一、日期时间函数

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1.1 简单使用

select current_date();

select current_time();

select current_timestamp();

select date(current_timestamp());

select date_add(current_timestamp(),interval 10 day);select date_add(current_timestamp(),interval 10 minute);

select date_sub(current_timestamp(),interval 10 second);select date_sub(current_timestamp(),interval 10 year);

select datediff('2000-1-1',current_date());select datediff('2050-1-1',current_date());

select now();

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1.2 案例实操

创建一张表，记录生日。

create table if not exists birthday(id int primary key auto_increment,birthday date not null
);

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二、字符串函数

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2.1 简单使用

select charset('LOVE');select charset('中国');

select concat('LOVE',' ','中国');

select instr('I love you,China!','China');select instr('I love you,China!','me');

select ucase('I love you,China!');select lcase('I love you,China!');

select left('I love you,China!',6);select right('I love you,China!',6);

select length('I love you,China!');select length('5201314');

select replace('I love you,China!','I','We');

select strcmp('Love','LOVE');select strcmp('Love','I');

select substring('I love you,China!',3,8);select substring('I love you,China!',11,8);

select ltrim('      Love       ') res ;select rtrim('      Love       ') res ;select trim('      Love       ') res ;

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2.2 案例实践

DROP database IF EXISTS `scott`;
CREATE database IF NOT EXISTS `scott` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;USE `scott`;DROP TABLE IF EXISTS `emp`;
CREATE TABLE `emp` (`empno` int(6) unsigned zerofill NOT NULL COMMENT '雇员编号',`ename` varchar(10) DEFAULT NULL COMMENT '雇员姓名',`job` varchar(9) DEFAULT NULL COMMENT '雇员职位',`mgr` int(4) unsigned zerofill DEFAULT NULL COMMENT '雇员领导编号',`hiredate` datetime DEFAULT NULL COMMENT '雇佣时间',`sal` decimal(7,2) DEFAULT NULL COMMENT '工资月薪',`comm` decimal(7,2) DEFAULT NULL COMMENT '奖金',`deptno` int(2) unsigned zerofill DEFAULT NULL COMMENT '部门编号'
);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7369, 'SMITH', 'CLERK', 7902, '1980-12-17', 800, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20', 1600, 300, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7521, 'WARD', 'SALESMAN', 7698, '1981-02-22', 1250, 500, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7566, 'JONES', 'MANAGER', 7839, '1981-04-02', 2975, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28', 1250, 1400, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7698, 'BLAKE', 'MANAGER', 7839, '1981-05-01', 2850, null, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7782, 'CLARK', 'MANAGER', 7839, '1981-06-09', 2450, null, 10);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7788, 'SCOTT', 'ANALYST', 7566, '1987-04-19', 3000, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7839, 'KING', 'PRESIDENT', null, '1981-11-17', 5000, null, 10);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7844, 'TURNER', 'SALESMAN', 7698,'1981-09-08', 1500, 0, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7876, 'ADAMS', 'CLERK', 7788, '1987-05-23', 1100, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7900, 'JAMES', 'CLERK', 7698, '1981-12-03', 950, null, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7902, 'FORD', 'ANALYST', 7566, '1981-12-03', 3000, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7934, 'MILLER', 'CLERK', 7782, '1982-01-23', 1300, null, 10);

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2.2.1 获取emp表的ename列的字符集

select ename,charset(ename) from emp;

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2.2.2 要求显示exam_result表中的信息，显示格式：“XXX的语文是XXX分，数学XXX分，英语XXX分”

select concat(name,'的语文是',chinese,',数学是',math,',英语是',english) from exam_result;

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2.2.3 求exam_result表中学生姓名占用的字节数

select name,length(name) from exam_result;

注意：
length函数返回字符串长度，以字节为单位，而非字符，中文一个字符就是多个字节，与字符集编码有关。

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2.2.4 将EMP表中所有名字中有S的替换成’上海’

select ename,replace(ename,'S','上海') from emp;

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2.2.5 截取EMP表中ename字段的第二个到第三个字符

select ename,substring(ename,2,2) from emp;

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2.2.6 以首字母小写的方式显示所有员工的姓名

select ename,concat(lcase(substring(ename,1,1)),substring(ename,2)) from emp;

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三、 数学函数

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3.1 简单使用

select abs(520);select abs(-520);

select bin(520);select bin(3);

select hex(520)select hex(16)

select conv(16,10,2);select conv(16,10,3);select conv(16,10,4);

select ceiling(3.14);select ceiling(-3.14);select floor(3.14);select floor(-3.14);

select format(3.1415926,2);select format(-3.1415926,2);

select rand();select format(rand()*100,0);select format(rand()*1000,0);

select mod(100,3);select mod(100,32);select mod(100,37);

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四、其他函数

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4.1 简单使用

select user();

select md5('1');select md5('I LOVE YOU!');

select database();

select ifnull('I Love you,China!','1314520');select ifnull(null,'1314520');

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结尾

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