Data conversion error when using CAST in a CASE statement - sql

Question

I am getting the error 'Error converting data type nvarchar to float' when running the following
declare
#completeCommand nvarchar (max) = 'x'
,#paramVal nvarchar (100)
,#paramName nvarchar (100)
,#paramType nvarchar (100)
,#tempParam sql_variant
declare #parameterList table (
RowID int identity (1,1)
,ParameterValue nvarchar (100)
,ParameterName nvarchar (100)
,ParameterType nvarchar (100)
)
insert into #parameterList
values
('10', 'Param1', 'int')
,('test', 'Param2', 'nvarchar')
-- Process each parameter one at a time
declare ParameterCursor cursor fast_forward for
select ParameterValue, ParameterName, ParameterType
from #parameterList
order by RowID
open ParameterCursor
fetch next from ParameterCursor into #paramVal, #paramName, #paramType
if ##FETCH_STATUS = 0
set #completeCommand = #completeCommand + ' '
while ##FETCH_STATUS = 0
begin
print #completeCommand
-- verify the datatype is correct
set #tempParam = case #paramType
when 'int' then CAST (#paramVal as int)
when 'float' then CAST (#paramVal as float)
when 'nvarchar' then CAST (#paramVal as nvarchar)
else 'NULL'
end
set #completeCommand = #completeCommand + #paramName + ' = ' + #paramVal + ','
fetch next from ParameterCursor into #paramVal, #paramName, #paramType
end
close ParameterCurosr
deallocate ParameterCursor
What I am trying to do is verify that user entered data matches the expected data type before the data is added to a command string. Any feedback on why the above code fails would be greatly appreciated.
Cheers,
Joe

Answer 1

The issue is that CASE will return the data type with the highest precidence, of which is float for this statement. see CASE (Transact-SQL) and Data Type Precedence (Transact-SQL).
To get this CASE to work add a bogus WHEN 'xzy' then CAST (#paramVal as sql_variant) which will cause CASE to use sql_variant as the return data type.
OR remove the CASE and use IF-ELSE, like:
IF #paramType='int'
set #tempParam = CAST (#paramVal as int)
ELSE IF #paramType='float'
set #tempParam = CAST (#paramVal as float)
ELSE IF #paramType='nvarchar'
set #tempParam = CAST (#paramVal as nvarchar)
ELSE
set #tempParam = NULL

Answer 2

What you used was Simple Case function. Searched Case function should work (see below).
I've just tried it in a very simple query:
DECLARE #type VARCHAR(100)
DECLARE #input VARCHAR(100)
DECLARE #value SQL_VARIANT
SET #type = 'varchar'
SET #input = 'test'
SET #value = CASE
WHEN #type = 'varchar' THEN #input
WHEN #type = 'int' THEN CAST(#input AS VARCHAR)
END
On SQL Server 2005 I am getting the same error, but the following is from SQL Server 2008 doc:
Simple CASE function:
Evaluates input_expression, and then in the order specified, evaluates input_expression = when_expression for each WHEN clause.
Returns the result_expression of the first input_expression = when_expression that evaluates to TRUE.
If no input_expression = when_expression evaluates to TRUE, the SQL Server 2005 Database Engine returns the else_result_expression if an ELSE clause is specified, or a NULL value if no ELSE clause is specified.
Searched CASE function:
Evaluates, in the order specified, Boolean_expression for each WHEN clause.
Returns result_expression of the first Boolean_expression that evaluates to TRUE.
If no Boolean_expression evaluates to TRUE, the Database Engine returns the else_result_expression if an ELSE clause is specified, or a NULL value if no ELSE clause is specified.
If this is true, then Simple Case will do all the CASTS and then decide qhich one to use, while Searched Case will evaluate the boolean expression and then decide which CAST to execute. This should be the solution.
Otherwise, you can probably use IF ... THEN ... ELSE, as proposed in another answer.