sql - MySQL Query help - Grouping and count problems -

i have mysql database trying specific count of data catch. best way explain showing you.

i have table has username , id_no, id not going unique there circumstances show duplication of (which expected), table might this

username    id_no  user1  1111111  user1  1111111  user1  1111111  user1  1111111  user1  2222222  user1  3333333  user1  3333333  user1  444444  user1  444444  user1  444444  user1  555555  user1  666666 

i need unique count of piece of cake

   select username, count(distinct id_no)     cases    group username 

that gives me count of 6. gets interesting need join table add category table might

username    cat id_no user1   category1   1111111 user1   category2   1111111 user1   category2   1111111 user1   category2   1111111 user1   category5   2222222 user1   category6   3333333 user1   category7   3333333 user1   category8   444444 user1   category8   444444 user1   category8   444444 user1   category11  555555 user1   category12  666666 

with need unquie count, catch here need match same number before (6), stuck how can in mysql because can see there categories overlap query simple looks this

   select username, cat, count(distinct id_no)     cases c    join case_details cd on c.id_no=cd.id_no    group username, cat 

because having group cat count of 8. if sql server inner query using top 1 on id (i think), mysql not let me use limit within subquery.

so clear need this

username    cat id_no user1   category1   1111111  user1   category5   2222222 user1   category6   3333333  user1   category8   444444  user1   category11  555555 user1   category12  666666 

you notice first category had id shown. total count 6 same first, looking for.

anyone got ideas here?