1.别名

给查询的字段改个名字，只对查询结果有影响，对表结构以及数据不会产生任何影响

先创建一张表并添加数据

CREATE DATABASE mahaiwuji;USE mahaiwuji;CREATE TABLE student (   sid INT PRIMARY KEY AUTO_INCREMENT,    sname VARCHAR (32),   sage INT  ) ENGINE = INNODB DEFAULT CHARSET = utf8;INSERT INTO student(sname,sage) VALUES ('码海无际1',18);INSERT INTO student(sname,sage) VALUES ('码海无际2',19);INSERT INTO student(sname,sage) VALUES ('码海无际3',20);

把sname改为studentname

SELECT sid,sname AS studentname,sage FROM student;

再把sage改为studentage

SELECT sid,sname AS studentname,sage AS studentage FROM student;

AS可以省略

SELECT sid,sname studentname,sage studentage FROM student;

where 后面不能用别名当筛选条件

-- okSELECT sid,sname studentname,sage studentage FROM student WHERE sage=18;-- errorSELECT sid,sname studentname,sage studentage FROM student WHERE studentage=18;

注释的内容不会被执行

单行注释

-- 要注释的内容，注意--后面有个空格

# 要注释的内容，#后面可以没有空格，但推荐加一个

-- INSERT INTO student(sname,sage) VALUES ('码海无际1',18);# INSERT INTO student(sname,sage) VALUES ('码海无际2',19);

-- INSERT INTO student(sname,sage) VALUES ('码海无际1',18);# INSERT INTO student(sname,sage) VALUES ('码海无际2',19);

多行注释

/* 要注释的内容 */

/*INSERT INTO student(sname,sage) VALUES ('码海无际1',18);INSERT INTO student(sname,sage) VALUES ('码海无际2',19);INSERT INTO student(sname,sage) VALUES ('码海无际3',20);*/