从两个不相关的表中选择不相关的列

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