数据库中没有主键的表？

Question

I have 4 tables in DB, and they are regularly connected. Now i have to make new table without new primary key, but it must have foreighn key from other table.

Is it possible?

Answer 1

if your relationship from your new table to other table is one to one, the foreign key can be used as your primary key. otherwise you will need a single or composite primary key to avoid redundancy of rows.

Answer 2

Yes. To do this you need an index on the column involved in the foreign key. But it need not be a unique index.

Note this. Primary keys, and all indexes, can have multiple columns in them. If your new table is a so-called join table showing many-to-many relationships between rows of two other tables, you can create a primary key from both columns. For example, if you have a student table with the primary key student_id , and a teacher table with the primary key teacher_id , you could create a table like this.

CREATE TABLE student_teacher (
    student_id INT NOT NULL,
    teacher_id INT NOT NULL,
    PRIMARY KEY (student_id, teacher_id),
    UNIQUE INDEX teacher_student (teacher_id, student_id)
)