PHP & MySql Login from Multiple Tables Issue

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PHP

Hi! I want to create login from multiple table like there are three table admin,employee and CRM. When they will enter their username and password it will check their department and will open the page responding that. like if admin put their username and password it will open admin/index.php page.

I have created one user login page but having problem now in creating multiple user login.

my one user login is like that....

$sql="SELECT username, password FROM admin where username='".$username."' and password='".$password."'";
$result=mysqli_query($conn,$sql);

// Mysql_num_row is counting table row
$count=mysqli_num_rows($result);

// If result matched $myusername and $mypassword, table row must be 1 row
if($count==1){

// Register $myusername, $mypassword and redirect to file "login_success.php"
$_SESSION['username'] = $username;
$_SESSION['password'] = $password;
header("location:admin/index.php");
}
else {
echo "Wrong Username or Password";
}