title: How to understand the cardinality in a combined primary in MySQL
body: <p>I am running the command: </p>
  
  <p>show index from <code>atpco_fare</code>.<code>atpco_r1_fare_cls</code> ; </p>
  
  <p>And the result is in the table </p>
  
  <pre><code>Table, Non_unique, Key_name, Seq_in_index, Column_name, Collation, Cardinality, Sub_part, Packed, Null, Index_type, Comment, Index_comment, Visible 
  atpco_r1_fare_cls   0   PRIMARY 1   tar_nbr A   7707    BTREE   YES 
  atpco_r1_fare_cls   0   PRIMARY 2   carr_cd A   46531   BTREE   YES 
  atpco_r1_fare_cls   0   PRIMARY 3   rule_nbr    A   426326  BTREE   YES 
  atpco_r1_fare_cls   0   PRIMARY 4   fare_cls    A   30087162    BTREE   YES 
  atpco_r1_fare_cls   0   PRIMARY 5   seq_nbr A   21610918    BTREE   YES 
  atpco_r1_fare_cls   0   PRIMARY 6   mcn A   31260526    BTREE   YES 
  atpco_r1_fare_cls   0   PRIMARY 7   eff_date    A   31260526    BTREE   YES 
  atpco_r1_fare_cls   0   PRIMARY 8   proc_ind    A   31260526    BTREE   YES 
  </code></pre>
  
  <p>I do not understand the cardinality of the combined primary key as it does not show  fully that with the increase of sequence it should be more unique rather than less. At least, it should not be less, based on my understanding of index.</p>
  
  <p>First question is why is the seq_nbr which is the sequence number has less cardinality. What does it indicate? </p>
  
  <p>Second, why are the last three with the same cardinality? What does it indicate? </p>