MySQL addition, deletion, modification, retrieval and constraint (detailed teaching)

  As we all know, the premise of adding, deleting, modifying, searching and constraints is to create a database first , Then create a table in the database , Here is an example , This is the raw data of the table .

One 、 Add data

  We need to add administrators 3、4、5,SQL The statement is as follows :

 

  Refresh the table , And it worked .

Two 、 Delete data

For example, we want to delete the administrator 5,SQL The statement is as follows :

  This indicates that the operation was successful .

3、 ... and 、 Modifying data

Will administrator 4 The password of is changed to 456123,SQL The statement is as follows :

Examples of success :

  Four 、 Query data

   Query Administrator 1 Information about , If it exists, query the information in the user table ,SQL The statement is as follows :

  5、 ... and 、 retrieval ( The above figure is an example )

1. Single column Select name from User table ;
2. Multiple columns Select id,name from User table ;
3. All columns Select*from User table ;

6、 ... and 、 constraint

1. Primary key constraint :

mysql>alter table User table

->ADD PRIMARY KEY (id);

2. Foreign key constraints ：
mysql>ALTER TABLE Cashier structure
->ADD CONSTRAINT fk_ Commodity structure
->FOREIGN KEY (sum)
->REFERENCES Commodity structure (id);

3. Unique constraint ：
mysql>ALTER TABLE User table
->ADD CONSTRAINT unique_id UNIQUE (id);

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Pay attention to , Knowledge is not lost