mysql函数的基础学习

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本文深入探讨了SQL语句的使用，包括数值四舍五入、系统时间获取、日期运算、表计数、创建与插入数据、聚合函数、空值处理、最大最小值查找、分组与聚合操作。通过具体实例，展示了如何高效地进行数据库操作。

select round(0.325,2) from dual;
select round(32.325,-2) from dual;
select round(52.325,-2) from dual;
select trunc(23.325,-2) from dual;
select sysdate from dual;oracle返回系统时间
select now() from dual;mysql返回系统时间

select to_number(now()-1) from dual;
select now()-now() from dual;
select now() - '2013-12-30' from dual;
select (now() - '2013-12-30')/1000/60/60/24 from dual;

select count(*) from account;
select * from account;

create table emp(id int,salary double);
insert into emp values(1,20.33);
insert into emp values(1,20.33);
insert into emp values(1,20.33);
insert into emp values(2,25.25);

select count(*) from emp;
select count(3) from emp;
select count(a) from emp;
select count(salary) from emp where id = 3;
select count(*) from emp where id = 2;

select sum(salary) from emp;

insert into emp(id) values(3);
select * from emp;
select avg(salary) from emp where id = 3;
select count(2) from emp where id = 3;

select id,nvl(salary,0) from emp;
select id,ifnull(salary,0) from emp;

select count(ifnull(salary,0)) from emp where id = 2;
select count(salary) from emp where id = 2;

select max(salary) from emp;
select min(salary) from emp;

select count(id),id,salary from emp group by id,salary;
select count(id),id from emp;
select id from emp;

select id,count(id) from emp group by id;
select id,count(id) from emp;
select id from emp;