameters.AddWithValue(paramName, value);
    return this;
}
public override string ToString()
{
    return _rq.ToString();
}
}

You are right, this is related to SQL injection, which is a vulnerability that allows a malicioius user to execute arbitrary statements against your database. This old time favorite XKCD comic illustrates the concept:

In your example, if you just use:

var query = "SELECT empSalary from employee where salary = " + txtSalary.Text;
// and proceed to execute this query

You are open to SQL injection. For example, say someone enters txtSalary:

1; UPDATE employee SET salary = 9999999 WHERE empID = 10; --
1; DROP TABLE employee; --
// etc.

When you execute this query, it will perform a SELECT and an UPDATE or DROP, or whatever they wanted. The -- at the end simply comments out the rest of your query, which would be useful in the attack if you were concatenating anything after txtSalary.Text.

The correct way is to use parameterized queries, eg (C#):

SqlCommand query =  new SqlCommand("SELECT empSalary FROM employee 
                                    WHERE salary = @sal;");
query.Parameters.AddWithValue("@sal", txtSalary.Text);

With that, you can safely execute the query.

For reference on how to avoid SQL injection in several other languages, check bobby-tables.com, a website maintained by a SO user.