MySQL学习笔记 2

MySQL + 学习笔记2
创建数据库：create database
查询所有的数据库：show databases;
选中数据库：use 数据库名；
查询当前所在数据库：select database();
– 单字锻主键约束
创建表单：
CREATE TABLE emp
(
id INT PRIMARY KEY,
name VARCHAR(20),
dept_id INT,
salary FLOAT
);
– 插入数据，验证主键
INSERT INTO emp VALUES(1,‘姓名’,4,5000);
– 单字锻主键约束
CREATE TABLE emp_1
(
id INT,
name VARCHAR(20),
dept_id INT,
salary FLOAT,
PRIMARY KEY(id)
);

– 多字锻联合主键约束
CREATE TABLE emp_2
(
id INT,
name VARCHAR(20),
dept_id INT,
salary FLOAT,
PRIMARY KEY(id,name)
);
– 插入数据，验证主键
INSERT INTO emp_2 VALUES(1,‘姓名’,4,5000);
INSERT INTO emp_2 VALUES(1,‘姓名’,4,5000);
– 外键约束
CREATE TABLE dept
(
id INT PRIMARY KEY,
name VARCHAR(20),
location VARCHAR(50)
)ENGINE = InnoDB;
CREATE TABLE emp_3
(
id INT PRIMARY KEY,
name VARCHAR(20),
dept_id INT,
salary FLOAT,
CONSTRAINT fk_dept FOREIGN KEY(dept_id) REFERENCES dept(id)
– REFERENCES dept(id)
)ENGINE = InnoDB;
INSERT INTO emp_3 VALUES(1,‘姓名’,4,5000);
INSERT INTO dept VALUES(4,‘大数据’,‘广州’);
INSERT INTO emp_3 VALUES(1,‘姓名’,4,5000);

– INSERT INTO dept(id,name,dept_id) VALUES(1,‘姓名’,1,5000);
– INSERT INTO emp_3(id,name,dept_id) VALUES(1,‘姓名’,1,5000);
– 非空约束
CREATE TABLE emp_4
(
id INT PRIMARY KEY,
name VARCHAR(20)NOT NULL,
dept_id INT,
salary FLOAT
);
---- 插入一条姓名null的语句
INSERT INTO emp_4 VALUES(1,’’,4,5000);
INSERT INTO emp_4 VALUES(1,NULL,4,5000);

– 唯一姓约束
CREATE TABLE emp_5
(
id INT PRIMARY KEY,
name VARCHAR(20)UNIQUE,
dept_id INT,
salary FLOAT DEFAULT 2000
);
– 插入一条姓名null的语句
INSERT INTO emp_5 VALUES(1,‘你好’,4,5000);
INSERT INTO emp_5 VALUES(2,NULL,4,5000);
INSERT INTO emp_5 VALUES(3,’’,4,5000);
– 默认约束
CREATE TABLE emp_6
(
id INT PRIMARY KEY,
name VARCHAR(20)NOT NULL,
dept_id INT,
salary FLOAT DEFAULT 2000
);
INSERT INTO emp_6(id,name,dept_id) VALUES(3,‘你好’,4);

– 属性值自动增长约束
CREATE TABLE emp_7
(
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(20),
dept_id INT,
salary FLOAT
);
INSERT INTO emp_7(name,dept_id) VALUES(‘你好’,4);
– 查看表单
SELECT * FROM emp_*;
默认约束；DEFAULT;
唯一性约束；UNIQUE;
自动增长约束;AUTO_INCREMENT;