MySQL : How to check uniqueness of pair

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);

如果我们想表示一个无向图，我们需要在对的唯一性上添加一个 CHECK 约束。

由于 SQL 标准不允许在 CHECK 约束中使用子查询，我如何检查对的唯一性？

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 ;

这是一个示例表:

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，其中 PRIMARY KEY 的列颠倒了

让我们创建表:

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 ;

这是一些示例数据:

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 条件可防止任何自循环

警告

这个触发器在以下情况下不起作用

• 你从一张空 table 开始
• 在第一行输入(3,3)

因为 BEFORE INSERT 触发器不会在空表上触发。

一旦您使用 A <> B 输入有效行，所有检查都会正确执行。

试一试!!!