Check if exactly one variable is not null - sql-server

Question

In a trigger in my sql-server 2008 database I need to check if exactly one variable is not null. This code does what I need, but can it be done in fewer lines and more readable?
DECLARE #string varchar
DECLARE #float float
DECLARE #bit bit
DECLARE #int int
Set #string=NULL -- Exactly one of these variables needs to be set
Set #float=NULL --
Set #bit=NULL --
Set #int=NULL --
IF( (#string is not null AND COALESCE(#float, #bit, #int) IS NULL)
OR (#float is not null AND COALESCE(#string, #bit, #int) IS NULL)
OR (#bit is not null AND COALESCE(#string, #float, #int) IS NULL)
OR (#int is not null AND COALESCE(#string, #float, #bit) IS NULL)
)
print ' ok'
ELSE
print ' not ok'

Answer 1

SELECT CASE WHEN COUNT(c) =1 THEN 'Y' ELSE 'N' END
FROM
(VALUES (CAST(#string AS SQL_VARIANT)),(#float),(#bit),(#int)) T (c)

Answer 2

I'm not necessarily sure it's any more readable (although I guess if you abstracted it to a function it might be) but
if((case when #string is null then 0 else 1 end +
case when #float is null then 0 else 1 end +
case when #bit is null then 0 else 1 end +
case when #int is null then 0 else 1 end) = 1)
....
is a little more flexible?

Answer 3

I thought I was being clever, but this will only work if none of the 4 varibles can have the exact same value, so it's not useful in many cases. I'll post it anyway:
IF(
(COALESCE(#string,#float, #bit, #int) IS NULL)
OR (COALESCE(#string, #float, #bit, #int) != COALESCE(#int, #bit, #float, #string))
)
print 'not ok'
ELSE print 'ok'
COALESCE evals from left to right until it hits a NOT NULL value, so if you invert the order you'll get a different result if you have more than one var. set (unless the varibles can have the same value at the same time)

Answer 4

I think I know the type of setup you're creating. In those circumstances, I usually define me data structures as:
CREATE TABLE DataItems (
DataItemID int IDENTITY(1,1) not null,
Name varchar(10) not null,
TypeRequired varchar(6) not null,
constraint PK_DataItems PRIMARY KEY (DataItemID),
constraint CK_TypeRequired CHECK (TypeRequired in ('STRING','FLOAT','BIT','INT'),
constraint UQ_DataItems_TypeCheck UNIQUE (DataItemID,TypeRequired)
)
note that I've made DataItemID,TypeRequired a superkey, so I can reference it in a foreign key constraint.
Now, in the table that's collecting the data:
CREATE TABLE Answers (
AnswerID int IDENTITY(1,1) not null,
/* Other columns to FK to e.g. Client, Users, Session, whatever */
DataItemID int not null,
Type varchar(6) not null,
StringValue varchar(max) null,
FloatValue float null,
BitValue bit null,
IntValue int null,
constraint PK_Answers PRIMARY KEY (AnswerID),
constraint FK_Answers_DataItems FOREIGN KEY (DataItemID) references DataItems (DataItemID),
constraint FK_Answers_DataItems_TypeCheck FOREIGN KEY (DataItemID,Type) references DataItems (DataItemID,TypeRequired),
constraint CK_Answers_TypeCheck CHECK (
(FloatValue is null or TypeRequired = 'FLOAT') and
(StringValue is null or TypeRequired = 'STRING') and
(BitValue is null or TypeRequired = 'BIT') and
(IntValue is null or TypeRequired = 'INT')),
constraint CK_Answers_NotNUll CHECK (
FloatValue is not null or StringValue is not null or BitValue is not null or IntValue is not null)
)
The second foreign key constraint ensures the type column matches the defined type for the data item, and the pair of check constraints ensure that exactly one column (and the right column) is not null.
If you need to hide the Type column from the users, then I'd suggest renaming the above table (e.g. _Answers) creating a view with an insert trigger:
CREATE VIEW Answers
WITH SCHEMABINDING
AS
SELECT
AnswerID,
DataItemID,
StringValue,
FloatValue,
BitValue,
IntValue
FROM
dbo._Answers
CREATE TRIGGER T_Answers_I
ON Answers
INSTEAD OF INSERT
AS
INSERT INTO _Answers (DataItemID,Type,StringValue,FloatValue,BitValue,IntValue)
SELECT i.DataItemID,di.Type,i.StringValue,i.FloatValue,i.BitValue,i.IntValue
FROM inserted i inner join DataItems di on i.DataItemID = di.DataItemID

Answer 5

I found an alternative solution but it isn't any fewer lines. It uses the bitwise XOR operator. I'm not sure if I like it or not - but it does mean that each variable only gets checked once rather than on each line so potentially it satisfies your readability requirement:
DECLARE #string varchar
DECLARE #float float
DECLARE #bit bit
DECLARE #int int
SET #string=NULL -- Exactly one of these variables needs to be set
SET #float=NULL --
SET #bit=NULL --
SET #int=NULL --
if ((case when #string is null then 1 else 0 end)
^ (case when #float is null then 1 else 0 end)
^ (case when #bit is null then 1 else 0 end)
^ (case when #int is null then 1 else 0 end)) = 1
print 'ok'
else
print 'not ok'
Comments? Criticisms? Also not sure how efficient the CASE statements are.

Answer 6

Can't we check by this:
IF ISNULL(#string, '') <> ''
OR ISNULL(#float, 0) <> 0
OR #bit IS NOT NULL
OR ISNULL(#int, 0) <> 0
PRINT 'There is atleast one value'
ELSE
PRINT 'ALL ARE NULL'