The basic use of a database search --select

- Create Student Table

create table students (
    id int unsigned not null auto_increment primary key,
    name varchar(20) default '',
    age tinyint unsigned default 0,
    high decimal(5,2),
    gender enum('男', '女', '中性', '保密') default '保密',
    cls_id int unsigned default 0,
    is_delete bit default 0
);

- to insert data table students

Students. INTO values INSERT 
(0, 'Bob', 18,180.00,1,1,0), 
(0, 'Satsuki', 19,180.00,1,2,0), 
(0, 'Eddie', 28,185.00,1,1 , 0), 
(0, 'Andy', 58,175.00,1,2,0), 
(0, 'Huang Rong', 108,160.00,2,1,0), 
(0, 'Feng', 44,150.00,4,2,1 ), 
(0, 'WangZuXian', 52,170.00,2,1,1), 
(0, 'Jay children', 34 is, null, 1,1,0), 
(0, '- Kun', 44,181.00,1,2 , 0), 
(0, 'and Shen', 55,166.00,1,2,0), 
(0, 'Crystal Liu', 29,162.00,2,3,0), 
(0, 'Venus', 45,180.00,3,4, 0), 
(0, 'Shizuka', 18,170.00,2,4,0), 
(0, 'Guo Jing', 22,167.00,1,5,0), 
(0, 'Zhou Jie', 33,178.00,1,1,0 ), 
(0, 'Chin Siu-ho', 56,178.00,1,1,0), 
(0, 'Nicholas', 38,175.00,1,1,0), 
(0, 'Edison', 38,175.00,1,1,　　0);

Simple query:

- query all the columns
　　--select * from table name

select * from students;

- certain conditions query (where)

select * from where id=5;

- formulate queries column

select id_name from students;

- from the field to use as an alias

select id_name as 'alias (Alias)' 
SELECT name AS 'name', age as 'aged', high, gender from students;

- Query by table name field
　　select the library name / Field / * from students;

select * from students;
select students.* from students;
eg: mysql
> select test.students.* from test.students;

- alias the query to the table

select s.id,s.name,s.age from students as s;

- Eliminate duplicate rows
　　- distinct

select distinct age from students;

Conditions inquiry

- comparison operator
　　- Query information older than 18 years

select * from students where age > 18;

　Between --18-year-old to 28 years old (and)

select * from students where age >= 18 and age =< 28;
select * from students where age >= 18 || age =< 28;(有时会报错)
select * from students where age between 18 and 28

　- People over 18 years of age or a height of more than 180 (or)

select * from students where age > 18 or high > 180;

　　- Query the age equal to 18 years of age and older people equal to 28

select * from students where age=18 or age=28;

Fuzzy query:

- like
　　% or alternatively a plurality of or no

　　- query name has 'small' all names

select * from students where name like '%小%';

　- Query the word person's name

select * from students where name like '_ _'; (_ represents a character)

　- query has at least two-word name

select * from students where name like '%_ _%';

Range queries

- in (1,3,8) represents a non-contiguous within the range

　　- queries for the young people of 18 and 34

select * from students where age in (18, 34);
select * from students where age=18 or age=34;

- Query young people 18 and 34 do not include the (inverted)

select * from students where age not in (18, 34);

　　- Query information between the ages of 17 and 34 years

select * from students where age between 17 and 34;

- Query not older 18-34-year-old information

select * from students where age not between 17 and 34;

Empty judgment

- judgment is null

　　- Height query information is empty

select * from students where high is null;

　- to determine non-empty is not null

select * from students where high is not null;

Sequence

- order by fields (EG: name, Age, the above mentioned id)
- asc ascending order, that is, in ascending order (the default is not written asc)
- desc descending order, namely in descending order

　　- the old inquiry 18-34 males between the age, according to age, from small to large

select * from students where gender=1 and age between 18 and 34 order by age;

- Query older women between 18-34 years of age, height from high to low

select * from students where gender=2 and age between 18 and 34 order by high desc;

- order by multi-field
　　- query in older women aged 18-34, height from high to low sort, from small to large sorted by age, if under the same circumstances height

select * from students where age between 18 and 34 and gender=2 order by high desc;

　　-- 按照年纪、身高排序（如果年纪相同的情况下按照身高从小到大排序）

select * from students where order by age，high asc;

　　-- 查询年纪在18到34岁的男性，身高从高到矮排序，如果身高相同的情况下按照年纪从小到大排序，

　　　　如果年龄也相等那么按照id从小到大排序；

select * from students where age between 18 and 34 and gender=1 order by high desc, age, id desc;

聚合函数

-- 总数
-- count
　　-- 查询有多少人

select count(name/age/high) from students;

　　（统计当前字段，如果要统计的数据为null（空），会出现统计错误，所以一般用*）

　-- 查询有多少人（并且显示中文）

select count(*) as '总人数' from students;

　-- 查询男性有多少人

select count(*) from students where gender=1;

-- 最大值max
　　-- 查询最大的年纪

select max(age) from students;

　　-- 查询女性的最高身高

select max(high) from students where gender=2;

-- 最小值min
　　-- 查询最高的人

select min(high) from students;

-- 求和sum
　　-- 计算所有人的年龄总和

select sum(age) from students;

-- 平均值avg
　　-- 计算平均年纪
　　-- 计算平均年纪 sum(age)/count(*)

select sum(age)/count(*) from students;
select avg(age) from students;

　-- 保留2位小数

select round(avg(age),2) from students;

分组（分组的优先级最高，一条语句中先分组）

-- group by
　　-- 按照性别分组，查询所有的性别

select gender from students group by gender;

　-- 计算每组性别的人数

select gender，count(*) from students group by gender;

--group_concat（函数，和group by配合使用）
　　-- 查询性别组中的男女平均年龄及姓名

select gender,group_concat(name),avg(age) from students group by gender;

-- having
　　-- 查询每个性别平均年纪超过30岁的性别，以及姓名 having avg(age) > 30

select gender, group_concat(name) from students group by gender having avg(age) > 30;

　-- 查询每种性别中的人数多于4个的组的信息

select gender,group_concat(name) from students group by gender having count(*)>4;

分页

　　-- 显示5页

select * from students limit 5;

　-- 查询年纪最大的人名(先按照年纪排序，再分页)

select * from students order by age desc limit 1;

　-- 分页显示，每页显示2条数据

select * from students limit 0,2;

　-- 按照身高从高到矮排序，查找出所有女性，并且分页显示，每页显示2条数据

select * from students where gender=2 order by high desc limit 0,2;