Advanced Series 5 external SQL connection usage

EDITORIAL

SQL itself is used as a data extraction tool appears, use SQL to generate a variety of customized reports and customized reports are not non-SQL features original purpose , but that does not mean that SQL can not achieve these functions.

Rows and columns with an external connection (1) (row → column): Production cross table

-- 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;

Rows and columns with an external connection (2) (column → row): duplicates are summarized in a

-- 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);

Production nested column in a front side cross table

/* 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;

As the connection multiplications

-- 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

Full outer join

Angle set for understanding the type of connection

• Left outer join LEFT OUTER JOIN
• A right outer join RIGHT OUTER JOIN
• Full outer connecting 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 (or INTERSECT) corresponds to an intersection, UNION (LEFT JOIN AND RIGHT JOIN, and then, or FULL JOIN) corresponds to the set and

Connection with an external set operation

Outer connecting differencing sets: AB

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;

Outer connecting differencing sets: BA

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;

Full divergent or connected with an outer set of

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;

Section Summary

• SQL language is not used to generate the report, not recommended format conversion Qi
• If necessary, can be considered external connections and CASE expressions to solve problems
• Sidebar generating nested table, if the master table into Mr. Cartesian product then is connected, it is easy to complete
• From the row numbers, it can be seen as connected to the multiplication table. Thus, when many relationship between a table, the connection does not increase the number of trailing
• Thought and set operations like external connection, connection may be implemented using various sets the outer operation

Exercises

-- 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);