1.CREATE TABLE stud(
sno VARCHAR(18) NOT NULL PRIMARY KEY,
sname VARCHAR(15) NOT NULL,
age INT,
saddress VARCHAR(30)
);
2.ALTER TABLE stud ADD COLUMN sex CHAR;
ALTER TABLE stud DROP COLUMN sex;
3.我们以后开发时，字段名都用英文，如果显示内容时要让表头变成中文，
用别名(AS可以省略)
SELECT * FROM stud;
SELECT sno,sname,age,saddress FROM stud;
SELECT sno AS 编号,sname AS 姓名,age,saddress FROM stud;
SELECT sno 编号,sname 姓名,age 年龄,saddress 地址 FROM stud; //省略AS
4.SELECT的复杂查询: WHERE子句, BETWEEN子句, IN子句
SELECT * FROM stud WHERE age>20;
SELECT * FROM stud WHERE age>20 AND age<25;
SELECT * FROM stud WHERE age BETWEEN 20 AND 24; //[20,24]BETWEEN-AND中的值是范围
SELECT * FROM stud WHERE age=18 OR age=25;
SELECT * FROM stud WHERE age IN(18,22,25,28); //IN 中的值是离散的
SELECT * FROM stud WHERE age NOT IN(18,22,25,28); //非NOT
5.模糊查询LIKE '%'通配所有字符 '_'通配单字符
SELECT * FROM stud WHERE sname LIKE '张%'; //姓张的学生
SELECT * FROM stud WHERE sname LIKE '张_'; //姓张,名为单字的学生
SELECT * FROM stud WHERE sname LIKE '张__'; //姓张,名为双字的学生
SELECT * FROM stud WHERE sname LIKE '%张%'; //姓名中含“张”字的学生
SELECT * FROM stud WHERE sname LIKE '%张%' AND age<24;
6.INSERT INTO stud(sno,sname,saddress) VALUES('1008','赵敏','新疆');
INSERT INTO stud(sno,sname,saddress) VALUES('1009','张无忌','昆明');
INSERT INTO stud(sno,sname,saddress) VALUES('1010','张三丰','五台山');
7.判断值为NULL的字段，不能用"="，而应用IS
//查出所有年龄为空的学生，并把他们的年龄设为18
UPDATE stud SET age=18 WHERE age=NULL; //bug版
UPDATE stud SET age=18 WHERE age IS NULL;
8.注意字符串为空和该字段没有设置值是不同的，判断方式不一样
INSERT INTO stud(sno,sname,age) VALUES('1011','鹰王',40);
SELECT * FROM stud WHERE saddress IS NULL; //未设置值
SELECT * FROM stud WHERE saddress=''; //空串
9.注意SQL中的"+"没有字符串连接功能
UPDATE stud SET saddress='湖南益阳' WHERE saddress='益阳';
UPDATE stud SET saddress='湖南'+saddress WHERE saddress='益阳'; //有bug，设置结果为0
10.注意,通配符只能在LIKE中使用
DELETE FROM stud WHERE sname='李四_'; //无法删除名为“李四a”的学生
11.句型:
select 字段名 from 表名 where 条件1 and 条件2 or 条件3
update 表名 set 字段名1=值 [，字段名2=值] where 子句[in 子句] [between子句]
delete from 表名 where 子句
create table 表名(
字段名 类型 primary key，
字段名 类型 unique
)
12.create view 视图名 as select 子句
视图 视图是虚拟表，该表在物理上不存在的，创建之后，我们写代码时可以当作跟表一样(如果进行数据修改删除，那么它是会
及时更新到物理表中)
CREATE VIEW oldersView AS SELECT * FROM stud WHERE age>=20;
SELECT * FROM oldersView;
SELECT sname,age,saddress FROM oldersView WHERE saddress='益阳';
UPDATE oldersview SET age =age+1;
DELETE FROM oldersview WHERE saddress='新疆';
视图的生命周期，创建之后会一直存在，要我们手动删除才消失。虽然物理上不是独立的，但它是由一段代码执行生成的，这
段代码一直存在，除非删除视图。
DROP VIEW 视图名; //删除视图
13.聚合函数
SELECT COUNT(*) FROM stud; //输出行数，一般要用别名
SELECT COUNT(*) AS rows FROM stud;
SELECT COUNT(1) AS ROWS FROM stud;//功能同上，据说效率更高
SELECT COUNT(saddress) AS sumNum FROM stud; //统计该字段值非NULL的数量
SELECT AVG(age) AS averageAge FROM stud;
SELECT AVG(age) AS averageAge FROM stud WHERE age<20;
SELECT SUM(age) AS sumAge FROM stud WHERE age<20;
SELECT COUNT(1) AS num FROM stud WHERE age<20;
SELECT MAX(age) AS maxAge FROM stud;
SELECT MIN(age) AS minAge FROM stud;
把各种年龄段的值输出---注意，DISTINCT不要放在最后。
SELECT DISTINCT age FROM stud;
选择年龄最小的那些学生的名字和年龄
SELECT sname,age FROM stud WHERE age=18; //写死了
SELECT sname,age FROM stud WHERE age=(SELECT MIN(age) FROM stud); //写活了
选择年龄最小和最大那些学生的名字和年龄
SELECT sname,age FROM stud WHERE age=17 or age=42;
SELECT sname,age FROM stud WHERE age=(SELECT MIN(age) FROM stud)
or age=(SELECT MAX(age) FROM stud);
SELECT sname,age FROM stud WHERE age IN ( SELECT MIN(age),MAX(age) FROM stud ); //WA!!!!!!
SELECT sname,age FROM stud WHERE age IN ( (SELECT MIN(age) FROM stud) ,(SELECT MAX(age) FROM stud) );
14.排序ORDER BY order by
SELECT * FROM stud ORDER BY age; //默认升序
SELECT * FROM stud ORDER BY age ASC;//升序
SELECT * FROM stud ORDER BY age DESC;//降序
15.exists()
由括号中的子查询返回结果集决定该方法的调用结果，如果子查询结果集为空则函数返回false,否则返回true
SELECT * FROM stud WHERE EXISTS( SELECT * FROM stud WHERE saddress IS NOT NULL);
//只要子查询结果非空则所有信息都输出，否则一条不输出
select * from stud where saddress is NULL;
16.group by
按saddress分组，把各组的年龄总和查询出来
SELECT saddress, sum(age) AS 年龄总和 FROM stud GROUP BY saddress;
按saddress分组，把各组的平均年龄输出
SELECT saddress, AVG(age) AS 平均年龄 FROM stud GROUP BY saddress;
按saddress分组，把平均年龄大于22的那些地方和平均年龄输出
SELECT saddress, AVG(age) AS 平均年龄 FROM stud GROUP BY saddress HAVING AVG(age)>=22 ;
句型(固定搭配):
order by子句要放在最后, distinct 放前面
1. select [distinct] ... from ... where ... order by ...
注意,group by子句中的条件要用having(不能用where)
2. select ... from ... group by ... having ....
字符串处理函数
1.SELECT * FROM stud WHERE saddress='aaa';
2.SELECT * FROM stud WHERE TRIM(saddress)='aaa';//去掉空格
3.SELECT * FROM stud WHERE LEFT(saddress,2)='湖南';//取左边两个字符
4.SELECT REVERSE(sname)AS 反名 FROM stud;//反转
5.SELECT UPPER(sname)AS 大写 FROM stud;//大写
6.SELECT CONCAT(sname,sno,saddress)AS 拼接串 FROM stud;//拼接
7.SELECT INSTR(sname,'a')AS 子串位置, sname FROM stud;//取位置