If value exists in table 1 insert into table 2 mysql

Question

For my ordering system, when an admin inserts an order i need to check if the customer he inserts for the order exists in my customer table.
Im trying something like this, but with no luck..

$sql = "IF EXISTS(SELECT * FROM customer WHERE customer_id = '$customer')
THEN insert into `order` (customer_id, product, quantity, creation_time, order_note, order_employee)
values ('$customer', '$product', '$quantity', 'now()', '$note', '$employee')";

Error: syntax error in first line. What is wrong here?
And is this the right approach? Is there a better way?

Answer 1

you might want to do it "backwards" - insert if exists:

$sql = "insert into `order` (customer_id, product, quantity, creation_time, order_note, order_employee)
select '$customer', '$product', '$quantity', now(), '$note', '$employee'
from dual
where EXISTS(SELECT * FROM customer WHERE customer_id = '$customer')";

Answer 2

Here's a neat trick you can do:

$sql = "INSERT INTO `order`
    (`customer_id`, `product`, `quantity`,
                                `creation_time`, `order_note`, `order_employee`)
    SELECT `customer_id`, '$product', '$quantity', now(), '$note', '$employee'
        FROM `customer` WHERE `customer_id`='$customer'";

Note that I'm trusting you have already properly sanitised your variables!

But this will only insert the order if the customer with that ID exists.

Answer 3

I think you should think your problem in another way.

Proceed with steps :

1 - I need to know if customer my customer exist?

$sql = "SELECT ID FROM customer WHERE customer_id = '$customer'";

2- I try my customer, does he exist?

    If(!empty($result_cust)){
$sql = insert into `order` (customer_id, product, quantity, creation_time, order_note, order_employee)
values ('$customer', '$product', '$quantity', 'now()', '$note', '$employee')";
}

With this way your code will be more easily to understand.

Divide and Rule

They said.