Question

I have a problem with one of my queries in SQLPLUS.

I have been asked to display the SuppCode, SuppName, and PostCode for all suppliers who satisfy the following contraints: Every stock item they supply has a price which is greater than the average of all stock items.

The three tables I’m working with are; Supppliers and Stocks and SupplyItems.

The Suppliers table looks looks like:

SUPPCODE SUPPNAME STREET TOWN COUNTRY POSTCODE TELNO FAXNO

With 8 seperate sample data (8 suppliers), and no null fields.

The Stocks table looks like:

ITEMNO STORECODE ITEMDESC QUANTITY UNITS REORDER PRICE SUPPCODE

With 40 seperate sample data (40 stock items), and no null fields.

The SupplyItems table looks like:

SUPPCODE CATNO STOCKNO PRICE

With 60 seperate sample data (60 supply items), and no null fields.

I have written the following query:

select distinct SU.SuppCode, Su.SuppName, PostCode
from Suppliers SU
LEFT JOIN SupplyItems SI ON SU.SuppCode = SI.SuppCode
where Price >
(select AVG(Quantity) from Stocks ST
where SI.StockNo = ST.StockNo);

I get the following output (4 values):

SUPPCODE SUPPNAME POSTCODE
S6       BSS LTD. B10 8SS
S2       ITX LTD. IT5 3TX
S3       FFG LTD. FY9 6FG
S4       OMM LTD. OM5 4MM

Now, I have been made aware that the output should contain either 2 or 3 records, so my query is wrong, though I can’t see why.

Any help would be appreciated.

Answer 1

This:

where Price >
(select AVG(Quantity) from Stocks ST

looks wrong – shouldn’t it rather be

AVG(price)

?

EDIT: also,

select distinct SU.SuppCode, Su.SuppName, PostCode
from Suppliers SU
LEFT JOIN SupplyItems SI ON SU.SuppCode = SI.SuppCode
where Price > ...

probably doesn’t do what you want – this will return all suppliers who offer at least one item whose price is above the average; to get the suppliers that offer only items with price above the average, you’ll have to use a different approach, e.g. (untested)

select distinct SU.SuppCode, Su.SuppName, PostCode
from Suppliers SU
where not exists (
  select null from SupplyItems si 
  where su.SuppCode = si.SuppCode
  and su.price <= (
      select AVG(price) from Stocks ST
      where SI.StockNo = ST.StockNo)
)