更改表以添加外键

Question

I am using MySQL.

I have an existing table named school with hundreds of rows data be populated. Now I have another table named student , its primary key is " sid ".

I would like to alter my school table to have a foreign key reference to a student .

I tried the following sql statement:

ALTER TABLE school ADD FOREIGN KEY (sid) REFERENCES student(sid);

But I get error:

ERROR 1072 (42000): Key column 'sid' doesn't exist in table

What is the correct way to alter table to add a foreign key to another table?

Answer 1

You have to add the column sid on your table first.

ALTER TABLE school ADD COLUMN sid [INT, VARCHAR2(10];
ALTER TABLE school ADD FOREIGN KEY (sid) REFERENCES student(sid);

PS: I put [INT, VARCHAR2(10] because I don't know what type student(sid) is. Just change to the correct one.

Answer 2

where do you want to link your foreign key?

it seems that you missed the key to link against in the school table:

As far as i can imagine you want to link a student to his school.

So what i'd do is to add a column to the student table:

ALTER TABLE STUDENT
ADD COLUMN SCHOOL_ID INT NOT NULL;

then i'd create the foreign key in STUDENT table to point to SCHOOL

ALTER TABLE STUDENT
ADD FOREIGN KEY (F_SCHOOL_ID) REFERENCES SCHOOL(ID);

This is the best way and not the other way round.