title: Why does it say column ambiguously defined?
body: <pre><code>select a.id_progdi, a.nama_progdi, avg(b.ipk)  
  from tb_ipk b 
  join tb_mahasiswa c on b.nim = c.nim 
  join tb_progdi a on c.id_progdi = a.id_progdi  
  group by id_progdi
  </code></pre>