Create a type

Question

hey all, whats the problem of the second followed type creation? when I execute it, it gave me an error"type create with a compilation error".
CREATE or replace TYPE WaterType AS object
name VARCHAR(20)
NOT INSTANTIABLE
NOT FINAL;
CREATE or replace TYPE RiverType UNDER WaterType
length INT,
meets REF WaterType SCOPE IS Rivers,
runs_through REF StateType MULTISET SCOPE IS States
INSTANCE METHOD leads_to_ocean() RETURNS OceanType;

Answer 1

There are many issues with your code. It looks like you are trying to create type RiverType based on type WaterType and limit name references to names contained in table Rivers. First of all, you can not use SCOPE in type definition.Object type defines structure and datatypes. SCOPE defines a restrictions on a an type instance values. Another word you SCOPE column (instance) of object type but not type definition. And what is INSTANCE METHOD? I know ORDER, MAP, constructor methods. Anyway:
SQL> CREATE OR REPLACE TYPE WaterType
  2    AS
  3      OBJECT
  4            (
  5             name VARCHAR(20)
  6            )
  7      NOT FINAL;
  8  /
Type created.
SQL> CREATE OR REPLACE TYPE StateType
  2    AS
  3      OBJECT
  4            (
  5             name VARCHAR(20)
  6            )
  7  /
Type created.
SQL> CREATE TABLE States OF StateType
  2  /
Table created.
SQL> CREATE OR REPLACE
  2    TYPE StateRefListType IS TABLE OF REF StateType
  3  /
Type created.
SQL> CREATE OR REPLACE
  2    TYPE RiverType
  3     UNDER WaterType(
  4                     length INT,
  5                     meets REF RiverType,
  6                     runs_through StateRefListType
  7                    )
  8  /
Type created.
SQL> CREATE TABLE Rivers OF RiverType(
  2                                   SCOPE FOR(meets) IS Rivers
  3                                  )
  4    NESTED TABLE runs_through STORE AS runs_through_tbl
  5  /
Table created.
SQL> ALTER TABLE runs_through_tbl ADD(SCOPE FOR(column_value) IS States)
  2  /
Table altered.
SQL> Will create Rivers table where "meets" can be a river that already exists in table Rivers and "runs_through" being a list of valid states:
SQL> INSERT INTO States
  2    VALUES(
  3           StateType('MISSOURI')
  4          )
  5  /
1 row created.
SQL> INSERT INTO States
  2    VALUES(
  3           StateType('MISSISSIPPI')
  4          )
  5  /
1 row created.
SQL> INSERT INTO States
  2    VALUES(
  3           StateType('MINNESOTA')
  4          )
  5  /
1 row created.
SQL> INSERT INTO Rivers
  2    VALUES(
  3           RiverType(
  4                     'MISSISSIPPI',
  5                     2340,
  6                     NULL,
  7                     CAST(
  8                          MULTISET(
  9                                   SELECT  REF(s)
10                                     FROM  States s
11                                     WHERE name IN (
12                                                    'MISSOURI',
13                                                    'MISSISSIPPI',
14                                                    'MINNESOTA'
15                                                   )
16                                  )
17                          AS StateRefListType
18                         )
19                    )
20          )
21  /
1 row created.
SQL> INSERT INTO Rivers
  2    VALUES(
  3           RiverType(
  4                     'MISSOURI',
  5                     2540,
  6                     (
  7                      SELECT  REF(r)
  8                        FROM  Rivers r
  9                        WHERE name = 'MISSISSIPPI'
10                     ),
11                     CAST(
12                          MULTISET(
13                                   SELECT  REF(s)
14                                     FROM  States s
15                                     WHERE name IN (
16                                                    'MISSOURI',
17                                                    'MONTANA'
18                                                   )
19                                  )
20                          AS StateRefListType
21                         )
22                    )
23          )
24  /
1 row created.However, see what happened. Montana does not exist in States table, therefore it was not added to runs_through and since no error is raised it can be easily overlooked:
SQL> SELECT  DEREF(rt.column_value)
  2    FROM  Rivers r,
  3          table(runs_through) rt
  4    WHERE r.name = 'MISSOURI'
  5  /
DEREF(RT.COLUMN_VALUE)(NAME)
STATETYPE('MISSOURI')After we add Montana:
SQL> DELETE  Rivers
  2    WHERE name = 'MISSOURI'
  3  /
1 row deleted.
SQL> INSERT INTO States
  2    VALUES(
  3           StateType('MONTANA')
  4          )
  5  /
1 row created.
SQL> INSERT INTO Rivers
  2    VALUES(
  3           RiverType(
  4                     'MISSOURI',
  5                     2540,
  6                     (
  7                      SELECT  REF(r)
  8                        FROM  Rivers r
  9                        WHERE name = 'MISSISSIPPI'
10                     ),
11                     CAST(
12                          MULTISET(
13                                   SELECT  REF(s)
14                                     FROM  States s
15                                     WHERE name IN (
16                                                    'MISSOURI',
17                                                    'MONTANA'
18                                                   )
19                                  )
20                          AS StateRefListType
21                         )
22                    )
23          )
24  /
1 row created.
SQL> SELECT  DEREF(rt.column_value)
  2    FROM  Rivers r,
  3          table(runs_through) rt
  4    WHERE r.name = 'MISSOURI'
  5  /
DEREF(RT.COLUMN_VALUE)(NAME)
STATETYPE('MISSOURI')
STATETYPE('MONTANA')
SQL> Also, even though you can not add tributary before adding river it flows into, you could create river that flows into itself:
SQL> DELETE Rivers
  2  /
2 rows deleted.
SQL> INSERT INTO Rivers
  2    VALUES(
  3           RiverType(
  4                     'MISSOURI',
  5                     2540,
  6                     (
  7                      SELECT  REF(r)
  8                        FROM  Rivers r
  9                        WHERE name = 'MISSISSIPPI'
10                     ),
11                     CAST(
12                          MULTISET(
13                                   SELECT  REF(s)
14                                     FROM  States s
15                                     WHERE name IN (
16                                                    'MISSOURI',
17                                                    'MONTANA'
18                                                   )
19                                  )
20                          AS StateRefListType
21                         )
22                    )
23          )
24  /
1 row created.
SQL> INSERT INTO Rivers
  2    VALUES(
  3           RiverType(
  4                     'MISSISSIPPI',
  5                     2340,
  6                     (
  7                      SELECT  REF(r)
  8                        FROM  Rivers r
  9                        WHERE name = 'MISSISSIPPI'
10                     ),
11                     CAST(
12                          MULTISET(
13                                   SELECT  REF(s)
14                                     FROM  States s
15                                     WHERE name IN (
16                                                    'MISSOURI',
17                                                    'MISSISSIPPI',
18                                                    'MINNESOTA'
19                                                   )
20                                  )
21                          AS StateRefListType
22                         )
23                    )
24          )
25  /
1 row created.
SQL> SELECT  r.name,
  2          r.meets.name
  3    FROM  Rivers r
  4  /
NAME                 MEETS.NAME
MISSISSIPPI
MISSISSIPPI          MISSISSIPPI
SQL>  I assume you could create a trigger to take care of that.
SY.