MySQL learning diary-database backup and restore / constraints

# # Database backup and restore

1. Command line:

* Backup: mysqldump -u username -p password database name> save path

* Restore:

1. Log in to the database

2. Create a database

3. Use the database

4. Execute the file. source file path

2. Graphical tools: unwritten

 

# # Constraint

Concept: Limit the data in the table to ensure the correctness, validity and integrity of the data. 

classification:

 

1. Primary key constraint: primary key

2. Non-null constraint: not null, the value cannot be null

3. Unique constraint: unique

4. Foreign key constraint: foreign key

# Non-empty constraints

1. Add a non-empty constraint when creating a table:

CREATE  TABLE stu ( 
id INT , 
NAME VARCHAR ( 20 ) NOT  NULL  - name is not empty 
);

2. After creating the table, add a non-empty constraint:

ALTER TABLE stu MODIFY NAME VARCHAR(20) NOT NULL;

3. Remove the non-null constraint of name:

ALTER TABLE stu MODIFY NAME VARCHAR(20);

 

# Unique constraint: unique, value cannot be repeated

1. When creating a table, add a unique constraint

CREATE  TABLE stu ( 
id INT , 
phone_number VARCHAR ( 20 ) UNIQUE  - added unique constraint 
);

* Note that in mysql, the value of a column restricted by a unique constraint can have multiple nulls

2. Remove the unique constraint

ALTER TABLE stu DROP INDEX phone_number;

3. After creating the table, add a unique constraint

ALTER TABLE stu MODIFY phone_number VARCHAR(20) UNIQUE;

 

 

# Primary key constraint: primary key.

1. Note:

1. Meaning: Not empty and unique

2. A table can only have one field as the primary key 3. The primary key is the unique identifier of the records in the table

2. When creating a table, add a primary key constraint

Create  Table STU ( 
id int  Primary  Key , - to add a primary key constraint id 
name VARCHAR ( 20 is ) 
);

3. Delete the primary key

-- 错误 alter table stu modify id int ;
ALTER TABLE stu DROP PRIMARY KEY;

4. After creating the table, add the primary key

ALTER TABLE stu MODIFY id INT PRIMARY KEY;

5. Automatic growth:

1. Concept: If a column is of numeric type, use auto_increment to complete the automatic growth

2. When creating the table, add the primary key constraint, and complete the primary key self-growth

　　Create  Table STU ( 
　　id int  Primary  Key AUTO_INCREMENT, - to add a primary key constraint id 
　　name VARCHAR ( 20 is ) 
　　);

　　3. Delete automatic growth

　　ALTER TABLE stu MODIFY id INT;

　　4. Add automatic growth

　　ALTER TABLE stu MODIFY id INT AUTO_INCREMENT;

 

# Foreign key constraint: foreign key, so that the table has a relationship with the table, thereby ensuring the accuracy of the data.

1. When creating a table, you can add a foreign key

create  table table name ( 
.... 
foreign key column 
constraint foreign key name foreign  key (foreign key column name) references main table name (main table column name) 
);

2. Delete the foreign key

ALTER  TABLE table name DROP  FOREIGN  KEY foreign key name;

3. After creating the table, add the foreign key

ALTER  TABLE table name ADD  CONSTRAINT foreign key name FOREIGN  KEY (foreign key field name) REFERENCES main table name (main table column name);

4. Cascade operation

　　1. Add cascading operations

　　ALTER  TABLE table name ADD  CONSTRAINT foreign key name 
　　FOREIGN  KEY (foreign key field name) REFERENCES main table name (main table column name) ON  UPDATE  CASCADE  ON  DELETE  CASCADE ;

　　2. Classification:

　　　　1. Cascade update: ON  UPDATE  CASCADE  
　　　　2. Cascade delete: ON  DELETE  CASCADE