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MySQL Database Basics

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MySQL Database Basics

1. Introduction to mysql database related knowledge

2. Operation of the database

2.1 Display the current database

2.2 Create a database

2.3 Using the database

2.4 Delete the database

3. Common data types

3.1 Numeric Types

3.2 String type

3.3 Date Types

4. Operation of the table

4.1 View the structure of the table

4.2 Create a table

4.3 Delete Table

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1. Introduction to mysql database related knowledge

①What is a database? ? ?

A database is a type of software. The software for managing data often stores data on the hard disk (persistent storage), and it is very convenient to add, delete, check and modify; the typical representatives are: MySQL, Oracle, SQL Server; MySQL is a software with a client-server structure

②What do server, client, request, and response refer to? ? ?

Any command entered on the console essentially interacts with the server through the network;
server: the party that passively accepts the request
Client: the end that actively initiates the request Request: the data response
sent by the client to the server
: The data returned by the server to the client

③How MySQL database manages data

a. There can be many data tables in a database;

b. Each data table has many rows, each row is called a "record", and each row has many columns, and each column is called a "field".

c. Databases such as MySQL require that each row and column be consistent; eg. the first row and the first column are an integer. The first column of the second row is then also an integer 

d. Not all data is organized in a tabular manner

④The actual storage form of MySQL

2. Operation of the database

The operations mentioned in this section are introductory operations that belong to the database, and the subsequent complex operations will be described in the following sections.

Notice:

① Operations in the database need to end with;.

②The words are separated by spaces. The number of spaces may not be fixed, but there must be at least one

③ Both upper and lower case can be used, and bloggers are accustomed to lowercase, so subsequent codes will be presented in lowercase

④When there is a mistake when typing, press the up key in the direction key of the keyboard to restore

2.1 Display the current database

①Format:

show+space+databases semicolon

②Demo:

2.2 Create a database

①Format:

create+space+database+database name; (the name of the database can be arbitrarily chosen, but note that it cannot be a keyword)

②Demo:

2.3 Using the database

①Format:

use+space+database name;

②Demo:

2.4 Delete the database

①Format:

drop+database+database name;

②Demo:

③Note:

Deleting a database is a very dangerous operation, and it must be meticulous, meticulous and meticulous! ! !

3. Common data types

Since MySQL was born earlier than languages ​​like Java, the data type keywords are slightly different

3.1 Numeric Types

①Detailed table

type of data	size	illustrate	Corresponding java type
BIT[ (M) ]	M specifies the number of digits, the default is 1	binary number, M ranges from 1 to 64 , Stores values ​​ranging from 0 to 2^M-1	Commonly used Boolean corresponds to BIT . At this time, the default is 1 bit, that is, only 0 and 1 can be stored.
TINYINT	1 byte		Byte
SMALLINT	2 bytes		Short
INT	4 bytes		Integer
BIGINT	8 bytes		Long
FLOAT(M, D)	4 bytes	Single precision, M specifies the length, and D specifies the number of decimal places. Loss of precision (significant digits) occurs	Float
DOUBLE(M, D)	8 bytes		Double
DECIMAL(M, D)	M/D max +2	Double precision, M specifies the length, and D represents the number of decimal places. Exact value (significant digits)	BigDecimal
NUMERIC (M, D)	M/D max +2	same as DECIMAL	BigDecimal

②Note:

a. The value type can be specified as unsigned ( unsigned), which means no negative number.

Try not to use unsigned . For the data that may not be stored in int type, int unsigned may not be able to store it. Instead, it is better to upgrade int type to bigint type when designing. \

b. 1 byte ( bytes ) = 8bit

3.2 String type

①Detailed table:

type of data	size	illustrate	Corresponding java type
VARCHAR (SIZE)	0-65,535 bytes	variable length string	String
TEXT	0-65,535 bytes	long text data	String
MEDIUMTEXT	0-16 777 215 bytes	medium length text data	String
BLOB	0-65,535 bytes	long text data in binary form	String

3.3 Date Types

①Detailed table:

type of data	size	illustrate	Corresponding Java type
DATETIME	8 bytes	The range is from 1000 to 9999 , and no time zone retrieval and conversion will be performed.	java.util.Date 、 java.sql.Timestamp
TIMESTAMP	4 bytes	The range is from 1970 to 2038 , the current timezone is automatically retrieved and converted.	java.util.Date 、 java.sql.Timestamp

②Note:

Time said it is best not to use varchar

4. Operation of the table

When you need to operate the tables in the database, you need to use the database first;

(i.e. refer to show databases first)

4.1 View the structure of the table

①Format: (This operation needs to select the database in advance)

show + tables;

②Demo:

③View the structure of the table: (View the type, name, etc. of each column) 

desc + table name;

4.2 Create a table

 ①Format: (note that the column name is in the front, and the type is in the back!!!)

create table table name (column name type, column name, type.....)

②Demo:

4.3 Delete Table

①Format:

drop+table+table name;

②Demo:

③Note:

Deleting a table is also a very dangerous operation and needs to be thought twice

Suggestion: When there is a lot of code, you can type on text documents or other compilers, which is easier to check~~~

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