Running SQL user defined function that returns boolean, in where clause

Question

I'm getting this error on a SQL user defined function:

An expression of non-boolean type specified in a context where a condition is expected, near ')'.

For this:

UPDATE LMI_Contact SET Phone = NULL WHERE dbo.LMI_IsSingleCharacterRepeated(Phone, '0') 

where the function can be created using:

-- ***this will also find NULL and empty string values*** CREATE FUNCTION LMI_IsSingleCharacterRepeated (@string varchar(max), @char char(1)) RETURNS bit AS  BEGIN     DECLARE @index int     DECLARE @len int     DECLARE @currentChar char(1)     SET @index = 1     SET @len= LEN(@string)      WHILE @index <= @len     BEGIN         SET @currentChar = SUBSTRING(@string, @index, 1)         IF @currentChar = @char             SET @index= @index+ 1         ELSE             RETURN 0     END     RETURN 1 END; GO 

This function is for checking if a string is any specified single character, repeated.

Answer 1

You must use comparison operators against functions even if the return type is bit.

UPDATE LMI_Contact SET Phone = NULL WHERE dbo.LMI_IsSingleCharacterRepeated(Phone, '0') = 1