rdavg (1) --- compute the average value of an attribute  07/01/82


          | _U_s_a_g_e

          |      rdavg [<selection expr>] <attr>


          | _D_e_s_c_r_i_p_t_i_o_n

          |      'Rdavg'  is  part of the toy relational data base management
          |      system, 'rdb'.  It computes the average value of a specified
          |      attribute over all rows of the  relation  that  satisfy  the
          |      optional  select  expression.   If  no  select expression is
          |      given then it computes the average of an attribute over  all
          |      rows  of the relation.  Standard input 1 must be directed to
          |      a file containing an 'rdb' relation.  The result is  written
          |      to standard output.

          |      The input relation must be a file containing a relation that
          |      was  created  by 'rdmake' or other 'rdb' programs; the rela-
          |      tion cannot be read from the terminal.  The  select  expres-
          |      sion  is  formed  from  the logical operators "&" (and), "|"
          |      (or), and "~" (not) connecting relational conditions involv-
          |      ing two domains or a domain and a literal.


          | _E_x_a_m_p_l_e_s

          |      p.rel> rdavg weight
          |      p.rel> rdavg "height>65&height<80" weight


          | _M_e_s_s_a_g_e_s

          |      "Sorry, a relation can't be read from the terminal"
          |      "relation is corrupted!!"
          |      "Cannot load input relation"
          |      "Usage:  rdavg [<selection expr>] <attr>"
          |      "Domain not found"
          |      "Strings can't be averaged"
          |      "Average is undefined for empty relation"
          |      "Invalid expression"
          |      "expected domain name or literal"


          | _S_e_e _A_l_s_o

          |      rdcat (1), rdextr (1), rdjoin (1), rdmake (1), rdprint  (1),
          |      rdproj  (1),  rdsel  (1), rdsort (1), rduniq (1), rdatt (1),
          |      rdcount (1), rddiff (1), rddiv (1), rdint  (1),  rdmax  (1),
          |      rdmin (1), rdnat (1), rdsum (1)









            rdavg (1)                     - 1 -                     rdavg (1)