MySQL automatic primary key

Test environment: winxp+sql server2000:

primary key means that this table is the primary key. If you only want to add columns automatically, you can leave this statement unnecessary.

create table #aa(id int identity(1,1) primary key,name char(20))

insert into #aa(name)values('aa')

insert into #aa(name)values('bb')

insert into #aa(name)values('cc')

select * from #aa

If your table already exists, e.g. test table, create table test (name char(20));

Now you need to add an auto-incrementing id, alter table test add id int primary key auto increment; That's it, and the id value will be incremented at the same time.

Here's how it works:

1. Create a table.

create table t5

id int auto_increment,name varchar(20) primary key,key(id));

The name field is the primary key, and the id field is an auto-incrementing field.

2. Try to insert data:

insert into t5 (name) values ('a');

Execution result: It can be seen that for the first time, the id field is 1.

3. Insert the second data:

insert into t5 (name) values ('b');

Result: The second insertion is 2, so that the self-increment is achieved.

Just set that field to the key, MySQL allows you to add autoincrement to the key, but only one column can be autoincremented. You add a key (index) to the original table as described below, and then you can auto-increment on the column you want.

alter table add index name ( column );

However, only one column can be self-incrementing. You add a key to the original table as follows the following statement.

MySQL can only have one autogrow field per table, and this autogrow field can be used as a primary key or as a non-primary key, but please note that when you use an autogrow field as a non-primary key, you must add a unique index to it, otherwise the system will report an error. For example:

1.Set the autogrow field as the primary key.

create table t1 (id int auto_increment primary key,sid int);

2.Set the autogrow field to a non-primary key, noting that the unique key must be added explicitly.

create table t2 (sid int primary key,id int auto_increment unique);

3.If you set the autogrow field to a non-primary key, an error will be reported if you do not add a unique index, as shown below.

create table t3 (sid int primary key,id int auto_increment)。

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