Knowledge points of MySQL (12)

One 、 On the column ：AUTO-INCREMENT

effect ： The value of a field increases automatically

Features and requirements ：

1. A table can only have at most one self growing column
2. When a unique identifier or sequential value needs to be generated , Settable growth
3. The column of the self growing column constraint must be a key column （ Primary key column 、 Unique key column ）
4. The data type of a column with a self increasing constraint must be an integer type
5. If the auto increment column specifies 0 and NULL, It will increase automatically based on the current maximum , If the auto increment column manually specifies a specific value , Direct assignment to a specific value

Code implementation ：

1, stay CREATE TABLE add

# Build table 
creat table test4(
id int primary key auto_increment,
last_name varchar(15)
);

# add to 
insert into test4(last_name)
values('TOM');

select * from test4;

( Be careful ： In development , Once the primary key field is declared to have AUTO_INCREMENT, When we add data , Don't assign values to the fields corresponding to the primary key ）

2, stay ALTER TABLE add

# Build table 
create table test5(
id int primary key,
last_name varchar(15)
);

# Check the information 
desc test5;

# Add self increasing 
alter table test5 modify id int auto_increment;

desc test5;

Two 、FOREIGN KEY（ Foreign keys ） constraint

effect ： Limit the referential integrity of a field in a table

1, stay CREATE TABLE add

# Create the main table first 
create table dept1(
dept_id int,
dept_name varchar(15)
);

# Add primary key constraint 
alter table dept1 add primary key(dept_id);
desc dept1;

# Creating slave tables 
create table emp1(
emp_id int primary key auto_increment,
emp_name varchar(15),
department_id int,

# Table level constraints 
constraint fk_emp1_dept_id foreign key(department_id) references dept1(dept_id)
);

desc emp1;

select * from information_schema.table_constraints where table_name='emp1';

demonstration ：

# Add failure 
insert into emp1 values(1001,'TOM',10);

# Add success （ In the main table dept1 Add 10 After department No , You can add 10 Employees of Department ）
insert into dept1 values(10,'IT');

insert into emp1 values(1001,'TOM',10);

# Delete failed 
delete from dept1 where dept_id=10;

# Update failed 
update dept1 set dept_id=20 where dept_id=10;

2, Constraint level

  demonstration ：on update cascade on delete set null

# Create the main table 
create table dept(
did int primary key,         # Department number 
 dname varchar(50)           # Department name 
);

# Create from table 
create table emp(
eid int primary key,         # Employee number 
ename varchar(5),            # Employee name 
deptid int,                  # Employee's Department 

foreign key (deptid) references dept(did) on update cascade on delete set null
# Set the modification operation to cascade modification level , Set the delete operation to set null Grade 
);

# Add data 
# First add the main table data  
insert into dept values(1001,' Academic Building ');
insert into dept values(1002,' Finance Building ');
insert into dept values(1003,' Consulting building ');

# Add data from table 
insert into emp values(1,' Zhang San ',1001);  # When adding this data , Required department table 1001 department 
insert into emp values(2,' Li Si ',1002);
insert into emp values(3,' Wang Wu ',1003);

select * from dept;

select * from emp;

# Update and modify 
update dept set did=1004 where did=1002;
select * from dept;

select * from emp;

# Delete 
delete from dept where did=1004;

select * from dept;

select * from emp;

Conclusion ： For foreign key constraints , It's better to use ：‘on update cascade on delete restrict’ The way

3、 ... and 、CHECK（ Check ） Constraints and DEFAULT（ The default value is ） constraint

CHECK Constraint action ： Check whether the value of a field matches XX requirement , Generally refers to the range of values .

# Build table 
create table test6(
id int,
last_name varchar(15),
salary decimal(10,2) check (salary>2000)
);

# Add data 
# Add failure 

insert into test6 values(1,'TOM',1500);
select * from test6;

# Add success 
insert into test6 values(1,'TOM1',2500);
select * from test6;

DEFAULT Constraint action ： Give a field / Specify a default value for a column , Once the default value is set , When inserting data , If this field does not show assignment , The default value is assigned .

Four 、 View

1, Common database objects

2, View understanding

View function ： On the one hand, it can help us use some tables instead of all tables , On the other hand, you can also make different query views for different users .

View understanding ：

• View is a virtual table , There is no data in itself , Takes up very little memory space , It is SQL An important concept in
• Views are built on existing tables , The tables on which the view is built are called base tables
• The creation and deletion of views only affect the view itself , It does not affect the corresponding base table , But when the data in the view is added 、 When deleting and modifying operations , The data in the data table will change accordingly , vice versa .
• The statement that provides data content to the view is SELECT sentence , Views can be understood as stored SELECT sentence
• View , It is a form of providing users with base table data . Usually , Small project database can not use views , But on big projects , And when the data table is complex , The value of view is highlighted , It can help us put the result set of frequent queries into the virtual table , Improve efficiency .

                                                     3, Create view （VIEW）

# preparation 
create database dbtest1;

use dbtest1;

create table emp as select * from atguigudb.employees;

select * from emp;

desc emp;

# For single table 

# How to determine the field names in the view 1：
create view vu_emp1 as select employee_id emp_id,last_name lname,salary from emp;
# The alias of the field in the query statement will be used as the name of the field in the view 

select * from vu_emp1;

# How to determine the field names in the view 2：
create view vu_emp2（emp_id,name,monthly_sal)  # The number of fields in parentheses is the same as SELECT The number of fields in the is the same 
as select employee_id ,last_name ,salary from emp;

select * from vu_emp2;


# For multiple tables 

create view vu_emp_dept
as
select e.employee_id,e.department_id,d.department_name
from emps e join depts d
on e.'department_id'=d.'department_id';

select * from vu_emp_dept;

  Create a view based on the view ：

create view vu_emp3
as
select employee_id,last_name
from vu_emp1;

select * from vu_emp3;

4, View view

grammar 1： View the table objects of the database 、 View objects

show tables; 

grammar 2： View view structure

describe View name ; 

grammar 3： View the attribute information of the view

show table status like ' View name '; 

grammar 4： View view Detailed definition information of

show create view View name ; 

5, Update the data in the view （ There must be a one-to-one relationship between the rows in the view and the rows in the underlying basic table ）

# Select View 
select * from vu_emp1;

select employee_id,last_name,salary from emps;

# Update view data , This will lead to the modification of data in the base table 
update vu_emp1 set salary=2000 where employee_id=101;

# Updating the data in the table will also lead to the modification of the data in the view 
update emps set salary=10000 where employee_id=101;

# Delete data from view , It will also cause the data in the table to be deleted 
delete from vu_emp1 where employee_id=101;

  Of course, there are some views that cannot be updated ：

 （ Be careful ： Although view data can be updated , But on the whole , View as virtual table , It is mainly used to facilitate query , It is not recommended to update the data of the view , Modification of view data , It is done by operating the data in the actual data table ）

6, Modify the view

# The way 1
create or replace view vu_emp1

as

select employee_id,last_name,salary,email from emps where salary>7000;

# The way 2

alter view vu_emp1

as

select employee_id,last_name,salary,email,hire_data from emps;