python学习之数据库操作
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一、数据库基本操作 数据库操作: 查看存储引擎: show engines; 查看数据库: show databases; 或者show create database oldboy\G 创建数据库: create database oldboy default charset=utf8; 删除数据库: drop database oldboy; 进入数据库: use oldboy; 数据表操作: 创建数据表: create table tb_emp1 ( -> id int(11), -> name varchar(25), -> deptId int(11), -> salary float -> )engine=innodb default charset=utf8; primary key: create table tb_emp2 ( -> id int(11) primary key, -> name varchar(25), -> deptId int(11), -> salary float -> )engine=innodb default charset=utf8; 多个primary key: create table tb_emp3 ( id int(11), name varchar(25), deptId int(11), salary float, primary key(name, deptId) )engine=innodb default charset=utf8; 外键约束foreign key: 主表: create table tb_dept1 ( -> id int(11) primary key, -> name varchar(22) not null, -> location varchar(50) -> )engine=innodb default charset=utf8; 从表: create table tb_emp5 ( -> id int(11) primary key, -> name varchar(25), -> deptId int(11), -> salary float, -> constraint fk_emp_dept1 foreign key(deptId) references tb_dept1(id) -> )engine=innodb default charset=utf8; 非空约束not null: create table tb_emp6 ( -> id int(11) primary key, -> name varchar(25) not null, #非空 -> deptId int(11), -> salary float, -> constraint fk_emp_dept2 foreign key(deptId) references tb_dept1(id) -> )engine=innodb default charset=utf8; 唯一性约束unique,要求该列唯一，允许为空，但只能有一个值为空: create table tb_dept2 ( -> id int(11) primary key, -> name varchar(22) unique, -> location varchar(50) -> )engine=innodb default charset=utf8; 默认值default: create table tb_emp7 ( -> id int(11) primary key, -> name varchar(25) not null, -> deptId int(11) default 1111, -> salary float, -> constraint fk_emp_dept3 foreign key(deptId) references tb_dept1(id) -> )engine=innodb default charset=utf8; 自增主键auto_increment: create table tb_emp8 ( -> id int(11) primary key auto_increment, -> name varchar(25) not null, -> deptId int(11), -> salary float, -> constraint fk_emp_dept5 foreign key(deptId) references tb_dept1(id) -> )engine=innodb default charset=utf8; 插入tb_emp8三条数据: insert into tb_emp8(name,salary) values(‘Lucy‘, 1000),(‘lura‘, 1200),(‘Kevin‘,1500); id自增 查看表结构: desc tb_emp8; 或者show create table tb_emp8\G 修改数据表alter 修改表名: alter table tb_dept2 rename tb_deptment3; 修改字段类型: alter table tb_dept1 modify name varchar(30); 修改字段名: alter table tb_dept1 change location loc varchar(50); 添加字段: alter table tb_dept1 add managerId int(10); 添加有约束条件的字段: alter table tb_dept1 add column1 varchar(12) not null; 在某个位置添加字段: alter table tb_dept1 add column2 int(11) first; 在某个字段后面添加新字段: alter table tb_dept1 add column3 int(11) after name; 删除字段: alter table tb_dept1 drop column2; 修改表的存储引擎: alter table tb_deptment3 engine=MyISAM; 删除外键约束: alter table tb_emp9 drop foreign key fk_emp_dept; 删除数据表: drop table if exists tb_dept2; 删除外键关联的主表，需要首先取消外键关联，否则删除主表失败 数据类型 整形: TINYINT 1个字节 2**8 - 1=255个值 整形: smallint 2个字节 整形: int 4个字节 整形: bigint 8个字节 浮点数 单精度: float 4个字节 双精度: double 8个字节 decimal: 不固定，一般用于财务系统 日期时间类型 year: 1个字节,格式 YYYY time: 3个字节,格式 HH:MM:SS date: 3个字节,格式 YYYY-MM-DD datetime: 8个字节，格式 YYYY-MM-DD HH:MM:SS timestamp: 4个字节，格式YYYY-MM-DD HH:MM:SS 字符串类型 char(n): 固定长度字符串 #浪费内存，但查询速度快 varchar(n): 非固定长度字符串 #节省内存，但查询速度慢 text: 存放文本 longtext: 存放大数据文本 between .. and .. 关键字使用 select 4 between 4 and 6, 4 between 4 and 6, 12 between 9 and 10; in, not in 关键字使用 select 2 in(1,3,5,‘thks‘), ‘thks‘ in(1,3,5,‘thks‘); like用来匹配字符串 ‘%‘: 匹配任何数目的字符 ‘_‘: 只能匹配一个字符 select查询数据 create table fruits ( -> f_id char(10) not null, -> s_id int not null, -> f_name char(255) not null, -> f_price decimal(8,2) not null, -> primary key(f_id) -> ) engine -> ) engine=innodb =utf8; 插入字段: insert into fruits(f_id,s_id,f_name,f_price) values -> (‘a1‘,101,‘apple‘,5.2), -> (‘b1‘,102,‘blackberry‘,10.2), -> (‘bs1‘,102,‘orange‘,11.2), -> (‘bs2‘,105,‘melon‘,8.2), -> (‘t1‘,102,‘banana‘,10.3), -> (‘t2‘,102,‘grape‘,5.3), -> (‘o2‘,103,‘coconut‘,9.2), -> (‘c0‘,101,‘cherry‘,3.2), -> (‘a2‘,103,‘apricot‘,2.2), -> (‘l2‘,104,‘lemon‘,6.4), -> (‘b2‘,104,‘lemon‘,7.6), -> (‘m1‘,106,‘mango‘,15.6), -> (‘m2‘,105,‘xbabay‘,2.6), -> (‘t4‘,107,‘xbababa‘,3.6), -> (‘m3‘,105,‘xxtt‘,11.6), -> (‘b5‘,107,‘xxxx‘,3.6); 单表查询: 查询表: select f_id,f_name from fruits; 查询条件where: select f_id,f_name from fruits where f_price=10.2; #等号 = select * from fruits where f_price < 10; #小于 select * from fruits where s_id in(101,102) order by f_name (desc); #in关键字,按f_name排序，desc降序，asc升序 select * from fruits where f_price between 2.00 and 10.20; #between and select * from fruits where f_name like ‘b%‘; #like关键字匹配, %匹配任何多个字符 select * from fruits where f_name like ‘_____y‘; #_匹配任意一个字符 select * from fruits where s_id=‘101‘ and f_price >=5; #and多条件匹配 select * from fruits where s_id=‘101‘ or s_id=102; #or多条件匹配 order by查询结果排序 select * from fruits order by f_name; select * from fruits order by f_price desc; #desc倒叙排列 group by分组 select s_id,count(*) as Total from fruits group by s_id; #根据s_id分组,s_id相同的数量 select s_id,count(*) as Total from fruits group by s_id having count(f_name) > 1; #having后面加上查询条件 limit限制查询的数量 select * from fruits limit 4; #查询四条 select * from fruits limit 4,3; #索引为4，从第五条开始返回3条 inner join 内连接,返回两表中都有的记录 create table suppliers ( -> s_id int(11) not null auto_increment primary key, -> s_name char(50) not null, -> s_city char(50), -> s_zip char(10), -> s_call char(50) not null -> )engine=innodb default charset=utf8; 以下操作是fruits和suppliers关联 select suppliers.s_id, s_name, f_name, f_price from fruits inner join suppliers on -> fruits.s_id = suppliers.s_id; #on后面是条件查询， left join 返回包括左表中的所有记录和右表连接字段的所有记录 select s_name,f_price from fruits left join suppliers on fruits.s_id = suppliers.s_id; 子查询: select s_id, f_name from fruits where s_id=(select s1.s_id from suppliers as s1 where s1.s_city=‘Tianjin‘); union合并查询结果并去重 union all合并查询不去重 select s_id ,f_name,f_price from fruits where f_price < 9.0 union all select s_id,f_name,f_price from fruits where s_id in(101,103);
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