7-1　表的加減法

什麼是集合運算

集合運算，就是對滿足同一規則的記錄進行的加減等四則運算

表的加法——UNION

集合運算子會除去重複的記錄。

-- 建立表Product2（商品2）
CREATE TABLE Product2
(product_id CHAR(4) NOT NULL,
product_name VARCHAR(100) NOT NULL,
product_type VARCHAR(32) NOT NULL,
sale_price INTEGER ,
purchase_price INTEGER ,
regist_date DATE ,
PRIMARY KEY (product_id))
DEFAULT CHARSET=utf8;

-- 將資料插入到表Product2（商品2）中
START TRANSACTION; 
INSERT INTO Product2 VALUES ('0001', 'T恤衫' ,'衣服', 1000, 500, '2008-09-20');
INSERT INTO Product2 VALUES ('0002', '打孔器', '辦公用品', 500, 320, '2009-09-11');
INSERT INTO Product2 VALUES ('0003', '運動T恤', '衣服', 4000, 2800, NULL);
INSERT INTO Product2 VALUES ('0009', '手套', '衣服', 800, 500, NULL);
INSERT INTO Product2 VALUES ('0010', '水壺', '廚房用具', 2000, 1700, '2009-09-20');
COMMIT;

-- 使用UNION對錶進行加法運算
SELECT product_id, product_name
FROM Product
UNION
SELECT product_id, product_name
FROM Product2;
 

集合運算的注意事項

• 作為運算物件的記錄的列數必須相同

-- 列數不一致時會發生錯誤
SELECT product_id, product_name
FROM Product
UNION
SELECT product_id, product_name, sale_price
FROM Product2;

• 作為運算物件的記錄中列的型別必須一致

-- 資料型別不一致時會發生錯誤
SELECT product_id, sale_price
FROM Product
UNION
SELECT product_id, regist_date
FROM Product2;

• 可以使用任何SELECT語句，但ORDER BY子句只能在最後使用一次

-- ORDER BY子句只在最後使用一次
SELECT product_id, product_name
FROM Product
WHERE product_type = '廚房用具'
UNION
SELECT product_id, product_name
FROM Product2
WHERE product_type = '廚房用具'
ORDER BY product_id;

包含重複行的集合運算——ALL選項

在集合運算子中使用ALL選項，可以保留重複行。

-- 保留重複行
SELECT product_id, product_name
FROM Product
UNION ALL
SELECT product_id, product_name
FROM Product2;
 

7-2　聯結（以列為單位對錶進行聯結）

什麼是聯結

就是將其他表中的列新增過來，進行“新增列”的運算

內聯結——INNER-JOIN

• 內聯結要點—FROM子句

-- 進行聯結時需要在FROM子句中使用多張表。
FROM ShopProduct AS SP INNER JOIN Product AS P

• 內聯結要點--ON子句

-- 進行內聯結時必須使用ON子句，並且要書寫在FROM和WHERE之間。
ON SP.product_id = P.product_id;

• 內聯結要點-—SELECT子句

-- 使用聯結時SELECT子句中的列需要按照“<表的別名>.<列名>”的格式進行書寫。
SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price

- 將兩張表進行內聯結
SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price
FROM ShopProduct AS SP INNER JOIN Product AS P 
ON SP.product_id = P.product_id;

-- 內聯結和WHERE子句結合使用
SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price
FROM ShopProduct AS SP INNER JOIN Product AS P 
ON SP.product_id = P.product_id
WHERE SP.shop_id = '000A';

外聯結——OUTER-JOIN

• 外聯結要點-選取出單張表中全部的資訊
• 外聯結要點-每張表都是主表嗎？
  • 使用 LEFT 時 FROM 子句中寫在左側的表是主表
  • 使用 RIGHT 時右側的表是主表

-- 將兩張表進行外聯結
SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price
FROM ShopProduct AS SP RIGHT OUTER JOIN Product AS P
ON SP.product_id = P.product_id;

-- 改寫後外聯結的結果完全相同
SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price
FROM Product AS P LEFT OUTER JOIN ShopProduct AS SP 
ON SP.product_id = P.product_id;

3張以上的表的聯結

-- 建立InventoryProduct表並向其中插入資料
-- DDL ：建立表
CREATE TABLE InventoryProduct
( inventory_id CHAR(4) NOT NULL,
product_id CHAR(4) NOT NULL,
inventory_quantity INTEGER NOT NULL,
PRIMARY KEY (inventory_id, product_id));

-- DML ：插入資料
START TRANSACTION; 
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P001', '0001', 0);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P001', '0002', 120);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P001', '0003', 200);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P001', '0004', 3);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P001', '0005', 0);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P001', '0006', 99);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P001', '0007', 999);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P001', '0008', 200);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P002', '0001', 10);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P002', '0002', 25);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P002', '0003', 34);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P002', '0004', 19);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P002', '0005', 99);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P002', '0006', 0);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P002', '0007', 0);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) 
VALUES ('P002', '0008', 18);
COMMIT;

-- 對3張表進行內聯結
SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price, IP.inventory_quantity
FROM ShopProduct AS SP INNER JOIN Product AS P ON SP.product_id = P.product_id
INNER JOIN InventoryProduct AS IP ON SP.product_id = IP.product_id
WHERE IP.inventory_id = 'P001';

交叉聯結——CROSS-JOIN 笛卡兒積

-- 將兩張表進行交叉聯結
SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name
FROM ShopProduct AS SP CROSS JOIN Product AS P;

聯結的特定語法和過時語法

-- 使用過時語法的內聯結
SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price
FROM ShopProduct SP, Product P
WHERE SP.product_id = P.product_id
AND SP.shop_id = '000A';

-- DDL ：建立表
CREATE TABLE Skills
(skill VARCHAR(32),
PRIMARY KEY(skill));
CREATE TABLE EmpSkills
(emp VARCHAR(32),
skill VARCHAR(32),
PRIMARY KEY(emp, skill))
DEFAULT CHARSET=utf8;

-- DML ：插入資料
START TRANSACTION; 
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('渡來', 'Oracle');
COMMIT;

-- 選取出掌握所有3個領域的技術的員工
SELECT DISTINCT emp
FROM EmpSkills ES1
WHERE NOT EXISTS
(SELECT skill
FROM Skills
EXCEPT
SELECT skill
FROM EmpSkills ES2
WHERE EP1.emp = ES2.emp);

練習題

7.1 請說出下述 SELECT 語句的結果。

-- 使用本章中的Product表
SELECT *
FROM Product
UNION
SELECT *
FROM Product
INTERSECT
SELECT *
FROM Product
ORDER BY product_id;

會將 Product 表中的 8 行記錄原封不動地選取出來。

7.2 7-2 節的程式碼清單 7-11 中列舉的外聯結的結果中，高壓鍋和圓珠筆 2 條 記錄的商店編號（ shop_id）和商店名稱（ shop_name）都是 NULL。
請使用字串“不確定”替換其中的 NULL。期望結果如下所示。
執行結果

將商店編號和商店名稱輸出為"不確定"

SELECT COALESCE(s.shop_id, '不確定') as shop_id, 
COALESCE(s.shop_name, '不確定') as shop_name, 
p.product_id, p.product_name, p.sale_price
FROM ShopProduct s RIGHT OUTER JOIN Product p 
ON s.product_id = p.product_id;