Question

I was wondering if a SELECT of a SELECT incurs any additional performance, or if the database is smart enough to optimize this.

For example, I have some selects I do like the following:

SELECT a.year, b.month, c.day, totalVolume from 
(SELECT DATEPART(year, transactionTime) as year, 
        DATEPART(month, transactionTime) as month,
        DATEPART(day, transactionTime) as day,
        SUM(volume) as totalVolume FROM DOW30
        GROUP BY DATEPART(year, transactionTime),
                 DATEPART(month, transactionTime),
                 DATEPART(day, transactionTime)) a
order by a.year, b.month, c.day

All this particular query does is group all the trades in the Dow30 by date, and sums up the total volume. I use this because the table stores every trade on a per second basis, and I want to roll it up on a per-day basis.

This isn’t the full query, but just an example. I know I could just simply doing this from a single SELECT, but I was wondering if doing a SELECT of a SELECT would incur some sort of performance penalty, or if this all gets optimized magically.

The reason why I formulate the query this way is because that’s how I visualize the query, and I actually join this top-level select with another table, and selecting out the columns from the underlying SELECT makes it easier for me to visualize the query. But having no formal DBA background, I’m not sure if this is wrong, and if I need to figure out a better way of writing my queries.

Answer 1

Nesting SELECTs like this are considered one statement to the optimiser.

You can see this here: Why use Select Top 100 Percent? and How can I query 'between' numeric data on a not numeric field? In these examples, the nesting here has no effect unless you force it which is generally a bad idea