Using Derived Tables and CTEs to Display Details? - tsql

Question

I am teaching myself T-SQL and am struggling to comprehend the following example..
Suppose you want to display several nonaggregated columns along with
some aggregate expressions that apply to the entire result set or to a
larger grouping level. For example, you may need to display several
columns from the Sales.SalesOrderHeader table and calculate the
percent of the TotalDue for each sale compared to the TotalDue for all
the customer’s sales. If you group by CustomerID, you can’t include
other nonaggregated columns from Sales.SalesOrderHeader unless you
group by those columns. To get around this, you can use a derived
table or a CTE.
Here are two examples given...
SELECT c.CustomerID, SalesOrderID, TotalDue, AvgOfTotalDue,
TotalDue/SumOfTotalDue * 100 AS SalePercent
FROM Sales.SalesOrderHeader AS soh
INNER JOIN
(SELECT CustomerID, SUM(TotalDue) AS SumOfTotalDue,
AVG(TotalDue) AS AvgOfTotalDue
FROM Sales.SalesOrderHeader
GROUP BY CustomerID) AS c ON soh.CustomerID = c.CustomerID
ORDER BY c.CustomerID;
WITH c AS
(SELECT CustomerID, SUM(TotalDue) AS SumOfTotalDue,
AVG(TotalDue) AS AvgOfTotalDue
FROM Sales.SalesOrderHeader
GROUP BY CustomerID)
SELECT c.CustomerID, SalesOrderID, TotalDue,AvgOfTotalDue,
TotalDue/SumOfTotalDue * 100 AS SalePercent
FROM Sales.SalesOrderHeader AS soh
INNER JOIN c ON soh.CustomerID = c.CustomerID
ORDER BY c.CustomerID;
Why doesn't this query produce the same result..
SELECT CustomerID, SalesOrderID, TotalDue, AVG(TotalDue) AS AvgOfTotalDue,
TotalDue/SUM(TotalDue) * 100 AS SalePercent
FROM Sales.SalesOrderHeader
GROUP BY CustomerID, SalesOrderID, TotalDue
ORDER BY CustomerID
I'm looking for someone to explain the above examples in another way or step through it logically so I can understand how they work?

Answer 1

The aggregates in this statement (i.e. SUM and AVG) don't do anything:
SELECT CustomerID, SalesOrderID, TotalDue, AVG(TotalDue) AS AvgOfTotalDue,
TotalDue/SUM(TotalDue) * 100 AS SalePercent
FROM Sales.SalesOrderHeader
GROUP BY CustomerID, SalesOrderID, TotalDue
ORDER BY CustomerID
The reason for this is you're grouping by TotalDue, so all records in the same group have the same value for this field. In the case of AVG this means you're guarenteed for AvgOfTotalDue to always equal TotalDue. For SUM it's possible you'd get a different result, but as you're also grouping by SalesOrderID (which I'd imagine is unique in the SalesOrderHeader table) you will only have one record per group, so again this will always equal the TotalDue value.
With the CTE example you're only grouping by CustomerId; as a customer may have many sales orders associated with it, these aggregate values will be different to the TotalDue.
EDIT
Explanation of the aggregate of field included in group by:
When you group by a value, all rows with that same value are collected together and aggregate functions are performed over them. Say you had 5 rows with a total due of 1 and 3 with a total due of 2 you'd get two result lines; one with the 1s and one with the 2s. Now if you perform a sum on these you have 3*1 and 2*2. Now divide by the number of rows in that result line (to get the average) and you have 3*1/3 and 2*2/2; so things cancel out leaving you with 1 and 2.
select totalDue, avg(totalDue)
from (
select 1 totalDue
union all select 1 totalDue
union all select 1 totalDue
union all select 2 totalDue
union all select 2 totalDue
) x
group by totalDue
select uniqueId, totalDue, avg(totalDue), sum(totalDue)
from (
select 1 uniqueId, 1 totalDue
union all select 2 uniqueId, 1 totalDue
union all select 3 uniqueId, 1 totalDue
union all select 4 uniqueId, 2 totalDue
union all select 5 uniqueId, 2 totalDue
) x
group by uniqueId
Runnable Example: http://sqlfiddle.com/#!2/d41d8/21263