Question

These two queries seem to return the same results. Is that coincidental or are they really the same?

1.

SELECT t.ItemNumber,
  (SELECT TOP 1 ItemDescription
   FROM Transactions
   WHERE ItemNumber = t.ItemNumber
   ORDER BY DateCreated DESC) AS ItemDescription
FROM Transactions t
GROUP BY t.ItemNumber

2.

SELECT DISTINCT(t.ItemNumber),
  (SELECT TOP 1 ItemDescription
   FROM Transactions
   WHERE ItemNumber = t.ItemNumber
   ORDER BY DateCreated DESC) AS ItemDescription
FROM Transactions t

A bit of explanation:
I’m trying to get a distinct list of items from a table full of transactions. For each item, I’m looking for the ItemNumber (the identifying field) and the most recent ItemDescription.

Answer 1

Your example #2 had me scratching me head for a while – I thought to myself: “You can’t DISTINCT a single column, what would that mean?” – until I realised what is going on.

When you have

SELECT DISTINCT(t.ItemNumber)

you are not, despite appearances, actually asking for distinct values of t.ItemNumber! Your example #2 actually gets parsed the same as

SELECT DISTINCT
  (t.ItemNumber)
  ,
  (SELECT TOP 1 ItemDescription
   FROM Transactions
   WHERE ItemNumber = t.ItemNumber
   ORDER BY DateCreated DESC) AS ItemDescription
FROM Transactions t

with syntactically-correct but superfluous parentheses around t.ItemNumber. It is to the result-set as a whole that DISTINCT applies.

In this case, since your GROUP BY groups by the column that actually varies, you get the same results. I’m actually slightly surprised that SQL Server doesn’t (in the GROUP BY example) insist that the subqueried column is mentioned in the GROUP BY list.