SQL> create table ta ( id int, cust int, yes_or_no varchar2(1) );

Table created.

SQL> create table tb ( cust int primary key ) ;

Table created.

SQL>
SQL> insert into ta
  2  select rownum, dbms_random.value(1,500000), null
  3  from dual
  4  connect by level <= 60000;

60000 rows created.

SQL>
SQL> insert into tb
  2  select rownum*2
  3  from dual
  4  connect by level <= 500000;

500000 rows created.

SQL>
SQL> commit;

Commit complete.

SQL>
SQL>
SQL> merge into ta
  2  using ( select * from tb ) b
  3  on ( ta.cust = b.cust )
  4  when matched then
  5   update set yes_or_no = 'Y';

30057 rows merged.

SQL>
SQL> update ta set yes_or_no = 'N' where yes_or_no is null;

29943 rows updated.

SQL>
SQL> select * from ta where rownum < 50;

        ID       CUST Y
---------- ---------- -
      2087     179958 Y
      2088     275279 N
      2089     187087 N
      2090      34816 Y
      2091     444243 N
      2092     468718 Y
      2093     325392 Y
      2094     397626 Y
      2095     253285 N
      2096      43035 N
      2097     168041 N
      2098     216580 Y
      2099      76296 Y
      2100     281676 Y
      2101     173866 Y
      2102      91198 Y
      2103     322284 Y
      2104     290060 Y
      2105     315403 N
      2106     423184 Y
      2107     386996 Y
      2108      32697 N
      2109     316326 Y
      2110     320719 N
      2111     438909 N
      2112     427618 Y
      2113      80531 N
      2114     131810 Y
      2115     104332 Y
      2116     122527 N
      2117     330385 N
      2118      61493 N
      2119      78733 N
      2120      17073 N
...
...