mysql - identity column

# Identity column 
/ * 
is also known from the growth column 
Meaning: You can not manually insert the value, the system provides a default sequence value 

characteristics: 
1. Expresses its columns and primary key must match it? Not necessarily, but it requires a Key 
2. A table can have only one identity column! 
3. Identify the type of column restrictions? Only numeric type (int, a float, Double) 
4. identity column can set auto_increment_increment = 3; setting step, 
may be manually inserted by a value set starting value 

* / 
# a provided identity column when creating the table 
create table tab_identity ( 
	id int Primary Key, 
	name VARCHAR (20) 
	); 
Show the Tables; 
# insert data 
iNSERT INTO tab_identity values (1, "john"); 
iNSERT INTO tab_identity values (2, "Lily"); 
# here's id number you need to manually enter 
select * from tab_identity; 

if you want to automatically insert id columns, you need to 
drop the Table IF EXISTS tab_identity; 
the Create the Table tab_identity (
	ID int Primary Key AUTO_INCREMENT, 
	name VARCHAR (20 is) 
	); 


# insert data 
INSERT INTO tab_identity values (null, "John"); 
INSERT INTO tab_identity values (null, "Lily"); 

INSERT INTO tab_identity (name) values ( "Lucy ") 


# View auto_increment variable parameters 
show variables like"% auto_increment% " ; 
can be modified by modifying the amplitude of the updated variable step size actually 
auto_increment_increment parameters: step 
auto_increment_offset parameters: starting position 
# changing the step 
SET = auto_increment_increment. 3; 

Show variables like "% auto_increment%"; 
the data table is deleted # 
TRUNCATE table tab_identity; 
SELECT * from tab_identity; 
# reinsert data 
insert into tab_identity values (null, " john");
INTO tab_identity values INSERT (null, "Lily"); 
INSERT INTO tab_identity (name) values ( "lucy") 
	); 
Show the Variables like " AUTO_INCREMENT%% "; 
SET = auto_increment_increment. 1;
) At this time in the table is the order id 1,4,7 ,. . . .

# Starting value is not supported by similar steps in the modification, how to modify it? 
# Delete the data in the table 
TRUNCATE Table tab_identity; 
SELECT * from tab_identity; 
# reinsert data 
INSERT INTO tab_identity values (10, "John"); 
INSERT INTO tab_identity values (null, "Lily"); 
INSERT INTO tab_identity (name) values ( "lucy"); 
at this time the table is the order id 10,13,16 ,. . . . 
The start value is modified to 


# two, provided an identity column (similar modifications constraint) is modified table 
# table is prepared 
drop Table IF EXISTS tab_identity; 
Create Table tab_identity ( 
	ID int, 
	name VARCHAR (20 is)
desc tab_identity; 
# Set ID column 
ALTER Table tab_identity Modify column ID int Primary Key AUTO_INCREMENT; 

# insert data 
INSERT INTO tab_identity values (null, "John"); 
INSERT INTO tab_identity values (null, "Lily"); 
INSERT INTO tab_identity (name ) values ( "lucy"); 


# Third, delete the identity column when you modify the table 
the ALTER tab_identity the modify the table column the above mentioned id int; 
Show index from tab_identity;