Question

I am trying to develop a unique index.

CREATE UNIQUE NONCLUSTERED INDEX NCI_NewUnique
ON [NewUnique]([U1])
WHERE (ISNULL([MyField], '') = '') 

My error is

Incorrect WHERE clause for filtered index 'NCI_NewUnique' on table 'NewUnique'.

Here is another attempt; This one I have removed the ISNULL(MyField, ”) part. Why cannot this one have an OR?

CREATE UNIQUE NONCLUSTERED INDEX NCI_NewUnique
ON [NewUnique]([U1])
WHERE (
         ([MyId] IS NULL) 
         OR 
         ([MyId] IS NOT NULL AND [MyField] IS NOT NULL)
      )

Error is:

Incorrect syntax near the keyword 'OR'.

Answer 1

I don’t know why but according to the documentation functions and or is not allowed.

<filter_predicate> ::= 
    <conjunct> [ AND <conjunct> ]

<conjunct> ::=
    <disjunct> | <comparison>

<disjunct> ::=
        column_name IN (constant ,...n)

<comparison> ::=
        column_name <comparison_op> constant

<comparison_op> ::=
    { IS | IS NOT | = | <> | != | > | >= | !> | < | <= | !< }