Select option issue

Hi All,

I have written following query.

   SELECT matnr werks charg
      FROM mcha
      INTO TABLE t_mcha
      WHERE matnr IN s_mat
      AND   werks EQ  p_plant
       AND charg IN s_charg.

According to me whole table should be fetched if s_mat or S_char is blank.

However, i am getting few entries even after s_charg is blank  instead of fetching whole table.

I will be thankful if anybody explains this behaviour of select query?