sql server - Get the count if specific column is NULL in table rows [Telerik] -

i created telerik report using telerik standalone report designer tool.

this structure of report

this sql query used

    select     [tbl_hei_student].[gender],     datediff(hour,[tbl_hei_student].[date_of_birth],getdate())/8766 age,  (([tbl_hei_student]  full outer join [tbl_hec_civil_status]  on [tbl_hei_student].[id] = [tbl_hec_civil_status].[id])  ([tbl_hei_student].[gender]='male' or [tbl_hei_student].[gender]='female') , ([tbl_hec_civil_status].[status]='married') 

i want count rows if [tbl_hei_student].[date_of_birth] null(age null)

which count age unknown section

how can

the below query should do,

select count(*) [tbl_hei_student] [date_of_birth] null 

if want answer part of above query itself, above can given sub query or have define function count.