一、DCL
(1) 查询用户
SELECT * FROM mysql.USER;
(2) 创建用户
创建用户 chenMingJiu, 只能够在当前主机localhost访问, 密码123456
CREATE USER 'chenMingJiu' @'localhost' IDENTIFIED BY '123456';
创建用户zgq, 可以在任意主机访问该数据库, 密码123456
CREATE USER 'zgq' @'%' IDENTIFIED BY '123456';
修改用户zgq的访问密码为1234
ALTER USER 'zgq' @'%' IDENTIFIED WITH mysql_native_password BY '1234';
删除 zgq@% 用户
DROP USER 'zgq' @'%';
二、DCL 权限控制
(1) 查询权限
show grants for 'chenMingJiu'@'localhost';
(2) 授予权限
grant all on study_mysql.* to 'chenMingJiu'@'localhost';
(3) 撤销权限
revoke all on study_mysql.* from 'chenMingJiu'@'localhost';
三、函数
(1) 字符串函数
SELECT concat( 'I', 'Love', 'You', '!' ) '字符串拼接'; SELECT lower( 'LOVE' ) '转换为小写'; SELECT upper( 'love' ) '转换为大写'; SELECT lpad( '55', 6, '8' ) '左填充'; SELECT rpad( '55', 6, '8' ) '右填充'; SELECT trim( ' Love You ' ) '去除首尾空格'; SELECT substring( 'ILoveYou', 2, 4 ) '截取字串';
UPDATE emp SET workno = lpad( workno, 5, '0' );
(2) 数值函数
select ceil(1.09) '向上取整, 结果是: 2'; select floor(2.999) '向下取整, 结果是: 2'; select mod(5, 3) '取余数, 结果是: 2'; select rand() '随机0到1之间的浮点数'; select round(1.289, 2) '四舍不入, 结果是: 1.29';
SELECT lpad( round( rand( ) * 1000000, 0 ), 6, '0' );
(3) 日期函数
select CURRENT_DATE() '年月日（如: 2020-05-20）'; select curdate() 'yyyy-mm-dd'; select CURRENT_TIME() '时分秒'; select curtime() 'hh::MM:ss'; select NOW() '年月日 时分秒';
SELECT YEAR(NOW()); SELECT MONTH(NOW()); SELECT DAY(NOW());
select DATE_ADD(now(),INTERVAL 7 DAY); select DATE_ADD(now(),INTERVAL 1 YEAR); select DATE_ADD(now(),INTERVAL 2 MONTH); select DATEDIFF(curdate(), '1999-09-30') '计算两个日期相差多少天（第一个参数的日期减去第二个参数的日期）';
SELECT `name`, datediff( curdate( ), entrydate ) 'entrydays' FROM emp ORDER BY entrydays DESC;
(4) 流程控制函数
select IFNULL('121','211'); select IFNULL('','211') '结果为空字符串'; select IFNULL(null,'me');
SELECT `name`, ( CASE workaddress WHEN '北京' THEN concat( workaddress, '(一线城市)' ) WHEN '上海' THEN concat( workaddress, '(一线城市)' ) ELSE concat( workaddress, '(二线城市)' ) END ) AS '工作地址' FROM emp;
CREATE TABLE student ( id INT COMMENT 'ID', `name` VARCHAR ( 20 ) COMMENT '姓名', math INT COMMENT '数学', english INT COMMENT '英语', chinese INT COMMENT '语文' ) COMMENT '学员成绩表'; INSERT INTO student ( id, NAME, math, english, chinese ) VALUES ( 1, 'Tom', 67, 88, 95 ), ( 2, 'Rose', 23, 66, 90 ), ( 3, 'Jack', 56, 98, 76 );
需求：统计 student 表中每个学生的成绩。展示规则如下：
① 分数大于等于85，在分数后面加上“优秀”；
② 分数大于等于60，在分数后面加上“及格”；
③ 否则，在分数后面加上“不及格”
SELECT id, `name`, ( CASE WHEN math >= 85 THEN concat( math, '(优秀)' ) WHEN math >= 60 THEN concat( math, '(及格)' ) ELSE concat( math, '(不及格)' ) END ) '数学', ( CASE WHEN english >= 85 THEN concat( english, '(优秀)' ) WHEN english >= 60 THEN concat( english, '(及格)' ) ELSE concat( english, '(不及格)' ) END ) '英语', ( CASE WHEN chinese >= 85 THEN concat( chinese, '(优秀)' ) WHEN chinese >= 60 THEN concat( chinese, '(及格)' ) ELSE concat( chinese, '(不及格)' ) END ) '语文' FROM student;
四、约束
(1) 约束概述
(2) 使用约束
CREATE TABLE people ( id INT AUTO_INCREMENT PRIMARY KEY COMMENT 'ID唯一标识', `name` VARCHAR ( 10 ) NOT NULL UNIQUE COMMENT '姓名', age INT COMMENT '年龄', `status` CHAR ( 1 ) DEFAULT '1' COMMENT '状态', gender CHAR ( 1 ) COMMENT '性别' );
(3) 外键约束
CREATE TABLE dept ( id INT auto_increment COMMENT 'ID' PRIMARY KEY, NAME VARCHAR ( 50 ) NOT NULL COMMENT '部门名称' ) COMMENT '部门表'; INSERT INTO dept ( id, NAME ) VALUES ( 1, '研发部' ), ( 2, '市场部' ), ( 3, '财务部' ), ( 4, '销售部' ), ( 5, '总经办' ); CREATE TABLE emp2 ( id INT auto_increment COMMENT 'ID' PRIMARY KEY, NAME VARCHAR ( 50 ) NOT NULL COMMENT '姓名', age INT COMMENT '年龄', job VARCHAR ( 20 ) COMMENT '职位', salary INT COMMENT '薪资', entrydate date COMMENT '入职时间', managerid INT COMMENT '直属领导ID', dept_id INT COMMENT '部门ID' ) COMMENT '员工表'; INSERT INTO emp2 ( id, NAME, age, job, salary, entrydate, managerid, dept_id ) VALUES ( 1, '金庸', 66, '总裁', 20000, '2000-01-01', NULL, 5 ), ( 2, '张无忌', 20, '项目经理', 12500, '2005-12-05', 1, 1 ), ( 3, '杨逍', 33, '开发', 8400, '2000-11-03', 2, 1 ), ( 4, '韦一笑', 48, '开 发', 11000, '2002-02-05', 2, 1 ), ( 5, '常遇春', 43, '开发', 10500, '2004-09-07', 3, 1 ), ( 6, '小昭', 19, '程 序员鼓励师', 6600, '2004-10-12', 2, 1 );
添加外键：
