T-SQL Case Statement in a JOIN ON Clause

Question

I am trying to build a case/if statement in a JOIN ON clause.

LEFT JOIN [CTSTRC] [Statuses] ON RIGHT([Statuses].[STRID], 3) = [CTE].[F61]


        

Answer 1

Wouldn't something like this work:

LEFT JOIN [CTSTRC] [Statuses] ON RIGHT([Statuses].[STRID], 3) = cast([CTE].[F61] as varchar(3))

All you really care about is whether you have a match, so why not convert the numeric to varchar? You would have to test both ideas to see which one is faster.

I do agree with @KM, fixing a bad design like this is the best solution. Having functions and Case statements in a join is an indicatior that your design is fatally flawed and should be fixed.

Answer 2

You're looking for the ISNUMERIC function (I believe it was introduced in SQL 2005):

LEFT JOIN [CTSTRC] [Statuses] ON 
    (CASE ISNUMERIC(RIGHT([Statuses].[STRID], 3)) WHEN 0 THEN 0 ELSE CAST(RIGHT([Statuses].[STRID], 3) AS INT) END) = [CTE].[F61] 

Answer 3

create a persisted computed column and add an index on it.

ALTER TABLE YourTable ADD
    NewIntID AS (CASE ISNUMERIC(RIGHT([Statuses].[STRID], 3) + '.0e0)
                     WHEN 1 THEN CAST(RIGHT([Statuses].[STRID], 3) AS INT) 
                     ELSE 0
                 END) PERSISTED
GO

CREATE INDEX IX_YourTable_NewIntID 
ON YourTable (NewIntID ); 
GO

you can now just join to the new NewIntID column as if it were the proper numeric ID now.

Answer 4

You're looking for IsNumeric but it doesn't always work (+,- and . are numeric) so you need to use the solution described by GBN which is to add .0e0 to your varchar

LEFT JOIN [CTSTRC] [Statuses] ON 
    (CASE WHEN ISNUMERIC(RIGHT([Statuses].[STRID], 3) + '.0e0) = 1 
          THEN  CAST(RIGHT([Statuses].[STRID], 3) AS INT) 
          ELSE 0  END) = [CTE].[F61] 

Answer 5

You can try to create an expression column in the Statuses table that converts the right 3 characters to a number and then try to join on the expression column.

Answer 6

You might want to try something like this

select ...
from CTE
inner join
(
    select ...
    from [Statuses]
    where ISNUMERIC(STRID + '.0e0') = 1
) rsNumeric on CTE.F61 = rsNumeric.STRID