How to call a calculated column in "from" in sql server - sql

Question

I'm trying to call this calculated column 'RelativeEffectiveSpreadAbsoluteValue' in SQL servers' FROM part.
, case when cast(sa.Mid_Price as float) = 0
then 0
else ((CAST(sa.Ask_Price as float)-cast(sa.Bid_Price as float))/CAST(sa.Mid_Price as float))/(0.01/100)
end As RelativeEffectiveSpreadAbsoluteValue
like this, but the SQL server won't recognize it
left join [RISK].[dbo].[FILiquidityBuckets] FB6
ON FB6.Metric = 'Relative spread ' AND (
((CAST(RelativeEffectiveSpreadAbsoluteValue AS FLOAT)>= 0 AND CAST(RelativeEffectiveSpreadAbsoluteValue AS FLOAT)< 1000000) AND
FB6.LiquidityScore = 5) OR
((CAST(RelativeEffectiveSpreadAbsoluteValue AS FLOAT)>= 1000000 AND CAST(RelativeEffectiveSpreadAbsoluteValue AS FLOAT)<10000000) AND
FB6.LiquidityScore = 4) OR
((CAST(RelativeEffectiveSpreadAbsoluteValue AS FLOAT)>= 10000000 AND CAST(RelativeEffectiveSpreadAbsoluteValue AS FLOAT)< 100000000) AND
FB6.LiquidityScore = 3) OR
((CAST(RelativeEffectiveSpreadAbsoluteValue AS FLOAT)>= 100000000 AND CAST(RelativeEffectiveSpreadAbsoluteValue AS FLOAT)<1000000000) AND
FB6.LiquidityScore = 2) OR
(CAST(RelativeEffectiveSpreadAbsoluteValue AS FLOAT) >= 1000000000 AND F65.LiquidityScore = 1)
)
So far I know by using 'Cross Apply' a calculated column can calculate another column in the same view,
like this example
Select
ColumnA,
ColumnB,
c.calccolumn1 As calccolumn1,
c.calccolumn1 / ColumnC As calccolumn2
from t42
cross apply (select (ColumnA + ColumnB) as calccolumn1) as c
but this is only for the select part, can we use it in the From part?
Please help thank you!

Answer 1

Put the apply operation which does the calculation prior to the join in your query:
create table t(a int);
create table u(b int);
select t.a,
t2.calculatedColumn,
u.b
from t
cross apply (select t.a * 2) as t2 (calculatedColumn)
left join u on u.b = t2.calculatedColumn
As Panagiotis observed, this may result in a slow join because the join predicate will not be able to use an index. But if the nature of your query demands it, the language supports it.
If you need this to be fast, create an indexed computed column on the table you have aliased as sa instead of calculating it in the query. Since your column will be of type float, you will need to mark the computed column as persisted before you can index it.