Mysql:如何检查对的唯一性
如果我们想要表示一个无向图,我们需要在对的唯一性上添加一个检查约束Mysql:如何检查对的唯一性,mysql,sql,Mysql,Sql,如果我们想要表示一个无向图,我们需要在对的唯一性上添加一个检查约束 由于SQL标准不允许在检查约束中使用子查询,如何检查对的唯一性?MySQL不支持检查约束 您可以创建一个BEFORE INSERT和BEFORE UPDATE触发器来检查这种情况,并在需要时抛出一个错误 例如: CREATE TABLE nodes ( id INTEGER PRIMARY KEY, name VARCHAR(10) NOT NULL, feat1 CHAR(
由于SQL标准不允许在检查约束中使用子查询,如何检查对的唯一性?MySQL不支持检查约束 您可以创建一个BEFORE INSERT和BEFORE UPDATE触发器来检查这种情况,并在需要时抛出一个错误 例如:
CREATE TABLE nodes (
id INTEGER PRIMARY KEY,
name VARCHAR(10) NOT NULL,
feat1 CHAR(1), -- e.g., age
feat2 CHAR(1) -- e.g., school attended or company
);
CREATE TABLE edges (
a INTEGER NOT NULL REFERENCES nodes(id) ON UPDATE CASCADE ON DELETE CASCADE,
b INTEGER NOT NULL REFERENCES nodes(id) ON UPDATE CASCADE ON DELETE CASCADE,
PRIMARY KEY (a, b)
);
CREATE INDEX a_idx ON edges (a);
CREATE INDEX b_idx ON edges (b);
另外,创建类似的更新前触发器以避免更新时出现新的错误值,或者只使用存储过程,因为代码是相同的。
CHECK
在MySQL的create TABLE
上不受支持,如
CHECK子句已解析,但被所有存储引擎忽略
事实上,自2004年以来,在这个问题上有一个新的解决方案(!)
我将采用的方法是在插入和更新时创建一个存储过程触发器,如果这对触发器存在,则会故意失败。您可以装配一个触发器,在看到
(a,B)
或(B,a)
时失败:
触发因素如下:
CREATE TABLE edges(
a INT(11) NOT NULL,
b INT(11) NOT NULL
);
DELIMITER $$
CREATE TRIGGER trigger1
BEFORE INSERT
ON edges
FOR EACH ROW
BEGIN
SET @cnt = NULL;
SELECT COUNT(*) INTO @cnt FROM edges
WHERE a = new.a AND b = new.b OR a = new.b AND b = new.a;
IF @cnt > 0 THEN
SIGNAL SQLSTATE '02000' SET MESSAGE_TEXT = 'Error: uniqueness of pair';
END IF;
END
$$
DELIMITER ;
mysql> DROP DATABASE if exists saurabh;
Query OK, 1 row affected (0.01 sec)
mysql> CREATE DATABASE saurabh;
Query OK, 1 row affected (0.00 sec)
mysql> USE saurabh
Database changed
mysql> CREATE TABLE edges
-> (
-> a INTEGER NOT NULL,
-> b INTEGER NOT NULL,
-> PRIMARY KEY (a,b),
-> UNIQUE KEY (b,a)
-> );
Query OK, 0 rows affected (0.12 sec)
mysql>
下面是一个示例表:
DELIMITER $$
CREATE TRIGGER edges_bi BEFORE INSERT
ON edges FOR EACH ROW
BEGIN
DECLARE found_count,dummy,diff,SomethingsWrong INT DEFAULT 0;
DECLARE errmsg VARCHAR(128);
SET diff = new.a - new.b;
IF diff = 0 THEN
SET errmsg = CONCAT('[',new.a,',',new.b,'] is Vertex, Not Edge');
SET SomethingsWrong = 1;
END IF;
SELECT COUNT(1) INTO found_count FROM edges
WHERE (a=NEW.a AND b=NEW.b) OR (a=NEW.b AND b=NEW.a);
IF found_count = 1 THEN
SET errmsg = CONCAT('[',new.a,',',new.b,'] Already Exists');
SET SomethingsWrong = 1;
END IF;
IF SomethingsWrong = 1 THEN
SELECT errmsg INTO dummy FROM edges WHERE 1=1;
END IF;
END; $$
DELIMITER ;
请注意,我有一个主键和一个主键列反转的唯一键
让我们创建一个表:
DROP DATABASE if exists saurabh;
CREATE DATABASE saurabh;
USE saurabh
CREATE TABLE edges
(
a INTEGER NOT NULL,
b INTEGER NOT NULL,
PRIMARY KEY (a,b),
UNIQUE KEY (b,a)
);
让我们创建触发器:
CREATE TABLE edges(
a INT(11) NOT NULL,
b INT(11) NOT NULL
);
DELIMITER $$
CREATE TRIGGER trigger1
BEFORE INSERT
ON edges
FOR EACH ROW
BEGIN
SET @cnt = NULL;
SELECT COUNT(*) INTO @cnt FROM edges
WHERE a = new.a AND b = new.b OR a = new.b AND b = new.a;
IF @cnt > 0 THEN
SIGNAL SQLSTATE '02000' SET MESSAGE_TEXT = 'Error: uniqueness of pair';
END IF;
END
$$
DELIMITER ;
mysql> DROP DATABASE if exists saurabh;
Query OK, 1 row affected (0.01 sec)
mysql> CREATE DATABASE saurabh;
Query OK, 1 row affected (0.00 sec)
mysql> USE saurabh
Database changed
mysql> CREATE TABLE edges
-> (
-> a INTEGER NOT NULL,
-> b INTEGER NOT NULL,
-> PRIMARY KEY (a,b),
-> UNIQUE KEY (b,a)
-> );
Query OK, 0 rows affected (0.12 sec)
mysql>
以下是一些示例数据:
mysql> DELIMITER $$
mysql> CREATE TRIGGER edges_bi BEFORE INSERT
-> ON edges FOR EACH ROW
-> BEGIN
-> DECLARE found_count,dummy,diff,SomethingsWrong INT DEFAULT 0;
-> DECLARE errmsg VARCHAR(128);
-> SET diff = new.a - new.b;
-> IF diff = 0 THEN
-> SET errmsg = CONCAT('[',new.a,',',new.b,'] is Vertex, Not Edge');
-> SET SomethingsWrong = 1;
-> END IF;
-> SELECT COUNT(1) INTO found_count FROM edges
-> WHERE (a=NEW.a AND b=NEW.b) OR (a=NEW.b AND b=NEW.a);
-> IF found_count = 1 THEN
-> SET errmsg = CONCAT('[',new.a,',',new.b,'] Already Exists');
-> SET SomethingsWrong = 1;
-> END IF;
-> IF SomethingsWrong = 1 THEN
-> SELECT errmsg INTO dummy FROM edges WHERE 1=1;
-> END IF;
-> END; $$
Query OK, 0 rows affected (0.11 sec)
mysql> DELIMITER ;
让我们尝试将它们加载到边缘表中:
INSERT INTO edges (a,b) VALUES (5,3);
INSERT INTO edges (a,b) VALUES (3,3);
INSERT INTO edges (a,b) VALUES (3,5);
INSERT INTO edges (a,b) VALUES (5,5);
SELECT * FROM edges;
请注意,阻塞A=B条件会阻止任何自循环
警告
如果
- 你从一张空桌子开始
- 输入
(3,3)
作为第一行
因为插入之前的触发器不会在空表上触发
使用a
B
输入有效行后,所有检查都会正确执行
试试看 我认为答案可能取决于您如何填充边
表,这在您的问题中并不清楚。如果是从节点
表填充的,则可以基于排除镜像对(即1,2和2,1)的SELECT查询构建视图。这也可以解决级联和删除需求 只是澄清一下-这里的目标是防止(1,2)和(2,1)出现在表格中-是吗?是的,是的。也不应该有任何自循环。