MySQL adds, modifies, and deletes table fields, field data types, and lengths (with various actual case statements)

1、 New fields

notes ：add It is often used to add a field to an existing table .

grammar ：

alter table Table name add Field name Field data type ( length ) [character set Field code collate Field sort code ] [NOT NULL] [DEFAULT 0] [COMMENT ‘ remarks ’] [(FIRST)|(AFTER The field... Already exists in the table )]

among , Has not been [] The parentheses include what is required , By [] The parentheses include settings that can be customized according to the field limits .

1. CHARACTER SET Field code COLLATE Field sort code ： Used to set the encoding value of a special field . for example ： If you want to store the string containing the expression in the table field , But the database code is utf-8, Then the expression will be garbled . At this point, you need to change the field code to utf8mb4.（alter table Table name add Field name Field data type ( length ) character set utf8mb4 collate utf8mb4_general_ci）
2. NOT NULL： Represents that the field cannot be empty .
3. DEFAULT value ： Set the default value . It can be a number or a string .
4. COMMENT ‘ remarks ’： Note information of this field .
5. (FIRST)|(AFTER Fields in the table )： Create the field in the first column or after the field in the table .

2、 Modify field name

notes ：change It is often used to modify field names , But with modify equally . You can also change the field type and length . And the modified field ,alter table There are no restrictions such as default values set in the statement , The limit of the original field will be reset . Here's an example name1 Fields do not inherit name Comments for the field 、 Default value and other information .

grammar ：

alter table [ Table name ] change [ Field name ] [ Field new name ] [ Type of field ]

for example ： take a Tabular name Renamed as name1

alter table a change name name1 varchar(20);

3、 Modify field type 、 length

notes ：modify It is often used to modify the type and length of fields in a table .

grammar ：

alter table [ Table name ] modify column [ Field name ] [ New data types ( Modified length )]

for example ： modify a In the table name The type of field is varchar, The length is 20. Then change the length to 30.

alter table a modify column name varchar(20);
alter table a modify column name varchar(30)

4、 Delete field

grammar ：

alter table Table name drop column Field name ;

for example ： Delete a In the table name Field

alter table a drop column name;

other

If the table field is modified , Not in force . Execute the following statement to refresh the table .

analyze table  Table name 

Actual case

1、 Set the field as the primary key ( Not commonly used )

In general, in practice , The primary key is specified during table creation and will not be modified , But if it needs to be modified later , You need to delete the existing primary key and set a new primary key .

Deleting directly will result in an error .Multiple primary key defined

Or if the deleted primary key is a self growing column, an error will be reported , First remove the self growth attribute , Delete the primary key .

--  remove a surface id The self growth property of the column 
alter table a modify id int(10) unsigned;
--  Delete id Primary key 
alter table a drop primary key;

--  Set up name Primary key 
alter table a add primary key(name); 
--  Or set up id、name It's a composite primary key , Also called double primary key 
alter table a add primary key(id,name);

2、 Add a non empty field in the table

--  surface a newly added name Field cannot be empty 
alter table a add name varchar(20) NOT NULL;

3、 Add a table default value field

--  surface a newly added createyear The default value of the field is 2021
alter table a add createyear varchar(20) DEFAULT '2021';

--  additional ： Delete field defaults 
alter table a alter createyear drop default;
--  additional ： Add default values to existing fields 
alter table a alter createyear set default '2021';

4、 Modify table self increment

alter table a auto_increment = 1;