级联truncate

12c之前的版本中，在子表引用一个主表以及子表存在记录的情况下，是不提供截断此主表操作的。而在 12c 中的带有 CASCADE 操作的TRUNCATE TABLE 可以截断主表中的记录，并自动对子表进行递归截断，并作为 DELETE ON CASCADE 服从外键引用。由于这是应用到所有子表的，所以对递归层级的数量是没有 CAP 的，可以是孙子表或是重孙子表等等。这一增强摈弃了要在截断一个主表之前先截断所有子表记录的前提。新的 CASCADE 语句同样也可以应用到表分区和子表分区等。
SQL> create table parent(id number primary key);

Table created.

SQL> create table child(cid number primary key,id number);

Table created.

SQL> insert into parent values(1);

1 row created.

SQL> insert into parent values(2);

1 row created.

SQL> insert into child values(1,1);

1 row created.

SQL> insert into child values(2,1);

1 row created.

SQL> insert into child values(3,2);

1 row created.

SQL> commit;

Commit complete.

SQL> select a.id,b.cid,b.id from parent a, child b where a.id=b.id;

    ID        CID         ID

---

     1          1          1
     1          2          1
     2          3          2

--添加约束，不附上 on delete cascade
SQL> alter table child add constraint fk_parent_child foreign key(id) references parent(id);

Table altered.

SQL> truncate table parent cascade;
truncate table parent cascade
*
ERROR at line 1:
ORA-14705: unique or primary keys referenced by enabled foreign keys in table
"HR"."CHILD"

SQL> col CONSTRAINT_NAME for a25;
SQL> col TABLE_NAME for a25;
SQL> col COLUMN_NAME for a25;
SQL> select CONSTRAINT_NAME,TABLE_NAME, COLUMN_NAME from user_cons_columns where TABLE_NAME='CHILD';

CONSTRAINT_NAME TABLE_NAME COLUMN_NAME

---

SYS_C0010458 CHILD CID
FK_PARENT_CHILD CHILD ID
-- 删除并添加约束，并附上 on delete cascade
SQL> alter table child drop constraint FK_PARENT_CHILD;

Table altered.

SQL> alter table child add constraint fk2_parent_child foreign key(id) references parent(id) on delete cascade;

Table altered.

SQL> truncate table parent cascade;

Table truncated.

SQL> select a.id,b.cid,b.id from parent a, child b where a.id=b.id;

no rows selected