sql server - Error when excuting SELECT CASE -

when run query error:

procedure sp_get_random_questions, line 40 1 expression can specified in select list when subquery not introduced exists. msg 116, level 16, state 1, procedure sp_get_random_questions, line 52 1 expression can specified in select list when subquery not introduced exists.

this stored procedure, meant give random questions, @easy, @medium, @hard number of questions that'll randomized. if user chooses give @num, i'll divide number 3 (the 3 dificulties) , random questions.

select  case when @num=0 (     select * (     select top (@easy) * tbl_perguntas p p.category=1 order newid())     union select *     (select top (@medium) * tbl_perguntas p p.category=2 order newid()) b     union select *     (select top (@hard) * tbl_perguntas p p.category=3 order newid()) c)     else (     select *     (select top (@num/3) * tbl_perguntas p p.category=1 order newid())     union select *     (select top (@num/3) * tbl_perguntas p p.category=2 order newid()) b     union select *     (select top (@num/3) * tbl_perguntas p p.category=3 order newid()) c ) end 

as comments have stated, need use if/else control-of-flow, try this:

if @num=0  begin     select top (@easy) *      tbl_perguntas p      p.category=1      order newid()      union       select top (@medium) *      tbl_perguntas p      p.category=2      order newid()      union       select top (@hard) *      tbl_perguntas p      p.category=3      order newid()) end else begin     select top (@num/3) *      tbl_perguntas p      p.category=1      order newid()      union       select top (@num/3) *      tbl_perguntas p     p.category=2      order newid()      union        select top (@num/3) *      tbl_perguntas p      p.category=3      order newid()) end