Cannot add foreign key constraint SQL when building schema

Question

I'm taking a intro to database class right now. For a homework assignment I was given a piece of SQL code and told to just run some queries and give the results back. The problem is I can't even build the schema given with the code my teacher gave me. Here is this code:

CREATE TABLE emp (
  name char(15),
  dno int,
  FOREIGN KEY (dno) REFERENCES dept(dno)
  ON DELETE SET NULL
  ON UPDATE CASCADE
  );

CREATE TABLE dept (
  dno int,
  location char(30)
 );

INSERT INTO emp(name,dno) VALUES
("Tom",111),
("Mary",111),
("Jack",222),
("Henry",222);

INSERT INTO dept(dno, location) VALUES
(111,"Irvine"),
(222,"LA"),
(333,"SF");

When this runs I get a error saying Cannot add foreign key constraint . I tried doing this via MySQL workbench and SQL Fiddle, both of which produce the same error. I don't really know whats wrong with the code given to me though and after looking online, couldn't seem to see anything obvious.

Answer 1

It may be part of the exercise for you to figure this out. However, assuming that it is not, here are important observations:

• The foreign key reference should be to a primary key, and neither table has primary keys.
• The table being referenced has to be defined before it can be referred to.

You can work on fixing these problems. Here is a SQL Fiddle with the right definitions.

Answer 2

The definition of the first table (emp) has a reference to a table that has not yeat been created (dept).You have to create first 'dept' to create a foreign key of it into another table.

Execute the queries in this order:

CREATE TABLE dept (
  dno int,
  location char(30)
 );

CREATE TABLE emp (
  name char(15),
  dno int,
  FOREIGN KEY (dno) REFERENCES dept(dno)
  ON DELETE SET NULL
  ON UPDATE CASCADE
  );

INSERT INTO emp(name,dno) VALUES
("Tom",111),
("Mary",111),
("Jack",222),
("Henry",222);

INSERT INTO dept(dno, location) VALUES
(111,"Irvine"),
(222,"LA"),
(333,"SF");

Answer 3

尝试：CONSTRAINT dno_fk外键（dno）参考DEPT（dno）删除级联