Question

我有以下表格：

CREATE TABLE BOOK_AUTHORS
(Book_id CHAR(20) NOT NULL,
AuthorName VARCHAR(30) NOT NULL,
PRIMARY KEY (Book_id, AuthorName),
FOREIGN KEY (Book_id) REFERENCES BOOK (Book_id));

CREATE TABLE BOOK_COPIES
(Book_id CHAR(20) NOT NULL,
Branch_id CHAR(20) NOT NULL,
No_of_copies NUMBER,
PRIMARY KEY (Book_id, Branch_id),
FOREIGN KEY (Book_id) REFERENCES BOOK (Book_id),
FOREIGN KEY (Branch_id) REFERENCES LIBRARY_BRANCH (Branch_id));

我想为这两者添加ON DELETE CASCADE个约束：

我第一次尝试它说它有效。该文件看起来像：

ALTER TABLE "BOOK_AUTHORS"
ADD CONSTRAINT "fk_test"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

然后我通过并为第二个表中的两个外键创建了两个单独的表：

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test1"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test2"
FOREIGN KEY ("Branch_id")
REFERENCES "LIBRARY_BRANCH" ("Branch_id")
ON DELETE CASCADE;

然而，这样做我得到了错误

“Book_id”无效标识符

然后

“Branch_id”无效标识符

我不知道我做错了什么。然后我又回去做了第一个alter table（我最初想过的那个），它给了我同样的错误信息（"Book_id" invalid identifier）。有人可以帮我添加这些约束吗？我还有五个其他表来添加这些约束。

Answer 1

如果您在标识符周围添加双引号（就像您在

中所做的那样）

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test1"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

）您的标识符（例如本例中的“Book_id”）会区分大小写。

所以要么你必须改变你的表定义并将列重命名为“Book_id”或者（更好的是恕我直言），只需删除约束定义中的双引号：

ALTER TABLE BOOK_COPIES
ADD CONSTRAINT fk_test1
FOREIGN KEY (Book_id)
REFERENCES BOOK (Book_id)
ON DELETE CASCADE;

Answer 2

首先让我澄清一件事，你不能添加删除级联到已经存在的外键约束，如docs所示，你只能改变其状态，这意味着启用或禁用，如果你需要先添加然后删除约束。这个问题被问了两次，仍然重复请主持人一眼就看出来。以下链接已经解决了您的问题。 first，second和third以及谁知道有多少被问及有关删除级联的问题。

Answer 3

在表名和列名周围加上双引号（“）。

我添加了表“BOOK”和表“LIBRARY BRANCH”：

CREATE TABLE "BOOK"
("Book_id" CHAR(20) NOT NULL,
"BookName" VARCHAR(30) NOT NULL,
PRIMARY KEY ("Book_id"));

CREATE TABLE "BOOK_AUTHORS"
("Book_id" CHAR(20) NOT NULL,
"AuthorName" VARCHAR(30) NOT NULL,
PRIMARY KEY ("Book_id", "AuthorName"));

CREATE TABLE "LIBRARY_BRANCH" 
("Branch_id" CHAR(20) NOT NULL, 
 "Branch_name" VARCHAR(50), 
 "Address" VARCHAR(100), 
 PRIMARY KEY ("Branch_id"));

CREATE TABLE "BOOK_COPIES"
("Book_id" CHAR(20) NOT NULL,
"Branch_id" CHAR(20) NOT NULL,
"No_of_copies" NUMBER,
PRIMARY KEY ("Book_id", "Branch_id"));


ALTER TABLE "BOOK_AUTHORS"
ADD CONSTRAINT "fk_test"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test1"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test2"
FOREIGN KEY ("Branch_id")
REFERENCES "LIBRARY_BRANCH" ("Branch_id")
ON DELETE CASCADE;

sqlfiddle demo

SQL ALTER TABLE ON DELETE CASCADE

3 个答案: