1. 建表SQL ``sql CREATE TABLE IF NOT EXISTSclass(idINT(10) UNSIGNED NOT NULL AUTO_INCREMENT,cardINT(10) UNSIGNED NOT NULL, PRIMARY KEY (id`) );

    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 book(card) ( select card from class )

    2. 2. 检索索引
    3. ```sql
    4. # 查询
    5. explain
    6. select * from class c
    7. left join book b on c.card=b.card
    9. # 创建索引
    10. create index idx_book_card    on book(card)
    1. 知识点 ```

    2. 左连接 索引需要加在右表

    3. 右链接 索引需要加在左表 ```