mysql-查询

select查询语句：
语法：
(
    select field_name ...
    from tb_name
    where 查询条件
    group by field_name ...   having 过滤条件  （分组查询和分组查询过滤）
    order by field_name asc(升序) desc(降序)   （查询结果排序）
    limit
)

where 和 having 都有条件判断的功能（过滤）
区别：
    where 是在分组之前进行过滤
    haing 是在分组之后进行过滤

1、条件查询：
select field_name ... from tb_name ... where field_name>values;  返回field_name 大于 values 的记录
select field_name ... from tb_name ... where field_name in(value1,value2， ... );  返回 field_name 等于 value 中的一个的记录。
select field_name ... from tb_name ... where field_name not in(value1,value2 ...);  返回field_name 不在其中的记录。
select field_name ... from tb_name ... where field_name between value1 and value2; 返回field_name 的value 在value1 和 value2 之间的记录。
select field_name ... from tb_name ... where field_name not between value1 and value2; 返回field_name 的value 不在value1 和value2 之间的记录。


2、模糊查询：
select field_name ... from tb_name ... where field_name like ‘a%b/a_b‘;  其中：% 表示多个任意字符，_ 表示一个任意字符。(查询field_name以a开头，以b结尾的所有记录)

3、空值查询：
select field_name ... from tb_name ... where field_name is null;

eg:
select * from massege where 总分 between 550 and 600;
select * from massege where 总分 not between 500 and 600;
select * from massege where 总分 like ‘5%‘;
select * from massege where 总分 like ‘__5‘;

4、查询结果去重：distinct
select distinct field_name from tb_name;

5、分组查询：
select field_name as total from tb_name group by field_name;

select age,count(2) as‘姓名‘ from stu_temp group by age;   count(1)/count(*) 都是统计数据的

select age,group_concat(name) as‘姓名‘ from stu_temp group by age; group_concat(field_name)显示该组中的全部field_name。

select age,group_concat(name) as‘姓名‘ from stu_temp group by age having count(name)>3;  having count(field_name)>values 过滤掉count(field_name)<values 的分组，只显示大于的分组。


select age,count(2) as‘姓名‘ from stu_temp group by age with rollup;  rollup 统计归纳。

6、分页查询：









create table stu_temp(
id int(10) unsigned primary key auto_increment,
name varchar(20) not null,
age int(10) not null
);

insert into stu_temp values(null,‘qqq‘,19),(null,‘www‘,20),(null,‘eee‘,18),(null,‘aaa‘,19)
,(null,‘sss‘,20),(null,‘ddd‘,18),(null,‘zzz‘,20),(null,‘xxx‘,19),(null,‘ccc‘,20),(null,‘vvv‘,18);


insert into product values(null,‘qqq‘,‘20170601‘),(null,‘www‘,‘20170602‘),(null,‘eee‘,‘20170603‘),(null,‘aaa‘,‘20170601‘),(null,‘qqq‘,‘20170601‘);

交叉查询
select *
from employee
cross join department;

等价于：
select * 
from employee,department;

inner join (内连接):基于联合条件进行的多表查询。匹配多个表中的行，并且允许你去查询在两个表中都包含的列的行。它被直接用在from的后面
inner join 的使用条件：
                    1、在from 后面指定主表
                    2、指定和主表连接的表，理论上，你可以联合多个表，但是为了更好的查询，应该限制表的个数
                    3、指定联合条件，联合条件应该在 inner join  的关键字 on 的后面。联合条件值匹配多表的规则。 
SELECT employee.`name` as ‘姓名‘ ,department.`name` as ‘部门‘
FROM employee
INNER JOIN department on department.id=employee.dep_id
WHERE department.id=4;

SELECT e.`name` as ‘姓名‘ ,d.`name` as ‘部门‘     这里是利用 as 为表其了别名（as 可以省略）
FROM employee as e
INNER JOIN department as d on d.id=e.dep_id
WHERE d.id=4;

select e.`name` as ‘姓名‘,d.name as ‘年龄‘
from employee as e
left join department as d on e.dep_id=d.id;

select e.`name` as ‘姓名‘,d.name as ‘年龄‘
from employee as e
right join department as d on e.dep_id=d.id;

select e.`name` as ‘姓名‘,d.name as ‘年龄‘
from employee as e
full join department as d on e.dep_id=d.id;
但：mysql 不支持 full [outer] join 语法，使用下列语法代替全外连接
(既：将左外连接和右外连接用 union 合并,就是 mysql 的全外连接)
select *
from `employee`
left join `department` on `employee`.dep_id=`department`.id
union
select * 
from `employee`
right join `department` on `employee`.dep_id=`department`.id;