CAVEAT

Question

   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);

如果我们想要表示无向图，我们需要在对的唯一性上添加CHECK约束。

由于SQL标准不允许CHECK约束中的子查询，我如何检查该对的唯一性？

Answer 1

您可以设置一个在看到(A,B)或(B,A)时失败的触发器：

这是触发器：

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 ;

以下是一个示例表：

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)
);

请注意，我有一个PRIMARY KEY和一个UNIQUE KEY，PRIMARY KEY的列被反转

让我们创建表格：

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>

让我们创建触发器：

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 ;

以下是一些示例数据：

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;

让我们尝试将这些加载到edges表中：

mysql> INSERT INTO edges (a,b) VALUES (5,3);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO edges (a,b) VALUES (3,3);
ERROR 1366 (HY000): Incorrect integer value: '[3,3] is Vertex, Not Edge' for column 'dummy' at row 1
mysql> INSERT INTO edges (a,b) VALUES (3,5);
ERROR 1366 (HY000): Incorrect integer value: '[3,5] Already Exists' for column 'dummy' at row 1
mysql> INSERT INTO edges (a,b) VALUES (5,5);
ERROR 1366 (HY000): Incorrect integer value: '[5,5] is Vertex, Not Edge' for column 'dummy' at row 1
mysql> SELECT * FROM edges;
+---+---+
| a | b |
+---+---+
| 5 | 3 |
+---+---+
1 row in set (0.00 sec)

请注意，阻止A = B条件会阻止任何自循环

CAVEAT

如果

，此触发器不起作用

你从空表开始
输入(3,3)作为第一行

因为BEFORE INSERT触发器不会在空表上触发。

输入A＆lt;＆gt;的有效行后B然后所有检查都正确执行。

试一试!!!

Answer 2

MySQL不支持CHECK约束。

您可以创建BEFORE INSERT和BEFORE UPDATE触发器来检查这种情况，并在需要时抛出错误。

示例：

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 ;

此外，创建类似的BEFORE UPDATE触发器以避免更新时出现新的错误值，或者仅使用存储过程，因为代码是相同的。

Answer 3

MySQL CHECK不支持

CREATE TABLE，因为documentation指示

CHECK子句被解析但被所有存储引擎忽略

事实上，自2004年以来，该问题就出现了open bug report（！）。

我将采用的方法是在插入和更新时创建存储过程触发器，如果该对存在则故意失败。

Answer 4

我认为答案可能取决于您填写edges表的方式，这在您的问题中并不清楚。如果从nodes表填充它，那么您可以基于排除镜像对的SELECT查询构建VIEW（即1,2和2,1）。这也可以解决级联和删除要求。

Mysql：如何检查对的唯一性

4 个答案:

CAVEAT