1、%和_的区别：

一个“_”代表一个字符

“%”不区分

SELECT * FROM applic_sessions WHERE suggestion LIKE '%超%';

SELECT * FROM applic_sessions WHERE suggestion LIKE '_超_';

SELECT * FROM applic_sessions WHERE suggestion LIKE '____超_';

2、值与其他值相加仍然为null

3、为查询出来的数据创建视图，并取前三条记录：

CREATE VIEW applic_X AS ( SELECT * FROM applic_sessions a WHERE a.platform = 'web' ORDER BY a.start_time ASC);

SELECT *  FROM  applic_X LIMIT 0,3;

4、DROP TABLES IF EXISTS applic_X; 若存在表applic_X，即删除该表。