how to set the validation error

Question

Dear All

 

I am develop the help Sp but i dont know how to set validation error . kindly give the suggetion.

 

Note : if select multiple options show error

Ex : select multiple  months in help show error message

 

if (@paramname= 'Month')

begin

select 'ALL' 'GRDPARAMVALUE'

union all

SELECT distinct DATENAME(month, schedule_date) 'GRDPARAMVALUE'

from tmscd_emp_gre_calendar_vw m

where master_ou=@ouinstance

end