Foreign keys, advanced operations

A foreign key (one to many)

Role:
       1 . Constraint
       2 save space. 
----------------------------------------- ---------------------------------------
        
        create table department (
            id  int auto_increment primary key, 
            depart_name varchar(32)  not null  default '',
            num int  not null default 0
        )engine=Innodb charset=utf8;
            
        create table userinfo (    
            id  int auto_increment primary key, 
            name varchar(32) not null default '',
            depart_id int not null  default 1,
            
            # Constraint foreign key name (fk_userinfo_depart) foreign key (column name (depart_id)) references the table name (Department) (column name associated (ID)), 
            # constraint fk_userinfo_depart Foreign Key (depart_id) References Department (ID) 
        ) Engine = Innodb = charset UTF8;
        
        
        ps:
            1 . You can not create a separate statement the foreign key out, the following ways
               
               alter table userinfo add constraint fk_userinfo_depart foreign key (depart_id) references department(id);
               alter table userinfo drop foreign key 外键名称(fk_userinfo_depart );
            
            2 . When a foreign key, must be associated with the table's primary key ID
            
            3 . Practice time, will write the statement in the text, and then perform the test in the past
            
            4. The primary key index: Find the acceleration can not be empty + + can not be repeated
            

1, the only index:

1 The only index:

create table t5(
    id int,
    num int,
    unique(num)
)engine=Innodb charset=utf8;


effect:    
    The value of the num column can not be repeated
    Find accelerate
    
create table t6(
    id int,
    num int,
    unique(id, num)
) Engine = Innodb charset = utf8; 


co-unique index action:    
    Id value of the num column and the column can not be repeated
    Find accelerate

create table t6(
    id int,
    num int,
    unique(id, num......)
)engine=Innodb charset=utf8;

2, one to many:

Sector Table:
    id       depart_name
     1           PR
      2           common part
      3           Security Department

Employees table:
    id    name  age   depart_id(外键)
    Eighty 12 1 2
    2     xxxx  13      1
    3     xxxx  13      2

 3, one-:

user table:
    id    name     age  
    1 intelligence 23  
    2 Eagon 34
    Eighty 3 45
    4      owen     83

Blog table:
    id url user_id (foreign key + unique constraint)
     . 1/2 linhaifeng
    2 / intelligence 1
    3/3 Eighty
    4/4 Eighty

4. many-:

user table:
    id    name    phone 
    1    root1    1234
    2    root2    1235
    3    root3    1236
    4 root4 1237
    5 root5 1238
    6 root6 1239
    7 root7 1240
    8    root8    1241
    
Host Table:

    id    hostname    
    1    c1.com    
    2    c2.com    
    3    c3.com    
    4    c4.com    
    5    c5.com    

To facilitate the query, the user has the following number of users on a certain number of hosts and host, we need to create the third table:
    user2host:
    
        id    userid    hostid
            1    1    1
            2    1    2
            3    1    3
            4    2    4
            5    2    5
            6    3    2
            734     
time of creation, userid and hostid must be a foreign key, and then co-unique index unique (userid, hostid)

 

Second, the operation of the data row:

increase:
    insert into table (1 column name, column name 2,) values ​​(value 1, value 2);
    insert into table (1 column name, column name 2,) values ​​(value 1, value 2), (value 1, value 2), (the value of n, n-value);
    
    insert into table (1 column name, column name 2,) select a column name, a column name 2 from table;
    
delete:
    delete from 表名; 
    
    Delete from table name WHERE ID> 10 
    Delete from table WHERE ID <10 
    Delete from table WHERE ID <= 10 
    Delete from the table name WHERE ID> = 10 
    Delete from table name WHERE ID! = 10  
    Delete from table where id = 10 and name = ' XXX ' ;   and : and the two conditions must be established
    Delete from table ID = 10 WHERE or name = ' XXX ' ;    or : or as long as a condition is satisfied

modify:
    update 表名  set name='zekai', age=23 where id > 10;
    
Inquire:
    
    Basic:
        select * from 表名;
        select name , age from 表名;

 1, where the query conditions:

select * from 表名 where  id=10;
    select * from 表名 where  id >10 and id<15;
    the SELECT * from table name the above mentioned id the WHERE> 10 ;
     =! : unequal and
     > = <= 
    
    
    the BETWEEN and : closed interval
        select * from t4 where id between 9 and 12;
    
    in : in one collection
        select * from t4 where id in (9,10,11....);
        
        select * from t4 where id in (select id from t3 where id between 2 and 4)
        

2. Tsuhaifu:

alex
  
    SELECT * from table name like WHERE ' ALE% '   - all at the beginning of the ALE (a plurality of strings)
    the SELECT * from table name like the WHERE ' ale_ '   - all at the beginning of ale (a character)

3, taking a few restrictions:

* SELECT from table index offset limit, the number of data extraction;


SELECT * from T3 limit 0, 10 ; a first page
SELECT * from T3 limit 10, 10 ; the second page

page = input('page:')

    the index page of data offset amount (offset)
      1         0              10
      2         10             10
      3         20             10
      4         30             10
      
      page   (page-1)*offset   offset

Page Core SQL:

    select * from t3 limit (page-1)*offset, offset;

4 Sort by:

order by

In descending order:
    select * from t4 order by 列名 desc; descending

Ascending:
    select * from t4 order by 列名 asc; ascending


Multi-row:
    
    create table t7(
    
        id int auto_increment primary key,
        num int not null default 0,
        age int not null default 0
    )charset=utf8;
    
    insert into t7 (num, age) values (2, 12),(3,13),(4, 12);

    select * from t4 order by num desc, name asc;
    
    If the previous value of the column equal, will be further sorted according to the value one.

5, grouping

select age, aggregate functions (COUNT (NUM) / SUM (NUM) / max (NUM) / min (NUM) / AVG (NUM)) from table group by the name of the column;


select age, avg(num) from t7 group by age;
 
select age, count(num) from t7 group by age;

Age SELECT, COUNT (NUM) as CNT from T7 by Age Group; Show Aliases as

having secondary deletion of selected:
    select age, count(num) as cnt from t7 group by age  having cnt>1;
    
where and having difference:
    . 1 ). HAVING similar and where, can be screened data
     2 ). Where role for columns in a table, query data
     3). Having play for the query result column, secondary screening data, and with the group by using

6, even the operating table

* SELECT from UserInfo, Department; (Cartesian product)

select * from userinfo, department where userinfo.depart_id=department.id;

Left connection:

    select * from userinfo left join department on userinfo.depart_id=department.id;
    The table below shows all the left, the right is not used does not show

Right Connections:
    
    select * from userinfo right join department on userinfo.depart_id=department.id;
    Table on the right shows all left no association is represented by null

En:
    The data are displayed both sides

ps: 
    a. Just remember to connect the left left join
    
    b. a plurality of tables can be connected via a particular condition
            
Note that the order of the query:
select name,sum(score) from 表 where id > 10 group by score having age> 12  order by age desc limit 2, 10