读SQL进阶教程笔记06_外连接
1. SQL的弱点
1.1. SQL语句的执行结果转换为想要的格式
-
1.1.1. 格式转换
-
1.1.2. SQL语言本来就不是为了这个目的而出现的
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1.1.3. SQL终究也只是主要用于查询数据的语言而已
1.2. 生成报表的功能
- 1.2.1. 窗口函数
1.3. SQL不是用来生成报表的语言,所以不建议用它来进行格式转换
- 1.3.1. 必要时考虑用外连接或CASE表达式来解决问题
2. 制作交叉表(行→列)
2.1. 示例
- 2.1.1.
- 2.1.1.1. --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) C0 --这里的C0是侧栏
LEFT OUTER JOIN
(SELECT name FROM Courses WHERE course = 'SQL入门’) C1
ON C0.name = C1.name
LEFT OUTER JOIN
(SELECT name FROM Courses WHERE course = 'UNIX基础’) C2
ON C0.name = C2.name
LEFT OUTER JOIN
(SELECT name FROM Courses WHERE course = 'Java中级’) C3
ON C0.name = C3.name;
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2.1.1.2. 一般情况下,外连接都可以用标量子查询替代
2.1.1.2.1. 需要增加或者减少课程时,只修改SELECT子句即可,代码修改起来比较简单
2.1.1.2.2. 利于应对需求变更,对于需要动态生成SQL的系统也是很有好处的
2.1.1.2.3. 性能不太好
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2.1.1.3. --水平展开(2):使用标量子查询
SELECT C0.name,
(SELECT '○'
FROM Courses C1
WHERE course = 'SQL入门’
AND C1.name = C0.name) AS "SQL入门",
(SELECT '○'
FROM Courses C2
WHERE course = 'UNIX基础’
AND C2.name = C0.name) AS "UNIX基础",
(SELECT '○'
FROM Courses C3
WHERE course = 'Java中级’
AND C3.name = C0.name) AS "Java中级"
FROM (SELECT DISTINCT name FROM Courses) C0; --这里的C0是表侧栏
-
2.1.1.4. 嵌套使用CASE表达式
2.1.1.4.1. CASE表达式可以写在SELECT子句里的聚合函数内部,也可以写在聚合函数外部
2.1.1.4.2. 其实在SELECT子句里,聚合函数的执行结果也是标量值,因此可以像常量和普通列一样使用
2.1.1.4.3. 和标量子查询的做法一样简洁,也能灵活地应对需求变更
-
2.1.1.5. --水平展开(3):嵌套使用CASE表达式
SELECT name,
CASE WHEN SUM(CASE WHEN course = 'SQL入门’THEN 1 ELSE NULL END) = 1
THEN'○'ELSE NULL END AS "SQL入门",
CASE WHEN SUM(CASE WHEN course = 'UNIX基础’THEN 1 ELSE NULL END) = 1
THEN'○'ELSE NULL END AS "UNIX基础",
CASE WHEN SUM(CASE WHEN course = 'Java中级’THEN 1 ELSE NULL END) = 1
THEN'○'ELSE NULL END AS "Java中级"
FROM Courses
GROUP BY name;
3. 汇总重复项于一列(列→行)
3.1. 示例
- 3.1.1.
- 3.1.1.1. --列数据转换成行数据:使用UNION ALL
SELECT employee, child_1 AS child FROM Personnel
UNION ALL
SELECT employee, child_2 AS child FROM Personnel
UNION ALL
SELECT employee, child_3 AS child FROM Personnel;
- 3.1.1.2. 视图
CREATE VIEW Children(child)
AS SELECT child_1 FROM Personnel
UNION
SELECT child_2 FROM Personnel
UNION
SELECT child_3 FROM Personnel;
child
-----
一郎
二郎
三郎
春子
夏子
3.1.1.2.1. --获取员工子女列表的SQL语句(没有孩子的员工也要输出)
SELECT EMP.employee, CHILDREN.child
FROM Personnel EMP
LEFT OUTER JOIN Children
ON CHILDREN.child IN (EMP.child_1, EMP.child_2, EMP.child_3);
4. 制作嵌套式表侧栏
4.1. 示例
- 4.1.1.
- 4.1.2.
-
4.1.3. 结果
- 4.1.3.1.
- 4.1.4. --使用外连接生成嵌套式表侧栏:错误的SQL语句
SELECT MASTER1.age_class AS age_class,
MASTER2.sex_cd AS sex_cd,
DATA.pop_tohoku AS pop_tohoku,
DATA.pop_kanto AS pop_kanto
FROM (SELECT age_class, sex_cd,
SUM(CASE WHEN pref_name IN (’青森’, ’秋田’)
THEN population ELSE NULL END) AS pop_tohoku,
SUM(CASE WHEN pref_name IN (’东京’, ’千叶’)
THEN population ELSE NULL END) AS pop_kanto
FROM TblPop
GROUP BY age_class, sex_cd) DATA
RIGHT OUTER JOIN TblAge MASTER1--外连接1:和年龄层级主表进行外连接
ON MASTER1.age_class = DATA.age_class
RIGHT OUTER JOIN TblSex MASTER2--外连接2:和性别主表进行外连接
ON MASTER2.sex_cd = DATA.sex_cd;
- 4.1.4.1. --停在第1个外连接处时:结果里包含年龄层级为2的数据
SELECT MASTER1.age_class AS age_class,
DATA.sex_cd AS sex_cd,
DATA.pop_tohoku AS pop_tohoku,
DATA.pop_kanto AS pop_kanto
FROM (SELECT age_class, sex_cd,
SUM(CASE WHEN pref_name IN (’青森’, ’秋田’)
THEN population ELSE NULL END) AS pop_tohoku,
SUM(CASE WHEN pref_name IN (’东京’, ’千叶’)
THEN population ELSE NULL END) AS pop_kanto
FROM TblPop
GROUP BY age_class, sex_cd) DATA
RIGHT OUTER JOIN TblAge MASTER1
ON MASTER1.age_class = DATA.age_class;
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4.1.5. 如果不允许进行两次外连接,那么调整成一次就可以了
-
4.1.6. 对于不支持CROSS JOIN语句的数据库,可以像FROM TblAge,TblSex这样不指定连接条件,把需要连接的表写在一起,其效果与交叉连接一样
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4.1.7. 如果先生成主表的笛卡儿积再进行连接,很容易就可以完成
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4.1.8. --使用外连接生成嵌套式表侧栏:正确的SQL语句
SELECT MASTER.age_class AS age_class,
MASTER.sex_cd AS sex_cd,
DATA.pop_tohoku AS pop_tohoku,
DATA.pop_kanto AS pop_kanto
FROM (SELECT age_class, sex_cd
FROM TblAge CROSS JOIN TblSex ) MASTER --使用交叉连接生成两张主表的笛卡儿积
LEFT OUTER JOIN
(SELECT age_class, sex_cd,
SUM(CASE WHEN pref_name IN (’青森’, ’秋田’)
THEN population ELSE NULL END) AS pop_tohoku,
SUM(CASE WHEN pref_name IN (’东京’, ’千叶’)
THEN population ELSE NULL END) AS pop_kanto
FROM TblPop
GROUP BY age_class, sex_cd) DATA
ON MASTER.age_class = DATA.age_class
AND MASTER.sex_cd = DATA.sex_cd;
5. 作为乘法运算的连接
5.1. 示例
- 5.1.1.
- 5.1.2. --解答(1):通过在连接前聚合来创建一对一的关系
SELECT I.item_no, SH.total_qty
FROM Items I LEFT OUTER JOIN
(SELECT item_no, SUM(quantity) AS total_qty
FROM SalesHistory
GROUP BY item_no) SH
ON I.item_no = SH.item_no;
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5.1.2.1. 以商品编号为主键的临时视图
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5.1.2.2. 无法利用索引优化查询
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5.1.3. --解答(2):先进行一对多的连接再聚合
SELECT I.item_no, SUM(SH.quantity) AS total_qty
FROM Items I LEFT OUTER JOIN SalesHistory SH
ON I.item_no = SH.item_no 一对多的连接
GROUP BY I.item_no;
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5.1.3.1. 代码更简洁
-
5.1.3.2. 没有使用临时视图,所以性能也会有所改善
5.2. 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
6. 全外连接
6.1. FULL OUTER JOIN
6.2. 相当于求集合的和(UNION,也称并集)
- 6.2.1. 内连接相当于求集合的积(INTERSECT,也称交集)
6.3. 示例
- 6.3.1. --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 A FULL OUTER JOIN Class_B B
ON A.id = B.id;
- 6.3.1.1. --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 A LEFT OUTER JOIN Class_B B
ON A.id = B.id
UNION
SELECT B.id AS id, A.name, B.name
FROM Class_A A RIGHT OUTER JOIN Class_B B
ON A.id = B.id;
6.3.1.1.1. 分别进行左外连接和右外连接,再把两个结果通过UNION合并起来
6.4. COALESCE是SQL的标准函数
- 6.4.1. 可以接受多个参数,功能是返回第一个非NULL的参数
6.5. 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
7. 用外连接求差集:B-A
7.1. 示例
- 7.1.1.
SELECT B.id AS id, B.name AS B_name
FROM Class_A A RIGHT OUTER JOIN Class_B B
ON A.id = B.id
WHERE A.name IS NULL;
7.2. 可以作为NOT IN和NOT EXISTS之外的另一种解法
7.3. 可能是差集运算中效率最高的
8. 用全外连接求异或集
8.1. SQL没有定义求异或集的运算符
8.2. 用集合运算符
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8.2.1. (A UNION B) EXCEPT (A INTERSECT B)
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8.2.2. (A EXCEPT B) UNION (B EXCEPT A)
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8.2.3. 性能开销大
8.3. 示例
- 8.3.1.
SELECT COALESCE(A.id, B.id) AS id,
COALESCE(A.name , B.name ) AS name
FROM Class_A A FULL OUTER JOIN Class_B B
ON A.id = B.id
WHERE A.name IS NULL
OR B.name IS NULL;
9. 用外连接进行关系除法
9.1. 示例
- 9.1.1. --用外连接进行关系除法运算:差集的应用
SELECT DISTINCT shop
FROM ShopItems SI1
WHERE NOT EXISTS
(SELECT I.item
FROM Items I LEFT OUTER JOIN ShopItems SI2
ON I.item = SI2.item
AND SI1.shop = SI2.shop
WHERE SI2.item IS NULL) ;
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