Insert into MYSQL Database if not exists - php

Question

I'm trying to add a customer to the MySQL database I created.
Whenever somebody orders an item on the online store, the customer is added to the database (I dont want duplicates). Here is my php code:
$sqlInsert = "INSERT INTO Customers (FirstName, Address, Phone)
VALUES (".$userName.",".$address.",".$phone.")";
if(mysqli_query($conn, $sqlInsert)) {
echo "new member registered successfully!";
} else {
echo "Error: " . $sqlInsert . "<br>" . $mysqli_error($conn);
}
I have looked into queries such as INSERT INTO... WHERE NOT EXISTS. But I don't understand the syntax for my case, and don't know if it would work.
here is my MYSQL customer table code:
CREATE TABLE IF NOT EXISTS Customers (
PersonID INT(11) NOT NULL AUTO_INCREMENT,
Email VARCHAR(100),
FirstName VARCHAR(100) NOT NULL,
LastName VARCHAR(100),
City VARCHAR(90),
Zip INT(10),
CustomerState VARCHAR(50),
Address VARCHAR(200),
Country VARCHAR(20),
Phone VARCHAR(50) NOT NULL,
PRIMARY KEY (PersonID)
);

Answer 1

INSERT INTO Customers (FirstName, Address, Phone)
SELECT * FROM (SELECT '$firstName', '$address', '$phone') AS tmp
WHERE NOT EXISTS (
SELECT FirstName from Customers WHERE FirstName= '$firstName'
) LIMIT 1;
This will prevent based on the first name, you may use all these columns for checking, I assume the matching column should be email, you can use that.
I just added the parameters within the query for you to get an idea, use parameter binding to avoid sql injection.
OR
select * from customers where .... //
Get the size of result set and if size > 0 that means there is a row already, so do not insert it.
Sql statement taken from MySQL: Insert record if not exists in table and modified.

Answer 2

Try select query before insert and check the rows...Try to use Mysqli Prepared statement. I do this code for your way...
<?php
$sqlselect = "SELECT * FROM Customers WHERE FirstName = ".$userName." AND Address = ".$address." AND Phone = ".$phone;
$exqry = mysqli_query($conn, $sqlselect);
$cnt = count($exqry);
if($cnt == 0){
$sqlInsert = "INSERT INTO Customers (FirstName, Address, Phone)
VALUES (".$userName.",".$address.",".$phone.")";
if(mysqli_query($conn, $sqlInsert)) {
echo "new member registered successfully!";
} else {
echo "Error: " . $sqlInsert . "<br>" . $mysqli_error($conn);
}
}else{
echo "Member already in table.";
//do your update or other stuff.
}
?>