Primary key conflict-a table can have two primary keys.

Primary key conflict-a table can have two primary keys. A new primary key statement can have two primary keys in the table: primary key for table xs 'is not unique: first, the two fields are converted into the primary key, which is inaccurate.
Strictly speaking, because the two fields you select constitute the primary key, that is, "student ID + course number"
The combination is a primary key. This combination cannot be repeated and cannot be empty at the same time, rather than "student ID"
"Course number" is a primary key. If you do not believe it, you can view the tb system table. In the table www.2cto.com, you can add a primary key for the existing primary key: Statement example: alter table AC04_blbc drop primary key;
Alter table AC04_blbc modify sbxz VARCHAR (8) not null,
Add constraint PK_AC04_blbc primary key (AAE002, AAC001, AAE041, sbxz );
Author chen11-1