Question

I’m building a database for a small bookshop with a PHP web interface. For this problem I want to display all records in the ‘products’ table (prod_core) without those that have a value in the products field ‘barcode’ equal to the ‘description’field in the ‘sales’ table (inc_core).

I actually managed to get this working for the first ‘sales’ mutation, but whenever I add a second record to the ‘sales’ table, the query doesn’t work any more. (except for the first record added). This is my SQL code:

SELECT prod_core.prodID, prod_core.title, prod_core.location, prod_core.genre,
       prod_core.price, prod_core.barcode
FROM   prod_core
WHERE  prod_core.barcode NOT IN (
         SELECT `description` 
         FROM inc_core
         GROUP BY prod_core.prodID)

I suppose it has something to do with SQL not looping (it stops searching the table for records that match the requirements) but I’m not sure. I would very much appreciate any help!

Answer 1

I would suggest a LEFT JOIN instead of a NOT IN:

SELECT
    prod_core.prodID, 
    prod_core.title, 
    prod_core.location, 
    prod_core.genre, 
    prod_core.price, 
    prod_core.barcode
FROM 
    prod_core 
    LEFT JOIN inc_core ON inc_core.`description` = prod_core.barcode
WHERE
    inc_core.`description` IS NULL

Also, since none of the columns in your SELECT statement are part of an aggregate function, I removed the GROUP BY and replaced it with DISTINCT… however, I don’t see why this table should include duplicates.

EDIT: I misread your query and though that the group by was on the outer query, and not on the sub-query… +1 to the other answers that caught this.