MySQL (II)

One 、 filter
1） Comparison operator

select * from student where id>2;
select * from student where id<>2;
select * from student where id is not Null;

2） Logical operators

select * from student where id=1 and name='fei';
select * from student where id=1 or name='fei';
select * from student where not name='fei';

Sort （order by）

  select * from student order by class; 
    select *from student order by class desc; 

. Limit （limit）

  select * from student limit 3;
  select * from student limit 5,3;

. duplicate removal （distinct）

select distinct * from student ;

Fuzzy query

select * from student where name like 'k%";
select * from student where name like 'k_";
  select * from student where name like 'k__";

Two 、 Aggregation and grouping

1） Common aggregate functions
1. Number of Statistics ：

count (column)
select count(name)from student;

2. Maximum ：

max (column)
select max(name)from student;

3. minimum value ：

min（column）
select min(name)from student

4. Sum up ：

sum (column)
select sum(name)from student;

5. Average ：

avg (column)
select avg(name)from student;

6. List all field values

：group_concat (column)
select group_concat(name)from student;

2） Group query

（group by ）    
  select class from student group by class;
   select class,group_concat(name) from student group by class;

3） Aggregate filter

1.（having）   
select class,group_concat(name) from student where id<5 group by class having class=2;
select class,group_concat(name) as gg from student where id<5 group by class having class=2

3、 ... and 、 Subquery

1） Definition

1. Leave the results of one query for the next query ( select Nested in select )
for example ： requirement ：1) Nested inside the query 2) Must always appear in parentheses
Find the average age of the students ``

select avg(age) from student;

Of the average age to be determined SQL Statement is used to find statements that are older than the average age

 Of the average age to be determined SQL Statement is used to find statements that are older than the average age 

Four 、 Link query

1） Internal connection （ inner join）

1. Unconditional internal connection ：
Unconditional internal connection , Also known as cross connect / Cartesian connection
Each item of the first table will be combined with each item of the other table in turn

Mysql> select * from student [inner] join detail;

2. Conditional inner connection ：
On the basis of unconditional inner link , Add a on Clause
When connected , Filter out those meaningful records for combination

Mysql> select * from student inner join detail
       on student.id =detail.id;
select student.id,name,age from student  join detail
       on student.id =detail.id;

2） External connection （{left | right} join）

1. The left outer join ： （ Based on the left table ）
When two tables are connected , When the connection conditions don't match
Leave the data in the left table , The data in the right table is represented by NULL fill

mysql>  select student.id,name,age from student right join detail  on student.id =detail.id;

2. Right connection ： （ Based on the right table ）
When connecting two tables , When the connection conditions don't match
Leave the data in the right table , The data in the left table is represented by NULL fill

mysql>   select student.id,name,age from student left join detail  on student.id =detail.id;