select CASE WHEN @P1 in ('a', 'd', 'z') THEN 1
WHEN @P1 in ('b', 't') THEN 2
ELSE 0
END
from your_table
or
select CASE WHEN @P1 = 'a' or @P1 = 'd' or @P1 = 'z' THEN 1
WHEN @P1 = 'b' or @P1 = 't' THEN 2
ELSE 0
END
from your_table