数据库学习第三天

※MySQL中的字符编码(注意，utf8中没有'-'，跟Java中不一样)SHOW VARIABLES; //查看系统变量//查询字符编码相关的系统变量SHOW VARIABLES WHERE variable_name LIKE 'character%';解决MySQK的中文乱码:character_set_client，character_set_connection和character_set_results这三者必须一致(GBK,GB2312,UTF8这三者都可以，但若采用其中一种则3个变量都要设这种)设置字符编码的系统变量:set character_set_client=utf8;  //独立设置某个变量set names utf8; //同时设置3个:client、 connection和results※其实，MySQL中的数据库、数据表 甚至 数据列 都可以有自己的编码，只是我们一般不专门设置。---从SQLyog中的更改表结构中可以查看出INSERT INTO stud VALUES(4,'湖南',20);CREATE TABLE person(  id INT,  NAME VARCHAR(30),  age INT);CREATE TABLE car(  id INT PRIMARY KEY,  //不允许重复，也不允许NULL  NAME VARCHAR(30),  NO INT UNIQUE  //不允许重复，允许NULL但最多只能一个);//除了在创建表时如上的方式直接指定主键，也可以通过修改表的方式单独创建主键：ALTER TABLE person ADD CONSTRAINT person_pk PRIMARY KEY(id);ALTER TABLE person ADD CONSTRAINT UNIQUE (NAME); //为name字段添加一个unique约束//添加和删除unique约束 --通过约束名称ALTER TABLE person ADD CONSTRAINT UNIQUE uniq_nm (NAME);DROP INDEX uniq_nm ON person;//删除主键ALTER TABLE person DROP PRIMARY KEY;//有关性别字段，比较好的表结构。DATE类型勉强用，建议不要用DATETIME。为考虑兼容性，最好用CHAR(19),VARCHAR(19)  //datetime格式： 2016-11-11 23:24:09CREATE TABLE s(  id INT,  sex CHAR(1) DEFAULT '0',  birth DATE);INSERT INTO s(id,birth) VALUES(3,'2008/8/8'); //因为sex字段设了default，所以该字段不手动赋值时，由默认值0来赋按理,date的规范格式是"yyyy-MM-dd"，但我们给其他格式如"yyyy/M/d"时，MySQL也会帮我们自动转换成规范格式。//有关性别字段的显示学习一个SQL语法： 手册-->函数和操作符-->控制流程函数-->Case-WhenSELECT id, (CASE sex WHEN '0' THEN '女' WHEN '1' THEN '男' ELSE '其它' END) AS 性别, birth FROM s; ※1无关子查询需求:查询具有同龄人的学生SELECT * FROM stud WHERE age IN(20,22,23); //过渡版："20,22,23" 写死了SELECT age FROM stud GROUP BY age HAVING COUNT(age)>=2; //这句可以动态输出:20,22,23综上:SELECT * FROM stud WHERE age IN( SELECT age FROM stud GROUP BY age HAVING COUNT(age)>=2 ) ORDER BY age ASC;※2相关子查询(子查询中用到了外面的查询结果表)需求:查询具有同龄人且年龄大于22的学生SELECT * FROM stud as s2 WHERE age IN( SELECT age FROM stud where s2.age>22 GROUP BY age HAVING COUNT(age)>=2 ) ORDER BY age ASC;select ... from ... where ... in( 子查询 )  order by ... asc(desc) select ... from ... where ... group by ... having ...※表与表之间的关系（1对1）1、CREATE TABLE person(   id INT,   NAME VARCHAR(10),   sex CHAR(1),   wife INT,   husband INT);INSERT INTO person VALUES(1,'小花','0',0,3);INSERT INTO person VALUES(2,'玉芬','0',0,4);INSERT INTO person VALUES(3,'张三','1',1,0);INSERT INTO person VALUES(4,'李四','1',2,0);INSERT INTO person VALUES(5,'王五','1',0,0);SELECT * FROM person WHERE sex='0';SELECT * FROM person WHERE sex='1';CREATE VIEW w AS SELECT * FROM person WHERE sex='0';CREATE VIEW m AS SELECT * FROM person WHERE sex='1';或CREATE VIEW w2 AS SELECT id,NAME,sex,husband FROM person WHERE sex='0';CREATE VIEW m2 AS SELECT id,NAME,sex,wife FROM person WHERE sex='1';SELECT w.name AS wn, m.name AS mn FROM w INNER JOIN m ON w.husband=m.id AND m.wife=w.id; SELECT * FROM w INNER JOIN m ON w.husband=m.id AND m.wife=w.id;