法則5-1

表中存儲的是實際數據，而視圖中保存的是從表中取出數據所使用的SELECT語句。

法則5-2

應該將經常使用的SELECT語句做成視圖。

創建視圖的方法

/*

CREATE VIEW 視圖名稱(<視圖列名1>, <視圖列名2>, ......) AS <SELECT語句>

*/

DELETE FROM shohin;

SELECT * FROM shohin;

INSERT INTO shohin VALUES (‘0001‘, ‘T恤衫‘, ‘衣服‘, 1000, 500, ‘2009-09-20‘);

INSERT INTO shohin VALUES (‘0002‘, ‘打孔器‘, ‘辦公用品‘, 500, 300, ‘2009-09-11‘);

INSERT INTO shohin VALUES (‘0003‘, ‘運動T恤‘, ‘衣服‘, 4000, 2800, NULL);

INSERT INTO shohin VALUES (‘0004‘, ‘菜刀‘, ‘廚房用具‘, 3000, 2800, ‘2009-09-20‘);

INSERT INTO shohin VALUES (‘0005‘, ‘高壓鍋‘, ‘廚房用具‘, 6800, 5000, ‘2009-01-15‘);

INSERT INTO shohin VALUES (‘0006‘, ‘叉子‘, ‘廚房用具‘, 500, NULL, ‘2009-09-20‘);

INSERT INTO shohin VALUES (‘0007‘, ‘擦菜飯‘, ‘廚房用具‘, 880, 790, ‘2008-04-28‘);

INSERT INTO shohin VALUES (‘0008‘, ‘圓珠筆‘, ‘辦公用品‘, 100, NULL, ‘2009-11-11‘);

CREATE VIEW shohinSum (shohin_bunrui, cnt_shohin) AS

SELECT shohin_bunrui, COUNT(*) FROM shohin GROUP BY shohin_bunrui;

SELECT * FROM shohinSum;

使用視圖的查詢

CREATE VIEW shohinSumJim (shohin_bunrui, cnt_shohin) AS SELECT shohin_bunrui, cnt_shohin

FROM shohinSum WHERE shohin_bunrui = ‘辦公用品‘;

法則5-3

應該避免在視圖的基礎上創建視圖

法則5-4

定義視圖時不要使用ORDER BY子句。

法則5-5

視圖和表需要同時進行更新，因此通過聚合得到的視圖無法進行更新。

刪除視圖

-- DROP VIEW 視圖名稱(<視圖列名1>, <視圖列名2>, ......);

DROP VIEW shohinSum;

子查詢和視圖

CREATE VIEW shohinSum (shohin_bunrui, cnt_shohin) AS SELECT shohin_bunrui, COUNT(*) FROM shohin GROUP BY shohin_bunrui;

-- 下面代碼也能實現此功能 這裏的內層 SELECT子句為 FROM子句中的select子句

SELECT shohin_bunrui, cnt_shohin FROM (SELECT shohin_bunrui, COUNT(*) AS cnt_shohin FROM shohin GROUP BY shohin_bunrui) AS shohinSum;

/*

首先執行from子句中的select語句（子查詢）

SELECT shohin_bunrui, COUNT(*) AS cnt_shohin FROM shohin GROUP BY shohin_bunrui;

根據上面的結果執行外層的select語句

SELECT shohin_bunrui, cnt_shohin FROM shohinSum;

*/

SELECT shohin_bunrui, cnt_shohin FROM shohinSum;

SELECT shohin_bunrui, cnt_shohin FROM (SELECT * FROM (SELECT shohin_bunrui, COUNT(*) AS cnt_shohin FROM shohin GROUP BY shohin_bunrui) AS shohinSum WHERE cnt_shohin = 4) AS shohinSum2;

法則5-6

子查詢作為內層查詢會首先執行。

標量子查詢（scalar subquery）

-- 在where子句中使用標量子查詢

-- 在where子句中不能使用聚合函數

-- 雖然這樣的select語句看上去能夠滿足我們的要求，但是由於在where子句中不能使用聚合函數，因此這樣的select語句是錯誤的。

SELECT shohin_id, shohin_mei, hanbai_tanka FROM shohin WHERE hanbai_tanka > AVG(hanbai_tanka);

-- 可以使用下面的代碼實現

-- 計算平均銷售單價的標量子查詢

SELECT AVG(hanbai_tanka) FROM shohin;

SELECT shohin_id, shohin_mei, hanbai_tanka FROM shohin WHERE hanbai_tanka > (SELECT AVG(hanbai_tanka) FROM shohin);

-- 在SELECT子句中使用標量子查詢

SELECT shohin_id, shohin_mei, hanbai_tanka, (SELECT AVG(hanbai_tanka) FROM shohin) AS avg_tanka FROM shohin;

SELECT shohin_bunrui, AVG(hanbai_tanka) FROM shohin GROUP BY shohin_bunrui;

-- 在HAVING子句中使用標量子查詢

SELECT shohin_bunrui, AVG(hanbai_tanka) FROM shohin GROUP BY shohin_bunrui HAVING AVG(hanbai_tanka) > (SELECT AVG(hanbai_tanka) FROM shohin);

法則5-7

標量子查詢就是返回單一值的子查詢

普通的子查詢和關聯子查詢的區別

-- 普通的子查詢和關聯子查詢的區別

SELECT AVG(hanbai_tanka) FROM shohin GROUP BY shohin_bunrui;

-- 發生錯誤的子查詢

SELECT shohin_id, shohin_mei, hanbai_tanka FROM shohin WHERE hanbai_tanka > (SELECT AVG(hanbai_tanka)

FROM shohin GROUP BY shohin_bunrui);

-- 正確的關聯子查詢書寫方法

SELECT shohin_id, shohin_mei, hanbai_tanka FROM shohin AS S1 WHERE hanbai_tanka >

(SELECT AVG(hanbai_tanka) FROM shohin AS S2 WHERE S1.shohin_bunrui = S2.shohin_bunrui GROUP BY shohin_bunrui);

-- 錯誤的關聯子查詢書寫方法：將關聯條件移到子查詢之外 該書寫方法究竟違法了什麽規則呢？那就是關聯名稱的作用域。也就是說關聯名稱存在一個有效作用域的限制。

SELECT shohin_bunrui, shohin_mei, hanbai_tanka FROM shohin AS S1 WHERE S1.shohin_bunrui = S2.shohin_bunrui AND hanbai_tanka >

(SELECT AVG(hanbai_tanka) FROM shohin AS S2 GROUP BY shohin_bunrui);

-- 正確的關聯子查詢書寫方法

SELECT shohin_bunrui, shohin_mei, hanbai_tanka FROM shohin AS S1 WHERE hanbai_tanka >

(SELECT AVG(hanbai_tanka) FROM shohin AS S2 WHERE S1.shohin_bunrui = S2.shohin_bunrui GROUP BY shohin_bunrui);

法則5-8

在細分的組內進行比較時，需要使用關聯子查詢。