I'm trying to create a web application about cooking, and I get weird behaviours when creating my MySQL database. I have the following tables: friends and user, described as below
CREATE TABLE IF NOT EXISTS user (
user_id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
username VARCHAR (100) UNIQUE NOT NULL,
password VARCHAR(255) NOT NULL
);
CREATE TABLE IF NOT EXISTS friends (
user_id int NOT NULL,
friend_id int NOT NULL,
PRIMARY KEY (user_id, friend_id),
FOREIGN KEY (user_id) REFERENCES user(user_id),
FOREIGN KEY (friend_id) REFERENCES user(user_id)
);
For some reason, in my friends table, the composite primary key gets created, but only the second field friend_id is a foreign key. I have the exact same problem when I create a table named recipe_composition, when I create a composite key recipe_id, ingredient_id. The composite primary keys gets created but only the second field ingredient_id gets the foreign key reference.
Any help would be appreciated, thank you.
Related
I have 3 tables. Courses,CourseDestails, CourseBooking.
Courses
CREATE TABLE Courses (
courseId int NOT NULL,
courseName varchar(255),
level varchar(255),
description varchar(255),
PRIMARY KEY (courseId)
);
CourseDetails
CREATE TABLE CourseDetails (
CourseStartDate int NOT NULL,
Location varchar(255) NOT NULL,
MaxNoPlaces int,
Length int,
Instructor varchar(255),
TotalPlacesBooked int,
NoPlacesCancelled int,
AdultPrice int,
ChildPrice int,
courseId int,
CONSTRAINT PK_CourseDetails PRIMARY KEY (CourseStartDate,Location,courseId),
FOREIGN KEY (courseId) REFERENCES Courses(courseId)
);
courseId from courses table is used as foreign key in the CourseDetails table.
Now I want to use CourseStartDate Location & courseId from CourseDetails table in another table called CourseBooking. I know how to add CourseStartDate Location as foreign keys. But I'm confused how to add courseId from courseDetails table as foreign key in the new CourseBookings Table.
I have tried the following
CREATE TABLE CourseBookings (
CourseStartDate int NOT NULL,
Location varchar(255) NOT NULL,
GuestNo int,
courseId int,
CONSTRAINT PK_CourseDetails PRIMARY KEY (CourseStartDate,Location,GuestNo,courseId),
FOREIGN KEY (courseId) REFERENCES CourseDetails.courseId(courseId),
FOREIGN KEY (CourseStartDate) REFERENCES CourseDetails(CourseStartDate),
FOREIGN KEY (Location) REFERENCES CourseDetails(Location),
FOREIGN KEY (GuestNo) REFERENCES Guest(GuestNo)
);
But there is an error saying
Can't create table 'b8040777_db1.CourseBookings'
Supports transactions, row-level locking, and foreign keys
There are several issues with the declaration of table CourseBookings.
ISSUE 1
This :
FOREIGN KEY (courseId) REFERENCES CourseDetails.courseId(courseId),
Should be written as :
FOREIGN KEY (courseId) REFERENCES CourseDetails(courseId),
ISSUE 2
One of the columns of CourseDetails, that is referenced by a foreign key of table CourseBookings, has no index.
From the documentation :
MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist.
In table CourseDetails :
courseId had an index automatically created because it references Courses(courseId)
CourseStartDate has no index but it is the first column in the index automatically generated by the primary key constraint declaration
Location has no index and it is only the second column in the primary key index --> it is not possible to create a foreign key referencing it, an error is raised when you try
SOLUTION 1
It is always possible to add the missing index to the table, by adding this to the CREATE TABLE CourseDetails statement :
INDEX idx_Location (Location)
SOLUTION 2
In your use case, I suspect that what you actually need is a multicolumn foreign key (supported in InnoDB only), that references the primary key of CourseDetails (an index already exists on these columns). This will allow you to associate each record of CourseBookings with a unique parent record in CourseDetails.
Suggestion of DDL :
CREATE TABLE CourseBookings (
CourseStartDate int NOT NULL,
Location varchar(255) NOT NULL,
GuestNo int,
courseId int,
CONSTRAINT PK_CourseDetails PRIMARY KEY (CourseStartDate,Location,courseId,GuestNo),
FOREIGN KEY (CourseStartDate,Location,courseId)
REFERENCES CourseDetails(CourseStartDate,Location,courseId),
FOREIGN KEY (GuestNo) REFERENCES Guest(GuestNo)
);
SOLUTION 3
If solution 2 fits for your use case, then it means that your schema can be optimized by creating an autoincremented integer primary key on table CourseDetails. The existing primary key can be turned into a UNIQUE constraint. Then, in table CourseBookings, you can just store a foreign key to that column.
This would give you a simple and efficient way to relate one table to the other, while avoiding duplicating information across tables (this actually leaves you with just 3 columns in CourseBookins).
In relational database design it is usually a good practice to create such a primary key on most tables. You could consider adding one to other tables too.
Example :
CREATE TABLE CourseDetails (
id int PRIMARY KEY AUTO_INCREMENT,
CourseStartDate int NOT NULL,
...
CONSTRAINT PK_CourseDetails UNIQUE (CourseStartDate,Location,courseId),
FOREIGN KEY (courseId) REFERENCES Courses(courseId)
);
CREATE TABLE CourseBookings (
id int PRIMARY KEY AUTO_INCREMENT,
courseDetailId INT NOT NULL,
GuestNo int,
CONSTRAINT PK_CourseBookings UNIQUE (courseDetailId,GuestNo),
FOREIGN KEY (courseDetailId) REFERENCES CourseDetails(id),
FOREIGN KEY (GuestNo) REFERENCES Guest(GuestNo)
);
PS, just in case :
FOREIGN KEY (GuestNo) REFERENCES Guest(GuestNo)
Does table Guest exists in your schema (you did not show the CREATE TABLE for it) ?
I want to know how can I create weak entities in mySql by Creating tables, I have the code like this:
CREATE TABLE users(
user_Id int AUTO_INCREMENT,
full_Name varchar(60),
email varchar(30),
password varchar(30),
reg_Date timestamp
);
CREATE TABLE personal_Infos(
...
);
These are the tables, all I want to Know is how can I Connect to each-other with foreign key and should I create another primary key at the second table? (Second table has some more informations for the first table)
should I create another primary key at the second table?
Yes you do need to create a primary key in your second table personal_Infos table and that primary key will become foreign key in your users table.
how can I Connect to each-other with foreign key ?
Suppose your primary key in personal_Infos table is P_id, then you can add it as foreign key in your users table like shown below
CREATE TABLE users(
user_Id int AUTO_INCREMENT,
full_Name varchar(60),
email varchar(30),
password varchar(30),
reg_Date timestamp,
p_id int not null,
FOREIGN KEY (P_Id) REFERENCES personal_Infos(P_Id)
);
What I am doing incorrect? Trying to create these tables in sqlfiddle
does not work gives
Cannot add foreign key constraint
create table product (
pid int NOT NULL,
name varchar(10),
PRIMARY KEY (pid)
);
create table trans (
tid int NOT NULL ,
productId int NOT NULL,
userId int NOT NULL,
PRIMARY KEY (tid),
FOREIGN KEY (productId) REFERENCES product(pid),
FOREIGN KEY (userId) REFERENCES user1(uid)
);
create table user1 (
uid int NOT NULL ,
location varchar(22),
PRIMARY KEY (uid)
);
As #BillKarwin mentioned, the definitions for tables containing primary keys referenced by the trans table should appear before the definition for the trans table. So you should move the definition for the trans table to last.
However, even doing this still results in an error in SQLFiddle:
SQLFiddle (uncomment the foreign key reference in trans)
SQLFiddle seems to have some sort of problem with accepting this table schema. This is not surprising, as the site seems to have such problems frequently.
What is order you create tables. You need first to create product and user1 and at the end - trans.
There are many similar question but this is bit different.
I have one table which has one foreign key that will reference to two tables.
I used below query for testing.
CREATE TABLE users
(
id int NOT NULL,
username varchar(255) NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE admins
(
id int NOT NULL,
username varchar(255) NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE info
(
id int NOT NULL,
fullname int NOT NULL,
user_id int,
PRIMARY KEY (id),
FOREIGN KEY (user_id) REFERENCES users(id),
FOREIGN KEY (user_id) REFERENCES admins(id)
);
Above queries works fine.
but when I try to draw model in mysql workbench it create on new field in info table that I don't want. I want user_id should work and show relation as foreign key for users and admins table.
One more thing, am I trying to do that is not well standard? Also suggests a correct way to do it.
Table names used only for example purpose. There is no logic here. I am trying to find solution for one key as foreign key for multiple table and faced issue with mysql work bench.
Try this:
Save your DDL in a file.
Create new model in MySQL Workbench
File > Import > Reverse Engineer MySQL Create Script
Browse to file created in step 1. Ensure that 'Place imported objects on diagram' is selected.
Click 'Execute'
From a data modelling point of view you might be better off specifying a user as an admin by including an extra column on the users table. Hence:
CREATE TABLE users
(
id int NOT NULL,
username varchar(255) NOT NULL,
isAdmin boolean not null default false,
PRIMARY KEY (id)
);
CREATE TABLE info
(
id int NOT NULL,
fullname int NOT NULL,
user_id int,
PRIMARY KEY (id),
FOREIGN KEY (user_id) REFERENCES users(id)
);
What would be the code for creating a table with two foreign keys?
I have a USER table and a PICTURE table. Since a USER can be in many PICTURE and many PICTURE can be from a USER, I need a third table with both primary keys.
Thank you SO, as usual you are invaluable for a learning novice. :)
I can't speak specifically for mySQL but in most databases I have worked with you can put as many foreign keys as you need on a table. But you can only have one primary key. A third table with both keys is the right choice. Make a foreign key to each of the other two tables and a primary key consisting of both ids in the table.
If I understood correctly, you may need to do something like the following:
CREATE TABLE users (
user_id INT NOT NULL PRIMARY KEY,
name VARCHAR(50) NOT NULL
) ENGINE=INNODB;
CREATE TABLE pictures (
picture_id INT NOT NULL PRIMARY KEY,
filename VARCHAR(255) NOT NULL,
posted_by INT NOT NULL,
FOREIGN KEY (posted_by) REFERENCES users(user_id)
) ENGINE=INNODB;
CREATE TABLE users_in_pictures (
user_id INT NOT NULL,
picture_id INT NOT NULL,
PRIMARY KEY (user_id, picture_id),
FOREIGN KEY (user_id) REFERENCES users(user_id),
FOREIGN KEY (picture_id) REFERENCES pictures(picture_id)
) ENGINE=INNODB;
Note that each picture can be posted by a user. In fact the posted_by field is constrained by a foreign key that references the users table.
In addition, I assume that you want to tag pictures ala-facebook. In this case, you can use the third table, which is using a composite primary key on (user_id, picture_id) and both fields are also constrained to the appropriate table.