So I'm working on a project in school in which we use MySQL to make a database and complete various tasks about school subjects and exam boards. One of my questions was to:
"Create and populate a third table called entries, again using query scripts. This table should contain foreign keys to allow sensible links to be made with the other two tables, together with the dates of each exam."
When carrying out this task I tried this code out
CREATE TABLE IF NOT EXISTS entries(
subject_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
subject_name VARCHAR(20) NOT NULL,
level_of_entry VARCHAR(10) NOT NULL,
exam_board VARCHAR(10) NOT NULL,
date_of_exam DATETIME NOT NULL,
PRIMARY KEY (date_of_exam),
FOREIGN KEY (subject_id) REFERENCES subjects(subject_id)
);
And that worked fine, but when I tried this:
CREATE TABLE IF NOT EXISTS entries(
subject_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
subject_name VARCHAR(20) NOT NULL,
level_of_entry VARCHAR(10) NOT NULL,
exam_board VARCHAR(10) NOT NULL,
date_of_exam DATETIME NOT NULL,
PRIMARY KEY (date_of_exam),
FOREIGN KEY (subject_id) REFERENCES subjects(subject_id),
FOREIGN KEY (subject_name) REFERENCES subjects(subject_name)
);
It threw up and error message stating:
"ERROR 1215 (HY000): Cannot add foreign key constraint"
Any clues on how to add multiple foreign key statements without it causing an error. Also I did try using the ALTER TABLE function and that didn't work,
Many thanks
Andrew.
Related
I know this question has been asked several times but it seemed like the problem was due to different data types between parent and child rows.
In my case, the data types are the same but I'm still getting the error. Here's my code
CREATE TABLE STUDENT_2(
StudentNumber INT NOT NULL AUTO_INCREMENT,
StudentName VARCHAR(50) NULL,
Dorm VARCHAR(50) NULL,
RoomType VARCHAR(50) NOT NULL,
CONSTRAINT STUDENTPK PRIMARY KEY(StudentNumber)
);
CREATE TABLE DORM_COST(
RoomType VARCHAR(50) NOT NULL,
DormCost DECIMAL(7,2) NULL,
CONSTRAINT DORM_COSTPK PRIMARY KEY(RoomType),
CONSTRAINT DORM_COST_FK FOREIGN KEY(RoomType)
REFERENCES STUDENT_2(RoomType)
ON UPDATE CASCADE
ON DELETE CASCADE
);
Where DORM_COSTS' foreign key cannot be added.
Thanks!
You want the foreign key reference on the table that has the foreign key, not the primary key. So, that would be:
CREATE TABLE DORM_COST (
RoomType VARCHAR(50) NOT NULL,
DormCost DECIMAL(7,2) NULL,
CONSTRAINT DORM_COSTPK PRIMARY KEY(RoomType)
);
CREATE TABLE STUDENT_2(
StudentNumber INT NOT NULL AUTO_INCREMENT,
StudentName VARCHAR(50) NULL,
Dorm VARCHAR(50) NULL,
RoomType VARCHAR(50) NOT NULL,
CONSTRAINT STUDENTPK PRIMARY KEY(StudentNumber),
CONSTRAINT fk_student2_roomtype FOREIGN KEY (RoomType) REFERENCES DORM_COST(RoomType)
);
Here is a db<>fiddle that shows that this works.
That said your data model seems quite strange.
I would expect a table called RoomTypes to have a primary key of RoomTypeId.
I would expect dorm_cost to have dates, because costs can vary from year to year.
I would expect different dorms to have similar room types -- singles, doubles, and so on.
I would expect those room types to vary, perhaps, by dorm.
Having trouble adding a foreign key to my table.
CREATE TABLE event (
id BIGINT(20) NOT NULL AUTO_INCREMENT,
issued DATETIME NOT NULL,
user VARCHAR(255) NOT NULL,
subject VARCHAR(255) NOT NULL,
attending BIGINT(255) NOT NULL,
attendees VARCHAR(255) NOT NULL,
organisers VARCHAR(255) NOT NULL,
place BIGINT(20) NOT NULL,
started DATETIME NOT NULL,
stopped DATETIME NOT NULL,
content LONGTEXT NOT NULL,
broadcasting TINYINT NOT NULL,
PRIMARY KEY (id),
FOREIGN KEY (place)
REFERENCES place (id),
FOREIGN KEY (user)
REFERENCES user (username)
)
The foreign key for place is executing fine but once I try adding user as a foreign key I keep getting the same error:
Error Code: 1005. Can't create table 'iservices.event' (errno: 150)
Can anyone help?
Picture of user table:
User table
Picture of place table:
Place table
Is there anyway of expanding the errors in MySQL Workbench?
a foreign key has to be unique, so make username unique (which is probably not the best idea) or choose something different like the user id.
My advice: add a primary auto-increment key to the user table and use it as the foreign-key.
Referenced column must be unique. It's OK in your case.
Columns in child and parent table must be of the same type. In your case they are different.
Could you please make sure user and place tables exist and have corresponding columns set as primary key? The below snipper works fine:
CREATE TABLE user(username VARCHAR(255) PRIMARY KEY);
CREATE TABLE place(id BIGINT(20) PRIMARY KEY);
CREATE TABLE event (
id BIGINT(20) NOT NULL AUTO_INCREMENT,
issued DATETIME NOT NULL,
user VARCHAR(255) NOT NULL,
subject VARCHAR(255) NOT NULL,
attending BIGINT(255) NOT NULL,
attendees VARCHAR(255) NOT NULL,
organisers VARCHAR(255) NOT NULL,
place BIGINT(20) NOT NULL,
started DATETIME NOT NULL,
stopped DATETIME NOT NULL,
content LONGTEXT NOT NULL,
broadcasting TINYINT NOT NULL,
PRIMARY KEY (id),
FOREIGN KEY (place)
REFERENCES place (id),
FOREIGN KEY (user)
REFERENCES user (username)
)
Here's SQL Fiddle.
I had encountered a similar error.
This can occur when you are trying to add a non-primary key from the primary table as the foreign key in the secondary table.
To avoid this, the foreign key in the secondary table has to be the primary key in the primary table.
I have researched thoroughly before asking this question including on this site.
I have a students table:
CREATE TABLE IF NOT EXISTS students(
student_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
first_name VARCHAR(20) NOT NULL,
middle_name VARCHAR(20),
last_name VARCHAR(40) NOT NULL,
email VARCHAR(60) NOT NULL,
password CHAR(40) NOT NULL,
reg_date DATETIME NOT NULL,
PRIMARY KEY (student_id),
UNIQUE (email));
I also have a subjects table:
CREATE TABLE IF NOT EXISTS subjects(
subject_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
subject_name VARCHAR(40) NOT NULL,
level_of_entry VARCHAR(20) NOT NULL,
exam_board VARCHAR(20) NOT NULL,
PRIMARY KEY (subject_id));
I am now creating a table to link the above tables:
CREATE TABLE IF NOT EXISTS entries(
exam_date DATETIME NOT NULL,
FOREIGN KEY (student_id) REFERENCES students (student_id),
FOREIGN KEY (subject_id) REFERENCES subjects (subject_id)
);
My problem is that when I try to declare the foreign keys in the third table called entries, I get an error stating that the subject_id foreign key is not in the table referenced.
ERROR 1072 (42000) : Key column 'student_id' doesn't exist in table, even though it is clearly contained inside the the students table and the same applies to 'subject_id' and the subject table.
I am certain that my syntax for declaring the foreign keys is correct so I am unsure how to fix the problem.
All help is appreciated. Thank you.
You forgot to create these two columns before applying your foreign key constraints :
CREATE TABLE IF NOT EXISTS entries(
exam_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
student_id INT UNSIGNED NOT NULL,
subject_id INT UNSIGNED NOT NULL,
exam_date DATETIME NOT NULL,
PRIMARY KEY (exam_id),
FOREIGN KEY (student_id) REFERENCES students (student_id),
FOREIGN KEY (subject_id) REFERENCES subjects (subject_id)
);
EDIT :
I advise you to add in every table a unique ID column (here : exam_id).
I would like to ask something that troubles me many many days...
Here is what I mean:
I create these two tables:
CREATE TABLE IF NOT EXISTS journal (
issn varchar(20) NOT NULL,
j_title varchar(100) NOT NULL,
j_publisher varchar(30) NOT NULL,
PRIMARY KEY (issn)
) ENGINE=InnoDB;
CREATE TABLE IF NOT EXISTS volume (
volume_no int(11) NOT NULL,
issn varchar(20) NOT NULL,
year int(11) NOT NULL,
PRIMARY KEY (issn,volume_no),
FOREIGN KEY (issn) REFERENCES journal(issn)
) ENGINE=InnoDB;
When I try to create this:
CREATE TABLE IF NOT EXISTS issue (
issue_no int(11) NOT NULL,
issue_pages varchar(10) NOT NULL,
issue_date varchar(10) NOT NULL,
issn varchar(20) NOT NULL,
volume_no int(11) NOT NULL,
PRIMARY KEY (issue_no,issn,volume_no),
FOREIGN KEY (issn) REFERENCES journal(issn),
FOREIGN KEY (volume_no) REFERENCES volume(volume_no)
) ENGINE=InnoDB;
it throws an error (errno 150)
The error is in the foreign key volume_no.
Without FOREIGN KEY (volume_no) REFERENCES volume(volume_no)
the table is created without a problem.... I can't explain what's going on... I have seen it many times again and again but nothing!! Does anybody know what's going on?
Thanks in advance!!
I could see that the foreign key doesnt include issn but which is actually included in primary key for volumn table.
CREATE TABLE IF NOT EXISTS issue ( issue_no int(11) NOT NULL,
issue_pages varchar(10) NOT NULL,
issue_date varchar(10) NOT NULL,
issn varchar(20) NOT NULL,
volume_no int(11) NOT NULL,
PRIMARY KEY (issue_no,issn,volume_no),
FOREIGN KEY (issn,volume_no) REFERENCES volume(issn,volume_no) ) ENGINE=InnoDB;
Look at the below sql fiddle.
http://sqlfiddle.com/#!2/55a63
maybe volume_no needs to be UNSIGNED
FOREIGN keys must reference a PRIMARY or a UNIQUE key in the parent table.
You need only one Foreign Key at table issue, not two. And it should reference the Primary Key of volume:
CREATE TABLE IF NOT EXISTS issue (
issue_no int(11) NOT NULL,
issue_pages varchar(10) NOT NULL,
issue_date varchar(10) NOT NULL,
issn varchar(20) NOT NULL,
volume_no int(11) NOT NULL,
PRIMARY KEY (issn, volume_no, issue_no),
FOREIGN KEY (issn, volume_no)
REFERENCES volume(issn, volume_no)
) ENGINE=InnoDB;
If the PK of the parent table is more than one field, the order of the fields in the FK must be the same as the order in the PK.
issue: FOREIGN KEY (issn, volume_no) REFERENCES volume(issn, volume_no)
These conditions must be satisfied to not get error 150:
The two tables must be ENGINE=InnoDB.
The two tables must have the same charset.
The PK column(s) in the parent table and the FK column(s) must be the same data type.
The PK column(s) in the parent table and the FK column(s), if they have a define collation type, must have the same collation type;
If there is data already in the foreign key table, the FK column value(s) must match values in the parent table PK columns.
source: MySQL Creating tables with Foreign Keys giving errno: 150
I had about the same issue with my database. It wasn't about the definition of the foreign key, actually it was the definition of the primary key field.
CREATE TABLE tblProcesses (
fldProcessesID SMALLINT(5) UNIQUE NOT NULL AUTO_INCREMENT ,
CREATE TABLE tblProcessesMessage (
fldProcesses TEXT NOT NULL,
fldProcessesID VARCHAR(15) NOT NULL DEFAULT ,
The primary key in tblProcesses (fldProcessesID) did not have the UNSIGNED keyword while the foreign key in tblProcessesMessage (fldProcessesID) had the UNSIGNED keyword. This keyword was causing the problem - inconsistent type of field. So i added the UNSIGNED keyword to fldProcessesID in tblPreocesses:
CREATE TABLE tblProcesses (
fldProcessesID SMALLINT(5) UNSIGNED UNIQUE NOT NULL AUTO_INCREMENT,
I hope that this will help you solve your problems.
Best regards,
Nicholas
Hi all I can't find the error in this table creation bit, seems really straight forward to be, here's what it's giving me:
ERROR 1064 at line 3: You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near 'FOREIGN KEY(courses_courseDepartmentAbbv))' at
line 8
DROP TABLE IF EXISTS courses;
CREATE TABLE courses(
courses_courseNumber INT NOT NULL AUTO_INCREMENT,
courses_courseTitle VARCHAR(25) NOT NULL,
courses_courseTeacher VARCHAR(30) NOT NULL,
courses_courseCostOfBooks DECIMAL(5,2) NOT NULL,
courses_courseDepartmentAbbv CHAR(4) NOT NULL,
PRIMARY KEY (courses_courseNumber),
FOREIGN KEY (courses_courseTeacher),
FOREIGN KEY (courses_courseDepartmentAbbv)
);
DROP TABLE IF EXISTS departments;
CREATE TABLE departments(
departments_departmentAbbv CHAR(4) NOT NULL,
departments_departmentFullName VARCHAR(15) NOT NULL,
PRIMARY KEY (departments_departmentAbbv),
FOREIGN KEY (departments_departmentAbbv) REFERENCES (courses_courseDepartmentAbbv)
);
DROP TABLE IF EXISTS teachers;
CREATE TABLE teachers(
teachers_teacherName VARCHAR(20) NOT NULL,
teachers_teacherHomeroom SMALLINT(3) NOT NULL,
teachers_teacherHomeroomGrade SMALLINT(1) NOT NULL,
teachers_teacherFullTime BOOL NOT NULL,
PRIMARY KEY (teachers_teacherName),
FOREIGN KEY (teachers_teacherName) REFERENCES (courses_courseTeacher)
);
You need to have a References after each Foreign key. You are missing that in the beginning setup of courses. Here is the documentation
I think this is more of what you want. Your order of creation was not correct. You had foreign keys in the wrong location due to this. You only set up foreign key mappings on the tables with the relations to the PK. You only need to set up the PK on your other tables. As long as you create the tables in the right order, then you can do this, as below.
So, teachers and departments have primary keys that are foreign keys within the courses table. teachers and departments does not need to worry about the foreign key. You leave that up to the table that actually has the reference (courses)
DROP TABLE IF EXISTS teachers;
CREATE TABLE teachers(
teachers_teacherName VARCHAR(20) NOT NULL,
teachers_teacherHomeroom SMALLINT(3) NOT NULL,
teachers_teacherHomeroomGrade SMALLINT(1) NOT NULL,
teachers_teacherFullTime BOOL NOT NULL,
PRIMARY KEY (teachers_teacherName)
--FOREIGN KEY (teachers_teacherName) REFERENCES courses (courses_courseTeacher)
--This is not where you set up the FK for courses
);
DROP TABLE IF EXISTS departments;
CREATE TABLE departments(
departments_departmentAbbv CHAR(4) NOT NULL,
departments_departmentFullName VARCHAR(15) NOT NULL,
PRIMARY KEY (departments_departmentAbbv)
--FOREIGN KEY (departments_departmentAbbv) REFERENCES courses (courses_courseDepartmentAbbv)
--This is not where you set up the FK for courses
);
CREATE TABLE courses(
courses_courseNumber INT NOT NULL AUTO_INCREMENT,
courses_courseTitle VARCHAR(25) NOT NULL,
courses_courseTeacher VARCHAR(30) NOT NULL,
courses_courseCostOfBooks DECIMAL(5,2) NOT NULL,
courses_courseDepartmentAbbv CHAR(4) NOT NULL,
PRIMARY KEY (courses_courseNumber),
FOREIGN KEY (courses_courseTeacher)
REFERENCES teachers (teachers_teacherName)
FOREIGN KEY (courses_courseDepartmentAbbv)
REFERENCES departments(departments_departmentAbbv)
);
You need to give a REFERENCES clause for your FOREIGN KEY clauses
FOREIGN KEY (courses_courseTeacher)
FOREIGN KEY (courses_courseDepartmentAbbv)
Define engine type that should be innodb which supports FOREIGN KEY constraints.
Follow Syntax defined here