“无法添加外键约束”

Question

First i created this table name DEPARTMENT and afterwards i created Dept_Locations which has the foreign key reference in this table at Dnumber.

create table DEPARTMENT(
    Dname               varchar(20)     NOT NULL,
    Dnumber             int             NOT NULL,
    Mgr_ssn             char(9)         NOT NULL,
    Mgr_start_date      date            NOT NULL,
    Primary key(Dnumber),
    Unique(Dname)
);

Code for Dept_Locations table:

create table Dept_Locations(
    Dnum char(4),
    Dlocation varchar(16),
    primary key(Dnum,Dlocation),
    foreign key(Dnum) references DEPARTMENT(Dnumber));

Now when i execute this code for creating Dept_Locations I get an error:

Cannot add foreign key constraint.

Answer 1

The foreign key must be the same data type as the column it's referring to.

The column Dnumber in DEPARTMENT is int , and the column Dnum is defined as char(4) - but it must be defined as int .

Answer 2

Think the datatypes differ.

Dnum char(4)

Dnumber int

Answer 3

You primary key and the column with the foreign key constraint are different sizes - try:

create table DEPARTMENT(
    Dname               varchar(20)     NOT NULL,
    Dnumber             char(4)          NOT NULL, # <-- Note the size change
    Mgr_ssn             char(9)         NOT NULL,
    Mgr_start_date      date            NOT NULL,
    Primary key(Dnumber),
    Unique(Dname)
);

(or update the other to match)