MySQL can only have one auto growth field per table. This auto growth field can be used as a primary key or a non primary key. However, please note that when you use an auto growth 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 auto grow field as the primary key.

create table t1 (id int auto_ increment Primary key,sid int);

2. Set the auto grow field to a non primary key. Note that the unique key must be added explicitly.

create table t2 (sid int primary key,id int auto_ increment Unique);

3. Set the auto growth field to a non primary key. If no unique index is added, an error will be reported * *, as shown in the following statement

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

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