Five constraints and three paradigms

One . Five constraints (z,w,f,m,w)

Memory method ：
Chase me, Fermat WOW ,
Primary key 、 only 、 Non empty 、 Default 、 Foreign keys
Primary key 、unique、not null、default、 Foreign keys

1.1 Primary key constraint ： Unique primary key

1. characteristic : only Non empty There is only one primary key in a table

1.2 Unique constraint unique

1. keyword unique

2. characteristic : only Can't repeat It can be for null

3. The difference between primary key constraint and unique constraint
A. Primary key constraint One There can only be One individual Unique constraint **** One There can be many individual
B. The primary key constraint cannot be empty only Constraint is You can set null
C. The unique constraint can be set as a joint unique constraint ( Multiple columns form a constraint )

4. There are two ways to create unique constraints
A. When creating a table, set
B. After the table is created, set

a: Set unique constraints when creating tables

CREATE TABLE p1(
 pid INT(11) PRIMARY KEY AUTO_INCREMENT, 
 pname VARCHAR(20) 	UNIQUE
 );

b: Set the unique constraint after creating the table

# Create table 
CREATE TABLE p1(
 pid INT(11) PRIMARY KEY AUTO_INCREMENT, 
 pname VARCHAR(20) 	
 );

# Add primary key constraint 
 grammar :
alter table  Table name  add CONSTRAINT  The name of the unique constraint  unique( Name )
# Add unique constraint 
 ALTER TABLE p2 ADD CONSTRAINT u_name UNIQUE(pname)

# Delete unique constraint 
ALTER TABLE p2 DROP INDEX u_name

c Union sole constraint

CREATE TABLE p3( 
pid INT(11) PRIMARY KEY AUTO_INCREMENT, 
pname VARCHAR(20), 
pwd VARCHAR(20), UNIQUE(pname,pwd) 
);
INSERT INTO p3(pname,pwd)VALUES(" Brother Mao ","123");

1.3 Non empty constraint not null

1. keyword : not null
2. effect : When inserting data, you must insert data Can't insert null
3. Set when creating a table

1.4 Default constraint default

1. keyword :default
2. effect : be used for Set the default value of the column
3. Set when creating a table

4. explain :
A. After setting the default constraint for the specified column No data inserted Show default
B. After setting the default constraint for the specified column Insert default It's also the default value
C. When inserting specific data Will replace the default value

1.5 Foreign key constraints

Foreign key constraints are used between two tables , Used to ensure the integrity of associated data .
Be careful ：

The key association should be noted ：
1. The foreign key must be the primary key of the main table ,
2. When deleting a table, delete the sub table first, and then delete the main table .
3. You can force the deletion of , Regardless of constraints ：drop table orders cascade constraint;
4. You can use cascading updates and cascading deletes , In this way, update or delete operations are performed in the main table , The data in the sub table will also be updated or deleted synchronously

Example ：
order surface

create table orders(

order_id number(10),

total_price number(10,2),

order_time date,
constraint orders_order_id_pk primary key (order_id)

);

order A detailed list

create table order_detail(

detail_id number(10),

order_id number(10),// stay order In the table is the primary key 

item_name varchar2(10),

quantity number(10),

constraint order_detail_detail_id_pk primary key (detail_id),

constraint order_detail_order_id_fk foreign key (order_id) 
referencs orders(order_id)|on delete cascade|on update cascade
　);

Two . Three paradigms

2.1

The three paradigms are also used to constrain the database Ensure data integrity And correctness

2.2

A. The first paradigm （ Atomicity ）: Each column in the table is the smallest atomic unit No further splitting
B. The second paradigm （ Dependence ）： A table must have a primary key ; Non primary key classes must rely entirely on primary keys , Instead of relying on only part of the primary key
C. The third paradigm （）: On the basis of following the first two paradigms No local dependencies （ Cannot pass dependencies ） You can only establish a dependency relationship with the primary key