sql server - Conditionally appending AND to where clause in SQL Select statement -

i have searched on , find questions on conditional where clauses nothing conditionally appending and piece onto where clause. i'm not sql/database person, i'm it area has been thrown "go try fix it" problem today because our regular database people gone. have basic programming concepts , knowledge sql foreign animal me.

i have simple:

select abc table sdate=@somedt 

but need add:

and otherdt=@otherdt 

but if @otherdt not null. if null don't want add and part @ all. i've tried few ways keeps giving me error. appreciated. last unsuccessful try:

select abc table  sdate=@somedt if @otherdt not null begin , otherdt=@otherdt end 

select abc table  sdate = @somedt , (@otherdt null or otherdt = @otherdt)