SQL进阶系列之5外连接的用法

写在前面

SQL本身是作为一种数据提取工具而出现，使用SQL生成各种定制化报表和非定制化报表并非SQL原本用途的功能，但这并不意味着SQL无法实现这些功能。

用外连接进行行列转换(1)(行 → 列)：制作交叉表

-- 建表语句
/* 用外连接进行行列转换（1）（行→列）：制作交叉表 */
CREATE TABLE Courses
(name   VARCHAR(32),
 course VARCHAR(32),
 PRIMARY KEY(name, course));

INSERT INTO Courses VALUES('赤井', 'SQL入门');
INSERT INTO Courses VALUES('赤井', 'UNIX基础');
INSERT INTO Courses VALUES('铃木', 'SQL入门');
INSERT INTO Courses VALUES('工藤', 'SQL入门');
INSERT INTO Courses VALUES('工藤', 'Java中级');
INSERT INTO Courses VALUES('吉田', 'UNIX基础');
INSERT INTO Courses VALUES('渡边', 'SQL入门');

-- 水平展开求交叉表(1):使用外连接
SELECT C0.name,
CASE WHEN C1.name IS NOT NULL THEN '○' ELSE NULL END AS “SQL入门",
CASE WHEN C2.name IS NOT NULL THEN '○' ELSE NULL END AS "UNIX基础",
CASE WHEN C3.name IS NOT NULL THEN '○' ELSE NULL END AS "Java中级"
FROM (SELECT DISTINCT name FROM Courses) AS C0
LEFT JOIN (SELECT name FROM Courses WHERE course = 'SQL入门') AS C1 ON C0.name = C1.name
LEFT JOIN (SELECT name FROM Courses WHERE course = 'UNIX基础') AS C2 ON C0.name = C2.name
LEFT JOIN (SELECT name FROM Courses WHERE course = 'Java中级') AS C3 ON C0.name = C3.name;
-- 评价：方法直观并容易理解，但大量用到内嵌视图和连接操作，随着列数增加，代码会臃肿，性能也会恶化

-- 水平展开求交叉表(2):使用标量子查询
SELECT C0.name,
(SELECT '○' FROM Courses AS C1 WHERE course = 'SQL入门' AND C1.name = C0.name) AS "SQL入门",
(SELECT '○' FROM Courses AS C2 WHERE course = 'UNIX基础' AND C2.name = C0.name) AS "UNIX基础",
(SELECT '○' FROM Courses AS C3 WHERE course = 'Java中级' AND C3.name = C0.name) AS "Java中级"
FROM (SELECT DISTINCT name FROM Courses) AS C0;
-- 在增加课程时，只需要修改SELECT子句后面的内容，代码修改比较简单，应对需求变更比较理想，缺点是性能不好，在SELECT子句中使用标量子查询和关联子查询时，计算开销较大

-- 水平展开求交叉表(3):嵌套使用CASE WHEN表达式
SELECT name,
CASE WHEN SUM(CASE WHEN course = 'SQL入门' THEN 1 ELSE 0 END) = 1 THEN '○' ELSE NULL END AS "SQL入门",
CASE WHEN SUM(CASE WHEN course = 'UNIX基础' THEN 1 ELSE 0 END) = 1 THEN '○' ELSE NULL END AS "UNIX基础",
CASE WHEN SUM(CASE WHEN course = 'Java中级' THEN 1 ELSE 0 END) = 1 THEN '○' ELSE NULL END AS "Java中级"
FROM Courses
GROUP BY name;

用外连接进行行列转换(2)(列 → 行)：汇总重复项于一列

-- 建表语句
/* 用外连接进行行列转换（2）（列→行）：汇总重复项于一列 */
CREATE TABLE Personnel
 (employee   varchar(32),
  child_1    varchar(32),
  child_2    varchar(32),
  child_3    varchar(32),
  PRIMARY KEY(employee));

INSERT INTO Personnel VALUES('赤井', '一郎', '二郎', '三郎');
INSERT INTO Personnel VALUES('工藤', '春子', '夏子', NULL);
INSERT INTO Personnel VALUES('铃木', '夏子', NULL,   NULL);
INSERT INTO Personnel VALUES('吉田', NULL,   NULL,   NULL);

-- 列数据转行数据(不去除空行)
SELECT employee,child_1 FROM Personnel
UNION ALL
SELECT employee,child_2 FROM Personnel
UNION ALL
SELECT employee,child_3 FROM Personnel;

-- 列数据转行数据(去除空行)
SELECT employee,child_1 FROM Personnel WHERE child_1 IS NOT NULL
UNION ALL
SELECT employee,child_2 FROM Personnel WHERE child_2 IS NOT NULL
UNION ALL
SELECT employee,child_3 FROM Personnel WHERE child_3 IS NOT NULL;

-- 按需自定义保留记录(不完全去除重复行)

-- 准备所有孩子的视图
CREATE VIEW Children(child) AS
SELECT child_1 FROM Personnel
UNION SELECT child_2 FROM Personnel
UNION SELECT child_3 FROM Personnel;

-- 获取员工子女列表的SQL语句(没有孩子的员工也要输出)
SELECT Personnel.employee,Children.child
FROM Personnel
LEFT JOIN Children
ON Children.child IN (Personnel.child_1,Personnel.child_2,Personnel.child_3);

在交叉表里制作嵌套式表侧栏

/* 在交叉表里制作嵌套式表侧栏 */
CREATE TABLE TblSex
(sex_cd   char(1),
 sex varchar(5),
 PRIMARY KEY(sex_cd));

CREATE TABLE TblAge
(age_class char(1),
 age_range varchar(30),
 PRIMARY KEY(age_class));

CREATE TABLE TblPop
(pref_name  varchar(30),
 age_class  char(1),
 sex_cd     char(1),
 population integer,
 PRIMARY KEY(pref_name, age_class,sex_cd));

INSERT INTO TblSex (sex_cd, sex ) VALUES('m',	'男');
INSERT INTO TblSex (sex_cd, sex ) VALUES('f',	'女');

INSERT INTO TblAge (age_class, age_range ) VALUES('1',	'21岁～30岁');
INSERT INTO TblAge (age_class, age_range ) VALUES('2',	'31岁～40岁');
INSERT INTO TblAge (age_class, age_range ) VALUES('3',	'41岁～50岁');

INSERT INTO TblPop VALUES('秋田', '1', 'm', 400 );
INSERT INTO TblPop VALUES('秋田', '3', 'm', 1000 );
INSERT INTO TblPop VALUES('秋田', '1', 'f', 800 );
INSERT INTO TblPop VALUES('秋田', '3', 'f', 1000 );
INSERT INTO TblPop VALUES('青森', '1', 'm', 700 );
INSERT INTO TblPop VALUES('青森', '1', 'f', 500 );
INSERT INTO TblPop VALUES('青森', '3', 'f', 800 );
INSERT INTO TblPop VALUES('东京', '1', 'm', 900 );
INSERT INTO TblPop VALUES('东京', '1', 'f', 1500 );
INSERT INTO TblPop VALUES('东京', '3', 'f', 1200 );
INSERT INTO TblPop VALUES('千叶', '1', 'm', 900 );
INSERT INTO TblPop VALUES('千叶', '1', 'f', 1000 );
INSERT INTO TblPop VALUES('千叶', '3', 'f', 900 );

-- 构造侧边栏
SELECT age_class,age_range,sex_cd,sex
FROM TblAge CROSS JOIN TblSex;
-- 处理TblPop表
SELECT age_class,sex_cd,
SUM(CASE WHEN pref_name IN ('秋田','青森') THEN population ELSE 0 END) AS "东北",
SUM(CASE WHEN pref_name IN ('东京','千叶') THEN population ELSE 0 END) AS "关东"
FROM TblPop
GROUP BY age_class,sex_cd;
-- 连接两张表
SELECT A.age_range,A.sex,B.东北,B.关东
FROM (SELECT age_class,age_range,sex_cd,sex FROM TblAge CROSS JOIN TblSex) AS A
LEFT JOIN (SELECT age_class,sex_cd,
SUM(CASE WHEN pref_name IN ('秋田','青森') THEN population ELSE 0 END) AS "东北",
SUM(CASE WHEN pref_name IN ('东京','千叶') THEN population ELSE 0 END) AS "关东"
FROM TblPop
GROUP BY age_class,sex_cd) AS B
ON A.age_class = B.age_class and A.sex_cd = B.sex_cd;

作为乘法运算的连接

-- 建表语句
/* 作为乘法运算的连接 */
CREATE TABLE Items
 (item_no INTEGER PRIMARY KEY,
  item    VARCHAR(32) NOT NULL);

INSERT INTO Items VALUES(10, 'FD');
INSERT INTO Items VALUES(20, 'CD-R');
INSERT INTO Items VALUES(30, 'MO');
INSERT INTO Items VALUES(40, 'DVD');

CREATE TABLE SalesHistory
 (sale_date DATE NOT NULL,
  item_no   INTEGER NOT NULL,
  quantity  INTEGER NOT NULL,
  PRIMARY KEY(sale_date, item_no));

INSERT INTO SalesHistory VALUES('2007-10-01',  10,  4);
INSERT INTO SalesHistory VALUES('2007-10-01',  20, 10);
INSERT INTO SalesHistory VALUES('2007-10-01',  30,  3);
INSERT INTO SalesHistory VALUES('2007-10-03',  10, 32);
INSERT INTO SalesHistory VALUES('2007-10-03',  30, 12);
INSERT INTO SalesHistory VALUES('2007-10-04',  20, 22);
INSERT INTO SalesHistory VALUES('2007-10-04',  30,  7);

-- 要求得到所有item即便没有销售记录的quantity求和
-- 方法一：连接前聚合，然后一对一进行连接
SELECT Items.item_no,SH.quantity
FROM Items LEFT JOIN (
SELECT item_no,sum(quantity) as quantity
FROM SalesHistory
GROUP BY item_no) AS SH
ON Items.item_no = SH.item_no;
-- 评价：从性能角度讲,通过聚合将SH上的item_no变成了非空不重复，但SH本身不存在主键索引，无法利用查询优化

-- 方法二：一对多进行连接不会增加行
SELECT Items.item_no,sum(SH.quantity) AS quantity
FROM Items
LEFT JOIN SalesHistory AS SH
ON Items.item_no = SH.item_no
GROUP BY Items.item_no;
-- 评价：这种方法没有使用临时视图，性能会有所改善

全外连接

面向集合的角度理解连接类型

• 左外连接 LEFT OUTER JOIN
• 右外连接 RIGHT OUTER JOIN
• 全外连接 FULL OUTER JOIN

/* 全外连接 */
CREATE TABLE Class_A
(id char(1),
 name varchar(30),
 PRIMARY KEY(id));

CREATE TABLE Class_B
(id   char(1),
 name varchar(30),
 PRIMARY KEY(id));

INSERT INTO Class_A (id, name) VALUES('1', '田中');
INSERT INTO Class_A (id, name) VALUES('2', '铃木');
INSERT INTO Class_A (id, name) VALUES('3', '伊集院');

INSERT INTO Class_B (id, name) VALUES('1', '田中');
INSERT INTO Class_B (id, name) VALUES('2', '铃木');
INSERT INTO Class_B (id, name) VALUES('4', '西园寺');

-- 全外连接保留全部信息
SELECT
COALESCE(A.id,B.id) AS id,A.name AS a_name,B.name AS b_name
FROM Class_A AS a FULL JOIN Class_B AS b
ON A.id = B.id;

-- 数据库不支持全外连接时的替代方案
SELECT A.id AS id,A.name,B.name FROM Class_A AS A LEFT JOIN Class_B AS B ON A.id = B.id
UNION
SELECT B.id AS id,A.name,B.name FROM Class_A AS A RIGHT JOIN Class_B AS B ON A.id = B.id;

INNER JOIN(或者INTERSECT)相当于交集，UNION(LEFT JOIN AND RIGHT JOIN ，再或者FULL JOIN)相当于并集

用外连接进行集合运算

用外连接求差集：A-B

SELECT A.id,A.name AS A_name,B.name AS B_name FROM Class_A AS A LEFT JOIN Class_B AS B ON A.id = B.id WHERE B.name IS NULL;

用外连接求差集：B-A

SELECT A.id,A.name AS A_name,B.name AS B_name FROM Class_A AS A RIGHT JOIN Class_B AS B ON A.id = B.id WHERE A.name IS NULL;

用全外连接求异或集

SELECT COALESCE(A.id,B.id) AS id,COALESCE(A.name,B.name) AS name FROM Class_A AS A FULL JOIN Class_B AS B ON A.id = B.id WHERE A.name IS NULL OR B.name IS NULL;

本节小结

• SQL不是用来生成报表的语言，不建议用齐进行格式转换
• 必要时可以考虑外连接和CASE表达式来解决问题
• 生成嵌套表侧栏时，如果先生成主表的笛卡尔积再进行连接，很容易就可以完成
• 从行数来看，表连接可以看成乘法。因此，当表之间是一对多的关系时，连接后行数不会增加
• 外连接的思想和集合运算很像，使用外连接可以实现各种集合运算

练习题

-- 练习题 1-5-1 ：先连接还是先聚合
SELECT master.age_range,master.sex,
SUM(CASE WHEN data.pref_name IN ('青森','秋田') THEN population ELSE NULL END) AS "东北",
SUM(CASE WHEN data.pref_name IN ('东京','千叶') THEN population ELSE NULL END) AS "关东"
FROM (TblAge CROSS JOIN TblSex) AS master
LEFT JOIN TblPop AS data
ON master.age_class = data.age_class
AND master.sex_cd = data.sex_cd
GROUP BY master.age_range,master.sex;

-- 练习题 1-5-2：请留意孩子的人数
SELECT Personnel.employee,COUNT(Children.child)
FROM Personnel
LEFT JOIN Children
ON Children.child IN (Personnel.child_1,Personnel.child_2,Personnel.child_3)
GROUP BY Personnel.employee;

-- 练习题 1-5-3 全外连接和merge运算符
MERGE INTO Class_A A
    USING (SELECT *
             FROM Class_B ) B
      ON (A.id = B.id)
    WHEN MATCHED THEN
        UPDATE SET A.name = B.name
    WHEN NOT MATCHED THEN
        INSERT (id, name) VALUES (B.id, B.name);