DB2：多个表之间的外键约束

Question

在DB2中，假设我有3个表：T1，T2，T3：

CREATE TABLE T1 (
  T1_ID INTEGER NOT NULL,
  PRIMARY KEY (ID)
);

CREATE TABLE T2 (
  T2_T1 INTEGER NOT NULL, -- parent entity ID from T1
  T2_ID INTEGER NOT NULL,
  PRIMARY KEY (T2_T1, T2_ID),
);

CREATE TABLE T3 (
  T3_T1 INTEGER NOT NULL, -- parent entity ID from T1
  T3_T2 INTEGER NOT NULL, -- parent entity ID from T2
  T3_ID INTEGER NOT NULL,
  PRIMARY KEY (T3_T1, T3_T2, T3_ID)
);

如果T3中有连接记录，我想以某种方式定义FK关系，禁止删除T2中的记录，但同时当我从T1删除记录时，它应该分别自动删除T2和T3中的记录。

是否可以定义FK约束来满足这些要求？或者，如果没有编写首先从T3中删除适当的记录记录，然后从T2中删除，然后从T1中删除的代码，就无法实现？

更新：我正在尝试运行的实际DB2 10.5代码：

CREATE TABLE t1 (
  t1_id INTEGER NOT NULL,
  PRIMARY KEY(t1_id)
)

CREATE TABLE t2 (
  t2_t1 INTEGER NOT NULL, 
  t2_id INTEGER NOT NULL, 
  PRIMARY KEY (t2_t1, t2_id), 
  CONSTRAINT t2_fk_t1 
    FOREIGN KEY (t2_t1) 
    REFERENCES t1 (t1_id) 
    ON DELETE CASCADE
)

CREATE TABLE t3 (
  t3_t1 INTEGER NOT NULL, 
  t3_t2 INTEGER NOT NULL, 
  t3_id INTEGER NOT NULL, 
  PRIMARY KEY (t3_t1, t3_t2, t3_id), 
  CONSTRAINT t3_fk_t1 
    FOREIGN KEY (t3_t1) 
    REFERENCES t1 (t1_id) 
    ON DELETE CASCADE, 
  CONSTRAINT t3_fk_t2 
    FOREIGN KEY (t3_t1, t3_t2) 
    REFERENCES t2 (t2_t1, t2_id) 
    ON DELETE RESTRICT
)

最后一个CREATE语句失败，出现以下错误：

SQL20255N FOREIGN KEY“T3_FK_T2”无效，因为它会导致后代表“S.T3”通过与冲突删除规则的多个关系删除连接到其祖先表“S.T1”。冲突在后代表上的约束“S.T3.T3_FK_T1”和“S.T3.T3_FK_T2”的删除规则之间。原因代码=“3”。 SQLSTATE = 42915

Answer 1

这应该工作。即使看起来有点奇怪，也不禁止重叠约束。

---

CREATE TABLE t1 (
  t1_id INTEGER NOT NULL PRIMARY KEY
);

CREATE TABLE t2 (
  t2_t1 INTEGER NOT NULL REFERENCES t1(t1_id) ON DELETE CASCADE
  , t2_id INTEGER NOT NULL
  , PRIMARY KEY (t2_t1, t2_id)
);

CREATE TABLE t3 (
  t3_t1 INTEGER NOT NULL REFERENCES t1(t1_id) ON DELETE CASCADE
  , t3_t2 INTEGER NOT NULL
        , FOREIGN KEY (t3_t1,t3_t2) REFERENCES t2(t2_t1,t2_id) ON DELETE RESTRICT
  , t3_id INTEGER NOT NULL
        , PRIMARY KEY (t3_t1, t3_t2, t3_id)
);
INSERT INTO t1 (t1_id) VALUES (11), (12),(13), (14), (15);
INSERT INTO t2 (t2_t1,t2_id) VALUES (11, 21), (12,22),(13,23), (14,24), (15,25);
INSERT INTO t3 (t3_t1,t3_t2,t3_id) VALUES (11, 21,31), (12,22,32),(13,23,33), (14,24,34), (15,25,35);


DELETE FROM t1 WHERE t1_id = 13; -- should succeed
DELETE FROM t2 WHERE t2_id = 24; -- should fail

SELECT * FROM t1;
SELECT * FROM t2;
SELECT * FROM t3;

Answer 2

对的，这是可能的。

只需创建一个从T3到T2和T1的外键，T2是删除时的限制，T1没有动作。

在T2中，T1的外键也带有级联删除。

CREATE TABLE T1 (
  T1_ID INTEGER NOT NULL unique,
  PRIMARY KEY (T1_ID)
);

CREATE TABLE T2 (
  T2_T1 INTEGER NOT NULL        constraint t2_t1_t1_id_fk
            references t1
                on delete cascade, -- parent entity ID from T1
  T2_ID INTEGER NOT NULL unique,
  PRIMARY KEY (T2_T1, T2_ID)
);

CREATE TABLE T3 (
  T3_T1 INTEGER NOT NULL constraint t3_t1_t1_id_fk
        references t1
            on delete cascade, -- parent entity ID from T1
  T3_T2 INTEGER NOT NULL constraint t3_t2_t2_id_fk
        references t2 (t2_id) on delete restrict, -- parent entity ID from T2
  T3_ID INTEGER NOT NULL,
  PRIMARY KEY (T3_T1, T3_T2, T3_ID)
);

请记住，如果您想拥有更严格的外键，您也可以从其他角度进行操作。 （来自ID的外键到引用！）

最糟糕的情况是总有触发器，一切皆有可能。 :)