Executing SQL Statements against a MySQL database

Once the connection and database selection is successfully performed, the PHP script can now proceed to operate on the database using standard SQL statements. The mysql_query() function is used for executing standard SQL statements against the database. In the following example, the PHP script queries a table called tbl_login in the previously selected database to determine if a username/password pair provided by the user is valid.

Assumption:

The tbl_login table has 3 columns named login, password, last_logged_in. The last_logged_in column stores the time that the user last logged into the system.

// The $username and $passwd variable should rightly be set by the login form
// through the POST method. For the purpose of this example, we’re manually coding it.
$username = “john”;
$passwd = “mypassword”;
// We generate a SELECT SQL statement for execution.
$sql="SELECT * FROM tbl_login WHERE login = '".$username."' AND password = '".$passwd."'";
// Execute the SQL statement against the currently selected database.
// The results will be stored in the $r variable.
$r = mysql_query($sql);
// After the mysql_query() command executes, the $r variable is examined to
// determine of the mysql_query() was successfully executed.
if(!$r) {
$err=mysql_error();
print $err;
exit();
}
// If everything went well, check if the query returned a result – i.e. if the username/password
// pair was found in the database. The mysql_affected_rows() function is used for this purpose.
// mysql_affected_rows() will return the number of rows in the database table that was affected
// by the last query
if(mysql_affected_rows()==0){
print "Username/password pair is invalid. Please try again.";
}
else {
// If successful, read out the last logged in time into a $last variable for display to the user
$row=mysql_fetch_array($r);
$last=$row["last_logged_in"];
print “Login successful. You last logged in at ”.$last.”.”;
}

The above example demonstrated how a SELECT SQL statement is executed against the selected database. The same method is used to execute other SQL statements (e.g. UPDATE, INSERT, DELETE, etc.) against the database using the mysql_query() and mysql_affected_rows() functions.

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