Question

With this standard authors/books setup:

CREATE TABLE authors (
    id int NOT NULL,
    name varchar(255) NOT NULL
)

CREATE TABLE books (
    id int NOT NULL,
    name varchar(255) NOT NULL,
    author_id int NOT NULL,
    sold int NOT NULL
)

INSERT INTO authors VALUES (1, 'author 1')
INSERT INTO authors VALUES (2, 'author 2')

INSERT INTO books VALUES (1, 'book 1', 1, 10)
INSERT INTO books VALUES (2, 'book 2', 1, 5)
INSERT INTO books VALUES (3, 'book 3', 2, 7)

this query somehow doesn’t work:

SELECT
    (
        SELECT
            count(*)
        FROM
            (
                SELECT
                    books.name
                FROM
                    books
                WHERE
                    books.author_id = authors.id
                    AND books.sold > (
                        SELECT
                            avg(sold)
                        FROM
                            books
                        WHERE
                            books.author_id <> authors.id
                    )
            ) AS t
    ) AS good_selling_books
FROM
    authors
WHERE
    authors.id = 1

The error message is:

SQL0204N “AUTHORS.ID” is an undefined name. SQLSTATE=42704

It looks like DB2 loses track of the outermost table after getting 3 levels deep into a subquery?

NOTE: This is just a fabricated query so it may not make much sense (and can be easily rewritten to only have 2 levels nesting which works fine). I just want to confirm if DB2 indeed has such a glaring shortcoming.

Answer 1

Just found the solution which is rather simple. DB2 has this LATERAL keyword which is needed for such query to work, e.g.

SELECT
    (
        SELECT
            count(*)
        FROM
            LATERAL( -- this is the only change
                SELECT
                    books.name
                FROM
                    books
                WHERE
                    books.author_id = authors.id
                    AND books.sold > (
                        SELECT
                            avg(sold)
                        FROM
                            books
                        WHERE
                            books.author_id <> authors.id
                    )
            ) AS t
    ) AS good_selling_books
FROM
    authors
WHERE
    authors.id = 1

The solution came from this blog post https://www.ibm.com/developerworks/mydeveloperworks/blogs/SQLTips4DB2LUW/entry/scoping_rules_in_db2125?lang=en, where the author also noticed the same shortcoming in DB2:

But DB2 also didn’t jump two levels up to S.c1. I suppose it could but, alas, it does not.