computation within same table - sql

Question

I have a table called factprojection which has the columns as shown in the image.
I want to multiply the 'value' column which have 'Total Lines & RSV Required'in the 'Measures' column with the 'value' column which have 'Line Average' in the 'Measures' column joining it by crew type. Should I create a temp table and then perform the multiplication? Could anyone help me with this?

Answer 1

N.B. I'm assuming all CrewType and DateKey values are in both tables, so all answers below use inner joins (any rows not matching on both sides are not returned)
Common Table Expressions (CTEs)
You can define Common Table Expressions that perform queries on the table, and then name those queries. You can then join these 'sub-tables', rather than generating any temporary data:
WITH LineAverage as (
select *
from factprojection
where Measure = 'Line Average'
)
,TotalLines as (
select *
from factprojection
where Measure = 'Total Lines & RSV Required'
)
select
LA.CrewType
,LA.DateKey
,LA.Value*TL.Value as newvalue
from LineAverage LA
inner join
TotalLines TL
ON LA.DateKey = TL.DateKey
and LA.CrewType = TL.CrewType
Self join
An equivalent but less visually obvious approach is a self-join, giving the tables different aliases:
select
LA.CrewType
,LA.DateKey
,LA.Value*TL.Value as newvalue
from factprojection LA,
factprojection TL
where LA.DateKey = TL.DateKey
and LA.CrewType = TL.CrewType
and LA.Measure = 'Line Average'
and TL.Measure = 'Total Lines & RSV Required'
Notice the use of restrictions on the Measure values alongside the join conditions in the WHERE clause. Although this query is marginally shorter to write it loses some readability over the CTE approach.
Here I've also used the comma form of an INNER JOIN, where the ON clause is replaced by placing the join conditions in the WHERE clause.