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: