1.準備數據

#分類 CREATE TABLE IF NOT EXISTS `class` ( `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, `card` INT(10) UNSIGNED NOT NULL, PRIMARY KEY (`id`) ); #圖書 CREATE TABLE IF NOT EXISTS `book` ( `bookid` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, `card` INT(10) UNSIGNED NOT NULL, PRIMARY KEY (`bookid`) );INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20))); INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));

2.left join左外連接

看這個分析結果發現：在 class 表上添加的索引起的作用不大。結論- **小表驅動大表**- 小表：相對來說記錄較少的表- 大表：相對來說記錄較多的表 - 驅動方式識別- left join：左邊驅動右邊（此時把小表放在左邊）- right join：右邊驅動左邊（此時把小表放在右邊） - 加索引的方式：通常建議在大表（被驅動）的表加索引，效率提升更明顯。 - 原因：- 原因1：被驅動表加了索引之后，收益更大。從 ALL -> ref- 原因2：外連接首先讀取驅動表的全部數據，被驅動只讀取滿足連接條件的數據。

3、inner join：MySQL會自動根據表中的數據選擇驅動表

4.總結：

- 保證被驅動表的 join 字段被索引。join 字段就是作為連接條件的字段。 - left join 時，選擇小表作為驅動表（放左邊），大表作為被驅動表（放右邊） - inner join 時，mysql 會自動將小結果集的表選為驅動表。 - 子查詢盡量不要放在被驅動表，衍生表建不了索引 - 能夠直接多表關聯的盡量直接關聯，不用子查詢

總結

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