MySQL（五）DQL数据查询语言（sql语句）

MySQL（五）DQL数据查询语言（sql语句）。

DQL:
首先来搭建测试环境：
show databases;
drop database test;
create database test CHARACTER SET ‘utf8’;
use test;
drop table if exists user ;

create table user(
id int primary key auto_increment,
name varchar(50),
department_id int,
hire DATE,
phone CHAR(20),
job VARCHAR(50),
sal float(8,2)
)ENGINE=InnoDB DEFAULT CHARSET=utf8;

alter table user rename to empl
alter table empl modify job varchar(40);
alter table empl add(note varchar(200))
alter table empl drop note

insert into empl VALUES(0,’刘备’,’1’,’2000-12-22’,123456789,’董事长’,8000.00);
insert into empl VALUES(0,’张飞’,’1’,’2001-10-2’,123422229,’经理’,16000.00);
insert into empl VALUES(0,’关于’,’1’,’2002-7-12’,123499989,’销售’,8700.00);
insert into empl VALUES(0,’赵云’,’1’,’2003-4-18’,123433389,’销售’,8400.00);
insert into empl VALUES(0,’曹操’,’2’,’2003-2-10’,123455789,’董事长’,1000.00);
insert into empl VALUES(0,’郭嘉’,’2’,’2001-8-17’,123458889,’经理’,6000.00);
insert into empl VALUES(0,’夏侯渊’,’2’,’2002-4-11’,123453789,’销售’,8000.00);
insert into empl VALUES(0,’夏侯惇’,’2’,’2003-2-10’,123455789,’销售’,19000.00);
insert into empl VALUES(0,’夏侯醇’,’2’,’2003-2-10’,123455789,’销售’,10000.00);

删除语句：
delete from empl where id=8;

函数：
去重：distinct
例如：select distinct job from empl;

数据拼接： concat
案例：select concat(‘我叫’,name,’，我的工作是’,job) as ‘描述’ from empl;

替换：如果空就用指定值代替：
select *,IFNULL(manager,’总经理’) from empl;