Question

What is the advantage of using select clause in the from clause over normal select clause ?. For ex.

select name, age, address from emp where 
 empid = 12.

what is the advantage of below query over above query.

 select A.name, A.age, A.address from (select
  * from emp where empid = 12) as A.

The inner query creates a temp view and from that result, the fields in the first query selected. Right ?. But the query mentioned in the top of this question can also be used to get the same result.

What is the advantage? Thanks.

Answer 1

One way this technique can be used to derive results in the inner query that you don’t want presented in the outer query. A simple example, I want to see the oldest person in each household, here is one way to do it:

SELECT name, age, address FROM
( 
  SELECT name, age, address, 
    rn = ROW_NUMBER() OVER (PARTITION BY address ORDER BY age DESC)
  FROM dbo.emp
) AS x
WHERE rn = 1;

This way the derived column (a ranking, essentially, of household members by age, oldest first) does not need to be a column in the result set (and this also makes it convenient for filtering).

And I’m sure there are plenty of others (such as not having to repeat elaborate expressions); this was just the first one that came to mind.

In the case you posted in your question, I don’t see an advantage at all. If you post a real example, where someone said there was an advantage, we might be able to explain why (or at least why they may have thought that).