Select unrelated columns from two unrelated tables

Question

Is there an easy way to select two columns from two unrelated tables, in order to use those columns in an insert?

Disclaimer: Ideally, I would never need to do this because the schema would have been set up with a little something called "foreign keys" and "referential integrity". But, it seems as though neither of these concepts existed on the planet on which this schema was created.

Here's a simplified version of what I need to do:

Customer Table

Id     Name
------------
1      Eddie
2      Stone
3      Mike

Product Table

Id     Name
---------------------
100    Drum sticks
101    Guitar strings
102    Amplifier

Invoice Table

Id     CustomerName      ProductName
---------------------------------------
1000   Eddie             Guitar Strings
1001   Mike              Amplifier

In my SQL code, I've got a :CustomerId and a :ProductId, and ideally I'd like to do something like this:

INSERT Invoice (
    Id,
    CustomerName,
    ProductName
)
SELECT
    :InvoiceId,
    Customer.Name,
    Product.Name
FROM
    Customer,
    Product
WHERE
    Customer.CustomerId = :CustomerId
    and Product.ProductId = :ProductId

This works fine and dandy if both the Customer and Product records exist, but I need to also cater for the scenario where one of them doesn't exist. (Yes, really.)

The only way I can think to do it is by declaring variables like :CustomerName and :ProductName and pre-populating them outside of the insert statement.

Is there a way to achieve this without going down the extra variables approach?

Answer 1

You could do this:

INSERT Invoice (
    Id,
    CustomerName,
    ProductName
)
SELECT
    :InvoiceId,
    (
        SELECT
            Customer.Name
        FROM
            Customer
        WHERE
            Customer.CustomerId = :CustomerId
    ),
    (
        SELECT
            Product.Name
        FROM
            Product
        WHERE
            Product.ProductId = :ProductId
    )
FROM RDB$DATABASE

Answer 2

Next to the answer provided by Arion, you could use a FULL OUTER JOIN with a join condition that is always true. This only works correctly if both subqueries produce a single row.

SELECT
   :InvoiceId,
   CustomerName,
   ProductName
FROM (
   SELECT CustomerName
   FROM Customer
   WHERE CustomerId = :CustomerId
) a
FULL OUTER JOIN (
   SELECT ProductName
   FROM Product
   WHERE Product.ProductId = :ProductId
) b
   ON 1 = 1