set serveroutput on
declare
TYPE rec IS RECORD (
   name varchar2(1000)
      );
TYPE ttbl IS TABLE OF rec;
p ttbl:=ttbl();
t ttbl:=ttbl();
x ttbl:=ttbl();
begin 
for i in 1..10
loop
p.extend;
p(i).name:=i;
end loop;

for j in 1..15
loop
t.extend;
t(j).name:=j;
end loop;

x:= t MULTISET UNION  p;

for k in x.first..x.last
loop
dbms_output.put_line(x(k).name);
end loop;
end;


Output:

PL/SQL procedure successfully completed.

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set serveroutput on
declare
TYPE rec IS RECORD (
   name varchar2(1000)
      );
TYPE ttbl IS TABLE OF rec;
p ttbl:=ttbl();
t ttbl:=ttbl();
x ttbl:=ttbl();
begin 
for i in 1..10
loop
p.extend;
p(i).name:=i;
end loop;

for j in 1..15
loop
t.extend;
t(j).name:=j;
end loop;

x:= t MULTISET UNION DISTINCT p;

for k in x.first..x.last
loop
dbms_output.put_line(x(k).name);
end loop;
end;

error Message:

Error report -
ORA-06550: line 22, column 5:
PLS-00306: wrong number or types of arguments in call to 'MULTISET_UNION_DISTINCT'
ORA-06550: line 22, column 1:
PL/SQL: Statement ignored
06550. 00000 -  "line %s, column %s:\n%s"
*Cause:    Usually a PL/SQL compilation error.
*Action:

 20  x:= t MULTISET UNION DISTINCT p;
 21  for k in x.first..x.last
 22  loop
 23  dbms_output.put_line(x(k).name);
 24  end loop;
 25* end;
 26  /
x:= t MULTISET UNION DISTINCT p;
    *
ERROR at line 20:
ORA-06550: line 20, column 5:
PLS-00306: wrong number or types of arguments in call to
'MULTISET_UNION_DISTINCT'
ORA-06550: line 20, column 1:
PL/SQL: Statement ignored


SQL> select * from V$Version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production

SQL> 

SCOTT@orcl> CREATE OR REPLACE TYPE rec AS OBJECT
  2    (name  VARCHAR2(1000),
  3  	MAP MEMBER FUNCTION sort_key RETURN VARCHAR2);
  4  /

Type created.

SCOTT@orcl> SHOW ERRORS
No errors.
SCOTT@orcl> CREATE OR REPLACE TYPE BODY rec
  2  AS
  3    MAP MEMBER FUNCTION sort_key RETURN VARCHAR2
  4    IS
  5    BEGIN
  6  	 RETURN name;
  7    END;
  8  END;
  9  /

Type body created.

SCOTT@orcl> SHOW ERRORS
No errors.
SCOTT@orcl> CREATE OR REPLACE TYPE ttbl AS TABLE OF rec;
  2  /

Type created.

SCOTT@orcl> SHOW ERRORS
No errors.
SCOTT@orcl> declare
  2    p ttbl:=ttbl();
  3    t ttbl:=ttbl();
  4    x ttbl:=ttbl();
  5  begin
  6    for i in 1..10
  7    loop
  8  	 p.extend;
  9  	 p(i) := rec(i);
 10    end loop;
 11  
 12    for j in 1..15
 13    loop
 14  	 t.extend;
 15  	 t(j) := rec(j);
 16    end loop;
 17  
 18    x:= t MULTISET UNION DISTINCT p;
 19  
 20    for k in x.first..x.last
 21    loop
 22  	 dbms_output.put_line(x(k).name);
 23    end loop;
 24  end;
 25  /
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PL/SQL procedure successfully completed.

SQL> declare
  2      r1 emp%rowtype;
  3      r2 emp%rowtype;
  4  begin
  5      if r1 = r2
  6        then
  7          dbms_output.put_line('Equal');
  8        else
  9          dbms_output.put_line('Not equal or null');
 10      end if;
 11  end;
 12  /
    if r1 = r2
          *
ERROR at line 5:
ORA-06550: line 5, column 11:
PLS-00306: wrong number or types of arguments in call to '='
ORA-06550: line 5, column 5:
PL/SQL: Statement ignored


SQL>