Database operation day 6

database ：

The data is in memory ：

• advantage ： Fast reading and writing .
• shortcoming ： Program end data loss .

Save the data to a file ：

• advantage ： Data can be stored permanently .
• shortcoming ：1. Frequent IO operation , Low efficiency .
  2. Data management is very inconvenient , You need to read all the data as a whole before you can operate .

database ：

• Data is permanently stored .
• Easy to manage .

Mysql

One ： Database operation ：

Create database

CREATE DATABASE school DEFAULT CHARACTER SET UTF8;

• SHOW DATABASES; Show all the databases .
  

Delete database

DROP DATABASE school;

Two ： Operation of database table ：

Create table

--  Build table 
CREATE TABLE student(
		id INT,
		`name` CHAR(10),
		age INT,
		gender CHAR(1)
)
-- Find all tables 
SHOW TABLES;

View table structure

--  View table structure 
DESC student;

Delete table

-- Delete table 
DROP TABLE student;

Change table structure

1. Add fields

--  Add fields 
ALTER TABLE student ADD COLUMN address CHAR(255);

2. Modify fields

--  Modify fields 
AlTER TABLE student CHANGE address addr char(20);

3. Delete field

--  Delete field 
ALTER TABLE student DROP COLUMN addr;

Modify the name of the table

--  Modify the name of the table 
ALTER TABLE student RENAME TO teacher;

Add primary key and set self increment ：
CREATE TABLE student(
id INT PRIMARY KEY AUTO_INCREMENT,
name CHAR(20),
age INT,
gender char(1)
)

3、 ... and ： Basic operation of the watch

1. increase

• 1

--  Insert column （ Insert one at a time ）
INSERT INTO student(`name`,age,gender) VALUES(" Zhang San ",1," male ");

• 2

-- Insert column （ Insert more than one at a time ）
INSERT INTO student(`name`,age,gender) VALUES(" Li Si ",2," Woman "),(" Wang Wu ",3," male ");

2. Change

-- repair 
UPDATE student SET gender=" male " WHERE `name`=" Li Si ";

3. Delete

-- Delete 
DELETE FROM student WHERE `name`=" Li Si ";

4. check

SELECT * FROM student;

All the contents of the query table , Not recommended , When the content is small, you can operate , It is suggested that you can find whichever column you need .

• 1

--  Query specified column 
SELECT `name`,age,gender FROM student WHERE id=1;

• 2

--  Merge columns when querying , Fields can be treated as java Inside the variable to operate 
SELECT id,`name`,(php+java) AS ' Total score ' FROM student;

• – Remove duplicate records during query
  SELECT DISTINCT address FROM student;

• Add constant columns when querying , adopt as It's a nickname
  SELECT id,name,age AS ‘ Age ’ FROM student;

expand ：

1. Aggregate query

2. Query sequence

3. Group query

• 1

-- Group query 
-- select The following queries are based on group by After that 
SELECT gender,COUNT(id) AS " Number " FROM student GROUP BY gender;

• 2

-- group by The following condition query uses having
SELECT gender,COUNT(id) AS " Number " FROM student GROUP BY gender HAVING COUNT(id)>2;