Use result of Case statement in another Case statement - sql-server

Question

I have quite a long SELECT query but I have pasted the relevant part here.
I need to use the result of the of my CASE statement to use in another CASE statement. I'm doing this in SQL Server.
Would be very grateful for help.
SELECT
CompanyContact.Name AS CompanyName,
CASE
WHEN SUBSTRING(HeadLease.TenantBreakNotice, LEN(HeadLease.TenantBreakNotice), 1) = 'M'
THEN CONVERT(VARCHAR(10), DATEADD(DD, -365 / (12 / SUBSTRING(HeadLease.TenantBreakNotice, 1, LEN(HeadLease.TenantBreakNotice) - 1)), HeadLease.TenantBreakDate), 103)
WHEN SUBSTRING(HeadLease.TenantBreakNotice, LEN(HeadLease.TenantBreakNotice), 1) = 'Y'
THEN CONVERT(VARCHAR(10), DATEADD(DD, -365 * (SUBSTRING(HeadLease.TenantBreakNotice, 1, LEN(HeadLease.TenantBreakNotice) - 1)), HeadLease.TenantBreakDate), 103)
ELSE HeadLease.TenantBreakNotice
END AS [TenantBreakNotice], <-- I need this to be used in the case statement below.
CASE
WHEN [TenantBreakNotice] < CONVERT(varchar(10), getdate(), 103)
THEN 'Expiry'
WHEN [TenantBreakNotice] IS NULL
THEN 'Expiry'
ELSE 'Break'
END AS [LeaseEventType]
FROM
HeadLease

Answer 1

You cannot use a column alias in the same select where it is defined. The usual solution is to repeat the logic (hard to maintain), use a subquery, or CTE. SQL Server offers another elegant solution:
SELECT hl.Name AS CompanyName, v.TenantBreakNotice,
(CASE WHEN v.TenantBreakNotice < CONVERT(varchar(10), getdate(), 103) THEN 'Expiry'
WHEN TenantBreakNotice IS NULL THEN 'Expiry'
ELSE 'Break'
END) AS [LeaseEventType]
FROM HeadLease hl OUTER APPLY
(VALUES (CASE WHEN SUBSTRING(hl.TenantBreakNotice, LEN(hl.TenantBreakNotice), 1) = 'M'
THEN CONVERT(VARCHAR(10), DATEADD(DAY, -365/(12/SUBSTRING(hl.TenantBreakNotice, 1, LEN(hl.TenantBreakNotice) -1)), hl.TenantBreakDate), 103)
WHEN SUBSTRING(hl.TenantBreakNotice, LEN(hl.TenantBreakNotice), 1) = 'Y'
THEN CONVERT(VARCHAR(10), DATEADD(DAY, -365*(SUBSTRING(hl.TenantBreakNotice,1, LEN(hl.TenantBreakNotice)-1)), hl.TenantBreakDate), 103)
ELSE hl.TenantBreakNotice
END) v(TenantBreakNotice);
Of course, the logic is incorrect, because you are comparing dates as strings. However, that is something you need to figure out yourself. Don't convert dates to strings for date operations. And, you should output the results as YYYY-MM-DD so the formats are unambiguous.

Answer 2

As #Juergen pointed out, you can't do exactly what you want, but you could compute the first CASE expression in a subquery, and then use it an outer wrapping query:
WITH cte AS (
SELECT
Name AS CompanyName,
CASE WHEN SUBSTRING(HeadLease.TenantBreakNotice,LEN(HeadLease.TenantBreakNotice), 1) = 'M'
THEN CONVERT(VARCHAR(10), DATEADD(DD,-365/(12/SUBSTRING(HeadLease.TenantBreakNotice,1,LEN(HeadLease.TenantBreakNotice)-1)),HeadLease.TenantBreakDate), 103)
WHEN SUBSTRING(HeadLease.TenantBreakNotice,LEN(HeadLease.TenantBreakNotice),1) = 'Y'
THEN CONVERT(VARCHAR(10), DATEADD(DD,-365*(SUBSTRING(HeadLease.TenantBreakNotice,1,LEN(HeadLease.TenantBreakNotice)-1)),HeadLease.TenantBreakDate), 103)
ELSE HeadLease.TenantBreakNotice
END AS [TenantBreakNotice]
FROM HeadLease
)
SELECT
Name,
TenantBreakNotice,
CASE WHEN TenantBreakNotice < CONVERT(varchar(10), getdate(), 103)
THEN 'Expiry'
WHEN TenantBreakNotice IS NULL THEN 'Expiry'
ELSE 'Break'
END AS [LeaseEventType]
FROM cte;

Answer 3

Use CTEs (common table expressions). In CTEs you can refer to the columns from the previous CTE, so you can split the CASE logic like you would like.
Example:
WITH
CTE_1 AS
(
SELECT
*
,CASE
WHEN SUBSTRING(HeadLease.TenantBreakNotice,LEN(HeadLease.TenantBreakNotice),1) = 'M'
THEN CONVERT(VARCHAR(10), DATEADD(DD,-365/(12/SUBSTRING(HeadLease.TenantBreakNotice,1,LEN(HeadLease.TenantBreakNotice)-1)),HeadLease.TenantBreakDate), 103)
WHEN SUBSTRING(HeadLease.TenantBreakNotice,LEN(HeadLease.TenantBreakNotice),1) = 'Y'
THEN CONVERT(VARCHAR(10), DATEADD(DD,-365*(SUBSTRING(HeadLease.TenantBreakNotice,1,LEN(HeadLease.TenantBreakNotice)-1)),HeadLease.TenantBreakDate), 103)
ELSE
HeadLease.TenantBreakNotice
END AS [TenantBreakNotice]
...
),
CTE_2 AS
(
SELECT
*
,CASE
WHEN [TenantBreakNotice] < CONVERT(varchar(10),getdate(),103) THEN 'Expiry'
WHEN [TenantBreakNotice] IS NULL THEN 'Expiry'
ELSE 'Break'
END AS [LeaseEventType]
FROM
CTE_1
)
SELECT * FROM CTE_2

Answer 4

You can move the first case expression into your from by using a derived table/subquery like so:
select
cc.Name as CompanyName
, convert(varchar(10),hl.[TenantBreakNotice],103) as TenantBreakNotice
, case
when hl.[TenantBreakNotice] < getdate() then 'Expiry'
when hl.[TenantBreakNotice] is null then 'Expiry'
else 'Break'
end as [LeaseEventType]
from (
select *,
case
when substring(HeadLease.TenantBreakNotice,len(HeadLease.TenantBreakNotice),1) = 'M'
then dateadd(day,-365/(12/substring(HeadLease.TenantBreakNotice,1,len(HeadLease.TenantBreakNotice)-1)),HeadLease.TenantBreakDate)
when substring(HeadLease.TenantBreakNotice,len(HeadLease.TenantBreakNotice),1) = 'Y'
then dateadd(day,-365*(substring(HeadLease.TenantBreakNotice,1,len(HeadLease.TenantBreakNotice)-1)),HeadLease.TenantBreakDate)
else HeadLease.TenantBreakNotice
end as [TenantBreakNotice]
from HeadLease
) as hl
inner join CompanyContact cc
on cc....
Notes:
Don't compare strings as dates, this is going to lead to problems; especially with the style you specified (103).
Bad Habits to Kick : Using shorthand with date/time operations - Aaron Bertrand
Bad habits to kick : mis-handling date / range queries - Aaron Bertrand