MySQL create and manage tables

Catalog

1. Basic knowledge of

1.1 A procedure for storing data

1.2 Identifier naming rules

1.3 MySQL Data types in

2. Create and manage databases

2.1 Create database

2.2 Using a database

2.2.1 View all current databases

2.2.2  Use / Switch database

2.2.3  View the database currently in use

2.2.4  View all tables under the specified library

2.2.5  View database creation information

 2.3 modify the database

2.3.1 Change the database character set

2.4 Delete database

3. Create table

3.1 How it was created 1

3.2 How it was created 2

3.2.1  practice 1： Create a table emplooyees_copy, Realize to employees Replication of tables , Including data

 3.2.2  practice 2： Create a table emplooyees_blank, Realize to employees Replication of tables , Excluding data

4. Modify table

4.1 Append a column

4.2 Modify a column

 4.3 Rename a column

4.4 Delete a column

 5. rename table

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1. Basic knowledge of

1.1 A procedure for storing data

         Storing data is the first step in processing data . Only by storing the data correctly , We can carry out effective processing and Analysis . otherwise , It can only be a mess , Do not know how to start .

         that , How can we put all kinds of business-related 、 Complex data , Orderly 、 Store it efficiently ？ stay MySQL in , A complete data stored procedure has a total of 4 Step , Create a database 、 Acknowledgement field 、 Create data table 、 insert data .

         We need to create a database first , Instead of creating a data table directly ？

         Because from the level of system architecture ,MySQL The order of database system from large to small is database server 、 database 、 Data sheet 、 Data table Rows and columns .

        MySQL The database server was previously installed . therefore , Let's start by creating a database .

1.2 Identifier naming rules

        1. Database name 、 The table name must not exceed 30 Characters , Variable names are limited to 29 individual

        2. Must contain only A–Z, a–z, 0–9, _ common 63 Characters

        3. Database name 、 Table name 、 Do not include spaces in object names such as field names

        4. The same MySQL In software , The database cannot have the same name ; In the same library , A watch cannot have the same name ; In the same table , The field cannot have the same name

        5. You must ensure that your field has no and reserved words 、 Database systems or common methods conflict . If you insist on using , Please be there. SQL Use in statement `（ mark of emphasis ） Lead up

        6. Keep field names and types consistent ： When naming fields and specifying data types for them, be sure to ensure consistency , If the data type is an integer in a table , Then don't turn into character in another table

1.3 MySQL Data types in

type	Examples of types
Integer types	TINYINT、SMALLINT、MEDIUMINT、INT( or INTEGER)、BIGINT
Floating point type	FLOAT、DOUBLE
Fixed point number type	DECIMAL
A type of	BIT
Date time type	YEAR、TIME、DATE、DATETIME、TIMESTAMP
Text string type	CHAR、VARCHAR、TINYTEXT、TEXT、MEDIUMTEXT、LONGTEXT
Enumeration type	ENUM
Collection types	SET
Binary string type	BINARY、VARBINARY、TINYBLOB、BLOB、MEDIUMBLOB、LONGBLOB
JSON type	JSON object 、JSON Array
Spatial data types	Single value ：GEOMETRY、POINT、LINESTRING、POLYGON; aggregate ：MULTIPOINT、MULTILINESTRING、MULTIPOLYGON、GEOMETRYCOLLECTION

among , Several common types are introduced as follows ：

data type	describe
INT	from -2^31 To 2^31-1 The integer data of . The storage size is 4 Bytes
CHAR(size)	Fixed length character data . If not specified , The default is 1 Characters , Maximum length 255
VARCHAR(size)	Variable length character data , Save according to the actual length of the string , Length... Must be specified
FLOAT(M,D)	Single precision , Occupy 4 Bytes ,M= Integer bit + Decimal places ,D= Decimal places . D<=M<=255,0<=D<=30, Default M+D<=6
DOUBLE(M,D)	Double precision , Occupy 8 Bytes ,D<=M<=255,0<=D<=30, Default M+D<=15
DECIMAL(M,D)	High precision decimal , Occupy M+2 Bytes ,D<=M<=65,0<=D<=30, The maximum value range is related to DOUBLE identical .
DATE	Date data , Format 'YYYY-MM-DD'
BLOB	Long text data in binary form , Up to 4G
TEXT	Long text data , Up to 4G

2. Create and manage databases

2.1 Create database

The way 1： Create database

        CREATE  DATABASE   Database name ;

The way 2： Create database and specify character set

        CREATE  DATABASE   Database name   CHARACTER  SET   Character set ;

The way 3： Determine whether the database already exists , If it does not exist, create the database （ recommend ）

        CREATE  DATABASE  IF  NOT  EXISTS   Database name ;

  

2.2 Using a database

2.2.1 View all current databases

grammar ：

        SHOW DATABASES;  # There is one S, Representing multiple databases

2.2.2  Use / Switch database

grammar ：

        USE Database name ;

2.2.3  View the database currently in use

grammar ：

         SELECT DATABASE(); # One used mysql Global functions in

2.2.4  View the table under the specified library

grammar ：

        SHOW TABLES 【FROM Database name 】; # see 【 Specify... In the database 】 All tables

        SHOW TABLE Table name 【FROM Database name 】;# see 【 Specify... In the database 】 Designated table

2.2.5  View database creation information

grammar ：     

        SHOW CREATE DATABASE Database name ;

         perhaps ：

        SHOW CREATE DATABASE Database name \G

 2.3 modify the database

2.3.1 Change the database character set

grammar ：

        ALTER DATABASE Database name CHARACTER SET Character set ; # such as ：gbk、utf8 etc.

         Be careful ：DATABASE You can't change your name . Some visualization tools can be renamed , It's building a new library , Copy all tables to the new library , Delete the old library again .

2.4 Delete database

The way 1： Delete the specified database , If the database does not exist , False report .

        DROP DATABASE Database name ;

The way 2： Delete the specified database （ recommend ）

        DROP DATABASE IF EXISTS Database name , If the database does not exist , It ends silently , No mistake. .

3. Create table

3.1 How it was created 1

Must have ：

        1.CREATE TABLE jurisdiction

        2. Storage space

Grammar format ：

        CREATE TABLE [IF NOT EXISTS] Table name (

         Field 1, data type [ constraint condition ] [ The default value is ],

         Field 2, data type [ constraint condition ] [ The default value is ],

         Field 3, data type [ constraint condition ] [ The default value is ],

        ……

        [ Table constraints ]

        );

         Combined with the IF NOT EXISTS keyword , said ： If the data table to be created does not exist in the current database , Then create a data table ;

         If the data table to be created already exists in the current database , The table creation statement is ignored , No longer create data table .

Must specify ：

        1. Table name

        2. Name ( Or field name ), data type , length

Optional designation ：

        1. constraint condition

        2. The default value is

notes ：

        1.MySQL When executing the table creation statement , take id The type of the field is set to int(11), there 11 It's actually int Type specifies the display width , The default display width is 11. You can also specify the display width of data when creating a data table .

        2. stay MySQL 8.x In the version , No longer recommended as INT Type specifies the display length , This syntax may be removed in future versions .

3.2 How it was created 2

         Use AS subquery Options , Combine creating tables with inserting data

         The specified column should correspond to the column in the subquery one by one

         Define columns by column names and default values

3.2.1  practice 1： Create a table emplooyees_copy, Realize to employees Replication of tables , Including data

 3.2.2  practice 2： Create a table emplooyees_blank, Realize to employees Replication of tables , Excluding data

4. Modify table

Modifying a table means modifying the structure of a data table that already exists in the database .

Use ALTER TABLE Statement can realize ：

        1. Add columns to existing tables

        2. Modify columns in existing tables

        3. Delete columns from existing tables

        4. Rename columns in an existing table

4.1 Append a column

The syntax is as follows ：

        ALTER TABLE Table name ADD 【COLUMN】 Field name Field type 【FIRST|AFTER Field name 】;

 

4.2 Modify a column

You can modify the data type , length 、 Default values and locations

Modify field data type 、 length 、 The default value is 、 The syntax format of the location is as follows ：

        ALTER TABLE Table name

        MODIFY 【COLUMN】 Field name 1 Field type 【DEFAULT The default value is 】【FIRST|AFTER Field name 2】;

 4.3 Rename a column

         Use CHANGE old_column new_column dataType Clause renames the column .

The syntax is as follows ：

        ALTER TABLE Table name CHANGE 【column】 Name New column names New data types ;

4.4 Delete a column

The syntax format of deleting a field in the table is as follows ：

        ALTER TABLE Table name DROP 【COLUMN】 Field name ;

 5. rename table

Mode one ： Use RENAME

         Grammar format ：

                REMANE TABLE Table name

                TO The new name of the table ;

  Mode two ： Use ALTER

          Grammar format ：

                ALTER TABLE Table name

                RENAME 【TO】 The new name of the table ;

6. Delete table

         stay MySQL in , When a data sheet No association with any other data table when , Can be Delete the current data table directly .

         Data and structures are deleted

         All running related transactions are committed

         All relevant indexes are deleted

        DROP TABLE Statement cannot be rolled back （ That is, it cannot be undone . If you delete it, you can't find it back , Unless there is a backup ）

Grammar format ：

        DROP TABLE [IF EXISTS] Data sheet 1 [, Data sheet 2, …, Data sheet n];

 

IF EXISTS Means ： If the corresponding data table exists in the current database , Delete the data table ; If there is no... In the current database In the corresponding data sheet , Delete statement is ignored , No longer delete the data table .

7. Clear the table

7.1 TRUNCATE TABLE sentence

TRUNCATE TABLE language sentence ：

        1. Delete all data in the table , Keep the table structure

        2. Free the storage space of the table

Grammar format ：

        TRUNCATE TABLE Table name ;

7.2 DELETE FROM sentence ：

Grammar format ：

        DELETE FROM Table name ;( You can add filter conditions , It means deleting some data )

7.3 contrast TRUNCATE TABLE And DELETE FROM

7.3.1  contrast TRUNCATE TABLE And DELETE FROM

The same thing ： All data in the table can be deleted , While preserving the data structure

Difference ：

        TRUNCATE TABLE： Once you do this , Delete all table data . meanwhile , Data cannot be rolled back

        DELETE FROM： Once you do this , All table data can be deleted （ No where）. meanwhile , data

                                      Rollback is possible

explain ：

        1.TRUNCATE TABLE Than DELETE Fast , It also uses less system and transaction log resources , but TRUNCATE nothing Transaction without triggering TRIGGER, Possible accidents , so This statement is not recommended in development code .

        2.TRUNCATE TABLE In function and without WHERE Clause DELETE Same statement .

7.3.2 DCL Medium COMMIT and ROLLBACK expand

COMMIT : Submit data . Once you do this , The data is permanently stored in the database , Means that the data is not

                  You can roll back .

ROLLBACK :  Once you do this , Data rollback can be realized . Roll back to the last COMMIT after .

7.3.3 DDL And DML explain

        1.DDL Once the operation is performed , You can't roll back . Instructions SET autocommit = FALSE Yes DDL Operation failure （ Because at the end of execution DDL After the operation , This must be done COMMIT. And this COMMIT The operation is not affected by

SET autocommit = FALSE  Affected ）

        2.DML Default operation of , Once executed , It cannot be rolled back . however , If in execution DML Before , Yes SET autocommit = FALSE , The rollback can be realized by executing the operation .

7.3.4 TRUNCATE TABLE And DELETE FROM Roll back the demo