写在前面
集合论是SQL语言的根基，因为这种特性，SQL也被称为面向集合语言
导入篇：集合运算的几个注意事项
- 注意事项1：SQL能操作具有重复行的集合(multiset、bag)，可以通过可选项ALL来支持
SQL的集合运算符提供了允许重复和不允许重复两种用法，UNION和INTERSECT结果里不会出现重复的行，UNION ALL则会保留重复行；ALL的作用和SELECT子句中的DISTINCT相反。ALL有助于优化查询性能，这是因为使用ALL后不再进行排序
- 注意事项2：集合运算符存在优先级
标准SQL规定：INTERSECT比UNION和EXCEPT优先级高
- 注意事项3：各个DBMS提供商在集合运算上的实现程度不同
MySQL不支持，Oracle使用MINUS替代EXCEPT
- 注意事项4：除法运算没有标准定义
四则运算的和(UNION)、差(EXCEPT)、积(CROSS JOIN)都被引入了标准SQL，商却迟迟没有进入SQL标准
比较表和表：检查集合相等性之基础篇
/* 比较表和表：检查集合相等性 */
CREATE TABLE Tbl_A
(keycol CHAR(1) PRIMARY KEY,
col_1 INTEGER,
col_2 INTEGER,
col_3 INTEGER);
CREATE TABLE Tbl_B
(keycol CHAR(1) PRIMARY KEY,
col_1 INTEGER,
col_2 INTEGER,
col_3 INTEGER);
/* 表相等的情况 */
DELETE FROM Tbl_A;
INSERT INTO Tbl_A VALUES('A', 2, 3, 4);
INSERT INTO Tbl_A VALUES('B', 0, 7, 9);
INSERT INTO Tbl_A VALUES('C', 5, 1, 6);
DELETE FROM Tbl_B;
INSERT INTO Tbl_B VALUES('A', 2, 3, 4);
INSERT INTO Tbl_B VALUES('B', 0, 7, 9);
INSERT INTO Tbl_B VALUES('C', 5, 1, 6);
/* B行不同的情况 */
DELETE FROM Tbl_A;
INSERT INTO Tbl_A VALUES('A', 2, 3, 4);
INSERT INTO Tbl_A VALUES('B', 0, 7, 9);
INSERT INTO Tbl_A VALUES('C', 5, 1, 6);
DELETE FROM Tbl_B;
INSERT INTO Tbl_B VALUES('A', 2, 3, 4);
INSERT INTO Tbl_B VALUES('B', 0, 7, 8);
INSERT INTO Tbl_B VALUES('C', 5, 1, 6);
/* 包含NULL的情况（相等） */
DELETE FROM Tbl_A;
INSERT INTO Tbl_A VALUES('A', NULL, 3, 4);
INSERT INTO Tbl_A VALUES('B', 0, 7, 9);
INSERT INTO Tbl_A VALUES('C', NULL, NULL, NULL);
DELETE FROM Tbl_B;
INSERT INTO Tbl_B VALUES('A', NULL, 3, 4);
INSERT INTO Tbl_B VALUES('B', 0, 7, 9);
INSERT INTO Tbl_B VALUES('C', NULL, NULL, NULL);
/* 包含NULL的情况（C行不同） */
DELETE FROM Tbl_A;
INSERT INTO Tbl_A VALUES('A', NULL, 3, 4);
INSERT INTO Tbl_A VALUES('B', 0, 7, 9);
INSERT INTO Tbl_A VALUES('C', NULL, NULL, NULL);
DELETE FROM Tbl_B;
INSERT INTO Tbl_B VALUES('A', NULL, 3, 4);
INSERT INTO Tbl_B VALUES('B', 0, 7, 9);
INSERT INTO Tbl_B VALUES('C', 0, NULL, NULL);如果两表相同，有如下逻辑：A UNION B = A = B ；还有 A \(\cup\) B = A \(\cap\) B
-- 判断两表是否完全相等(判断之前可以看看行数相不相同)
SELECT COUNT(*) AS row_cnt FROM
((SELECT * FROM Tbl_A )
UNION
(SELECT * FROM Tbl_B)) AS Total;对上面的表，我们发现对任意的表S都有如下的公式成立：S UNION S = S 这称之为幂等性，同一个集合加多少次结果都相同。
比较表和表：检查集合相等性之进阶篇
集合论里判断两个集合相等一般使用下面两个方法：
- A \(\subset\) B 且 A \(\supset\) B \(\Leftrightarrow\) A = B
- A \(\cup\) B = A \(\cap\) B \(\Leftrightarrow\) A = B
-- A union B = A intersect B means A = B,不难发现intersect也是一个幂等运算符
-- 两张表相等时返回"相等",否则返回"不相等"
SELECT CASE WHEN COUNT(*) = 0
THEN '相等' ELSE '不相等' END AS result
FROM ((SELECT * FROM tbl_A) UNION (SELECT * FROM tbl_B)
EXCEPT
(SELECT * FROM tbl_A) INTERSECT (SELECT * FROM tbl_B)) AS TMP;-- 查看量表不一样的记录
(SELECT * FROM Tbl_A EXCEPT SELECT * FROM Tbl_B)
UNION ALL
(SELECT * FROM Tbl_B EXCEPT SELECT * FROM Tbl_A);用差集实现关系除法运算
- 嵌套使用NOT EXISTS
- 使用HAVING子句转换成一对一关系
- 把重复变成减法
-- 建表语句
/* 用差集实现关系除法运算 */
CREATE TABLE Skills
(skill VARCHAR(32),
PRIMARY KEY(skill));
CREATE TABLE EmpSkills
(emp VARCHAR(32),
skill VARCHAR(32),
PRIMARY KEY(emp, skill));
INSERT INTO Skills VALUES('Oracle');
INSERT INTO Skills VALUES('UNIX');
INSERT INTO Skills VALUES('Java');
INSERT INTO EmpSkills VALUES('相田', 'Oracle');
INSERT INTO EmpSkills VALUES('相田', 'UNIX');
INSERT INTO EmpSkills VALUES('相田', 'Java');
INSERT INTO EmpSkills VALUES('相田', 'C#');
INSERT INTO EmpSkills VALUES('神崎', 'Oracle');
INSERT INTO EmpSkills VALUES('神崎', 'UNIX');
INSERT INTO EmpSkills VALUES('神崎', 'Java');
INSERT INTO EmpSkills VALUES('平井', 'UNIX');
INSERT INTO EmpSkills VALUES('平井', 'Oracle');
INSERT INTO EmpSkills VALUES('平井', 'PHP');
INSERT INTO EmpSkills VALUES('平井', 'Perl');
INSERT INTO EmpSkills VALUES('平井', 'C++');
INSERT INTO EmpSkills VALUES('若田部', 'Perl');
INSERT INTO EmpSkills VALUES('渡来', 'Oracle');-- 用求差集的方法进行关系除法运算(有余数)
SELECT DISTINCT emp
FROM EmpSkills ES1
WHERE NOT EXISTS
(SELECT skill FROM Skills EXCEPT SELECT skill FROM EmpSkills ES2 WHERE ES1.emp = ES2.emp);寻找相等的子集
/* 4.寻找相等的子集 */
CREATE TABLE SupParts
(sup CHAR(32) NOT NULL,
part CHAR(32) NOT NULL,
PRIMARY KEY(sup, part));
INSERT INTO SupParts VALUES('A', '螺丝');
INSERT INTO SupParts VALUES('A', '螺母');
INSERT INTO SupParts VALUES('A', '管子');
INSERT INTO SupParts VALUES('B', '螺丝');
INSERT INTO SupParts VALUES('B', '管子');
INSERT INTO SupParts VALUES('C', '螺丝');
INSERT INTO SupParts VALUES('C', '螺母');
INSERT INTO SupParts VALUES('C', '管子');
INSERT INTO SupParts VALUES('D', '螺丝');
INSERT INTO SupParts VALUES('D', '管子');
INSERT INTO SupParts VALUES('E', '保险丝');
INSERT INTO SupParts VALUES('E', '螺母');
INSERT INTO SupParts VALUES('E', '管子');
INSERT INTO SupParts VALUES('F', '保险丝');-- 生成供应商的全部组合
SELECT SP1.sup AS s1,SP2.sup AS s2
FROM SupParts SP1,SupParts SP2
WHERE SP1.sup < SP2.sup
GROUP BY SP1.sup,SP2.sup;SELECT SP1.sup AS s1,SP2.sup AS s2
FROM SupParts SP1,SupParts SP2
WHERE SP1.sup < SP2.sup
AND SP1.part = SP2.part
GROUP BY SP1.sup,SP2.sup
HAVING COUNT(*) = (SELECT COUNT(*) FROM SupParts SP3 WHERE SP3.sup = SP1.sup)
AND COUNT(*) = (SELECT COUNT(*) FROM SupParts SP4 WHERE SP4.sup = SP2.sup);用于删除重复行的高效SQL
/* 5.用于删除重复行的高效SQL */
/* 在PostgreSQL中，需要把“with oids”添加到CREATE TABLE语句的最后 */
CREATE TABLE Products
(name CHAR(16),
price INTEGER);
INSERT INTO Products VALUES('苹果', 50);
INSERT INTO Products VALUES('橘子', 100);
INSERT INTO Products VALUES('橘子', 100);
INSERT INTO Products VALUES('橘子', 100);
INSERT INTO Products VALUES('香蕉', 80);-- 删除重复行：使用关联子查询
DELETE FROM Products
WHERE rowid < (SELECT MAX(P2.rowid) FROM Products P2 WHERE Products.name = P2.name AND Product.price = P2.price);-- 用于删除重复行的高效SQL语句(1):通过EXCEPT求补集
DELETE FROM Products
WHERE rowid IN (SELECT rowid FROM Products EXCEPT SELECT MAX(rowid) FROM Products GROUP BY name,price)-- 用于删除重复行的高效SQL语句(2):通过NOT IN求补集
DELETE FROM Products
WHERE rowid NOT IN (SELECT MAX(rowid) FROM Products GROUP BY name,price)小结
- 集合运算方面，SQL的标准化进行的非常缓慢，使用时需要注意
- 如果集合运算符不指定ALL选项，重复行会被排除掉，而且这种情况下还会发生排序，所以性能方面不够好
- UNION和INTERSECT都具有幂等性，而EXCEPT不具有幂等性
- 标准SQL没有关系除法的运算符，需要自己实现
- 判断两个集合是否相等，可以通过幂等性或一一映射两种方式
- 使用EXCEPT可以很简单地求得补集
练习题
/* 练习题1-7-1：改进“只使用UNION的比较” */
SELECT CASE WHEN COUNT(*) = (SELECT COUNT(*) FROM tbl_A )
AND COUNT(*) = (SELECT COUNT(*) FROM tbl_B )
THEN '相等'
ELSE '不相等' END AS result
FROM ( SELECT * FROM tbl_A
UNION
SELECT * FROM tbl_B ) TMP;/* 练习题1-7-2：精确关系除法运算 */
SELECT DISTINCT emp
FROM EmpSkills ES1
WHERE NOT EXISTS
(SELECT skill
FROM Skills
EXCEPT
SELECT skill
FROM EmpSkills ES2
WHERE ES1.emp = ES2.emp)
AND NOT EXISTS
(SELECT skill
FROM EmpSkills ES3
WHERE ES1.emp = ES3.emp
EXCEPT
SELECT skill
FROM Skills );/* 练习题1-7-2：精确关系除法运算 */
SELECT emp
FROM EmpSkills ES1
WHERE NOT EXISTS
(SELECT skill
FROM Skills
EXCEPT
SELECT skill
FROM EmpSkills ES2
WHERE ES1.emp = ES2.emp)
GROUP BY emp
HAVING COUNT(*) = (SELECT COUNT(*) FROM Skills);原文地址：https://www.cnblogs.com/evian-jeff/p/11580818.html
时间： 2024-10-04 03:23:58