Question

I am running into some difficulty with the following query.

SELECT maker, speed 
FROM  
(
    SELECT * 
    FROM product 
    NATURAL JOIN laptop
) 
WHERE hd > 30;

I am trying to find the maker and speed of laptops with hard drives greater than 30 gigabytes.

I have two relations laptops and products. Laptops has the tuples (maker, model, type) and laptop has the tuples (model, speed, ram, hd, screen, and price).

What I think I am doing.

• Joining product with laptop with natural join, which I think and does
  (when submitted by itself) give me a relation of laptop but with two
  more columns maker and type.
• I then am trying to just select maker and speed from that table where
  the size of the hd is greater than 30.

Answer 1

A subquery in the FROM clause requires a table alias:

SELECT maker, speed 
FROM  
(
    SELECT * 
    FROM product 
    NATURAL JOIN laptop
  /* include an alias with AS */
) AS products_sub
WHERE hd > 30;

From the docs:

A table_subquery is also known as a subquery in the FROM clause. Such subqueries must include an alias to give the subquery result a table name. A trivial example follows; see also Section 12.2.9.8, “Subqueries in the FROM Clause”.

However for your example, the subquery isn’t needed at all.

SELECT maker, speed 
FROM products NATURAL JOIN laptop
WHERE hd > 30;

Note that NATURAL JOINs are not usually recommended, and it is best to be explicit about the columns joined in an ON clause.