How can I make my system not to insert value to database if there is a same user with the same username in PHP?

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PHP

<pre>           $Username_controller=$connection->query("SELECT * FROM progmeetusers WHERE Username='$Username_value'");

            if($Username_controller->num_rows==0){

                $Username_Updater=$connection->query("UPDATE progmeetusers SET Username='$Username_value' WHERE User_ID_value='$User_ID'");

            }

            

            else{

                $Username_Updater=" ";

                die("<script>alert('The Username that you selected is used by antoher user.Please go back and try again.')</script>");

What I have tried:

I am trying to make a registration page and I am trying to make the system not add users to the database if there is any account with the same username. I coded something but there is a problem. If the username value is the same as any account in the database. The system takes the values to the database and makes the Username column empty. How can I fix this?

Posted 30-Jul-22 0:32am

Member 15722220

Updated 30-Jul-22 1:35am

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OriginalGriff · Answer 1 · 2022-07-30T01:35:00

First off, don;t do it like that. Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'

The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'

Which SQL sees as three separate commands:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';

A perfectly valid SELECT

SQL

DROP TABLE MyTable;

A perfectly valid "delete the table" command

SQL

--'

And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

Secondly, that code doesn't do what you want: you specifically check that there are no users with that name, and if there aren't, you try to update all rows that match the ID to the new name. You can't add a new user - which is what a registration page is for - with an UPDATE, because the user doesn't exist yet! Instead, use an INSERT to add a row, but make sure that your ID column is either IDENTITY or UNIQUEIDENTIFIER or you are creating massive problems for yourself when you go live.