Learn MySQL from scratch - database and data table operations

​ Preface

Today we will learn the core content , Learn and practice how to modify database tables and contents in tables , Delete , Rename and so on .（ I want to see how many kids love learning on weekends , Where are you ,O(∩_∩)O ha-ha ~）

1、 Catalog

• Database operation ： Delete database 、 modify the database .

• Data table operation ： rename table 、 Delete table 、 Add a column of data at the specified position 、 Delete a column 、 Rename a column 、 Change data type 、 Delete a row of records .

2、 step

This section will be operated through practice , Lead you to be familiar with the modification and deletion of the database .

Before starting this experiment , You need to download the relevant code first .

This code can create two new databases , Respectively called test and mysql_shiyan , And in mysql_shiyan Built in database 4 Tables （department,employee,project,table_1）, Then insert data into it .

The statement of creating database and data table can be downloaded here .

https://github.com/rongweihe/MoreThanCode/tree/master/mysql_shiyan

When the download is complete , Enter the command to turn on MySQL Service and use root The user login ：

# open  MySQL  service 
sudo service mysql start

# Use  root  The user login 
mysql -u root

Download the SQL5 Under the table of contents , Yes 1 File MySQL_05_01.sql .

Be careful ： If the previous mysql_shiyan There is still , You can use it first drop database mysql_shiyan Delete . Load the data in the file you downloaded , If you download in home Under the table of contents , You need to in MySQL Enter the command in the console , Complete experimental preparation ：

source /home/MySQL_05_01.sql;

3、 Database operation

Use command SHOW DATABASES; You can see that just now MySQL_05_01.sql Two databases generated by the file ：

Now let's run the command to delete the file named test The database of ：DROP DATABASE test; Now use the command again SHOW DATABASES; You can find ,test The database has been successfully deleted ：

​ About modifying the database ： at present MySQL There is no method to modify the database name , Because this has led to a series of safety problems . In the old version RENAME DATABASE You can modify the database name , In subsequent experiments , Officials should find out the danger of this order , So at the moment MySQL 5.1.23 This command has been removed in version and later .

Someone on the Internet said that if it is MySQL The storage engine is MyISAM Words , Just modify DATA The name of the folder with the database name under the directory can be modified , I haven't tried this , You can try to see if you can modify .

in fact , Database names rarely have to be modified , If you have to , It is safer to rebuild a new library , Then transfer the data from the old library to the new library , And don't delete the old database for the time being , In case of data loss .

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​3.1 rename table

There are many forms of statements to rename a table , following 3 The effect of the two formats is the same ：

RENAME TABLE  Original name  TO  New name ;
ALTER TABLE  Original name  RENAME  New name ;
ALTER TABLE  Original name  RENAME TO  New name ;

Access to database mysql_shiyan ：

use mysql_shiyan

Use the command to try to modify table_1 The name is table_2 ：

RENAME TABLE table_1 TO table_2;

​3.2 Delete table

Statement to delete a table , Similar to the statement used just now to delete the database , This is the format ：

DROP TABLE  Table name ;

For example, we put table_2 Table delete ;

DROP TABLE table_2;

​3.3 Add a row

Modification of table structure , It will actually affect the structure of the data table , Sometimes small mistakes can have irreparable consequences , So operate carefully . In the actual production environment , Be sure to get the permission of your superiors , Otherwise, do not modify it easily .

Let's see , Add a column to the table in the form of ：

ALTER TABLE  Table name  ADD COLUMN  Name   data type   constraint ;
ALTER TABLE  Table name  ADD  Name   data type   constraint ;

Now? employee Table has id、name、age、salary、phone、in_dpt this 6 Columns , We try to join height ( height ) A column and specify DEFAULT constraint ：

ALTER TABLE employee ADD height INT(4) DEFAULT 170;

​ You can find ： Newly added columns , It is placed on the far right of this table by default . If you want to insert the added column in the specified position , It needs to be used at the end of the statement AFTER key word (“AFTER Column 1” Indicates that the new column is placed in “ Column 1” Behind ).

remind ： Statement INT(4) Not the number of bytes representing an integer , Instead, it represents the display width of the value , If the fill character is set to 0, be 170 Is shown as 0170

For example, we add a new column weight( weight ) Put in age( Age ) Behind ：

ALTER TABLE employee ADD weight INT(4) DEFAULT 120 AFTER age;

The above effect is to add the new column after a certain position , If you want to put it in the first column , Then use FIRST key word , If statement ：

ALTER TABLE employee ADD test INT(10) DEFAULT 11 FIRST;

The effect is as follows ：

​3.4 Delete a column

Deleting a column in the table is very similar to the statement format of adding a column just used , Just put the key words ADD Change it to DROP , The statement does not need to be followed by a data type 、 Constraint or location information . Specific sentence format ：

ALTER TABLE  Table name  DROP COLUMN  Name ;
 or ：ALTER TABLE  Table name  DROP  Name ;

Let's take what we just added test Delete ：

ALTER TABLE employee DROP test;

​3.5 Rename a column

To be exact , Rename this statement, which is to modify a column (CHANGE) ：

ALTER TABLE  Table name  CHANGE  The original name   New column names   data type   constraint ;

Be careful ： After this rename statement “ data type ” Don't omit , Otherwise, renaming fails .

When the original column name is the same as the new column name , Specify a new data type or constraint , It can be used to modify data types or constraints . It should be noted that , Changing the data type may result in data loss , Therefore, it also needs to be used carefully .

We use this statement to “height” Rename a column to Hanyu Pinyin “shengao” , The effect is as follows ：

ALTER TABLE employee CHANGE height shengao INT(4) DEFAULT 170;

3.6 Change the data type

To change the data type of a column , In addition to using the CHANGE Out of statement , You can also use this MODIFY sentence ：

ALTER TABLE  Table name  MODIFY  Name   New data types ;

Here we need a very special reminder , You must be careful when modifying data types , Because this may cause your data loss .

It is best to create an online database before , Consider the type of data , You can create a temporary database to operate the data and see if it meets the expectations , Confirm that there is no problem in creating an online database . Before trying to modify the data type , Be sure to carefully consider the possible consequences of the operation .

3.7 Modify a value in the table

Most of the time, what we need to modify is not the whole database or the whole table , It's one or more data in the table , This requires us to use the following command to achieve precise modification ：

UPDATE  Table name  SET  Column 1= value 1, Column 2= value 2 WHERE  Conditions ;

for instance , Recently, what about Xiao Wei , Due to outstanding work performance , The company gave him a raise , Then we should xiaowei Of salary Change it to 20000, You can write like this ：

UPDATE employee SET age=25 ,salary=20000 WHERE name='xiaowei';

Be careful ： Be sure to have WHERE Conditions , Otherwise, there will be unwanted consequences

3.8 Delete a row of records

Delete a row of data in the table , You have to add WHERE Conditions , Otherwise, the whole column of data will be deleted . Delete statements ：

DELETE FROM  Table name  WHERE  Conditions ;

We tried to xiaohong Data deletion ：

DELETE FROM employee WHERE name='xiaohong';

​ There is no more in the database xiaohong 了 .

4、 summary

In this experiment, we learned about the database 、 Database table 、 Modification and deletion methods of database table items and records .

Reference material

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original text ： Learn from scratch MySQL -- Database and data table operations