Select a row field with a minimum value of another column using sub query and aggregate function

Question

Attempting to complete exercises from the following link for the northwind example database in MySQL.

http://laerer.rhs.dk/andersb/basicprog/database/exercises/northwindselect.htm

The Question "Using SUBQUERY...find the minimum unitPrice from the table products. Use this price to find productID and productName for the product(s) which has the minimal price." has me a little confused.

The following Query

    SELECT ProductID, ProductName 
    FROM products 
    ORDER BY UnitPrice
    LIMIT 1;

seems to produce the correct answer but doesn't use a SUB QUERY. Is this standard practice to produce fields from a row having a MIN/MAX value from another column? I.e. without using the aggregate functions MIN()/MAX().

I would guess that this method would be computationally expensive; having the DBMS sort the table before printing just the top row. Would a SUB QUERY or any other method be more efficient? Perhaps using the appropriate aggregate function rather than ORDER BY. If so, what query should I use?

Answer 1

Your method finds one product with the minimum price. If there are more than one, then you are missing them.

So, the more general solution is to use something a lot like your query as a subquery:

select p.*
from products p join
     (SELECT UnitPrice
      FROM products 
      ORDER BY UnitPrice
      LIMIT 1
     ) pmin
     on p.UnitPrice = pmin.UnitPrice;

Actually, the subquery would more commonly be written:

     (SELECT min(UnitPrice)
      FROM products 
     ) pmin

Both produce the same answer.