I would like to know if it's mandatory for my database to have the same tables as my ERD diagram?
In my ERD I have 5 tables...:
I have a database that only has 2 tables:
Booking (C_Name, C_Phone, B_Date, S_Time, Ach,)
C_NAME: customer name
C_Phone: customer phone number
B_Date date of booking
S_Time: booking start time
Ach: achieved
Food(Meal_Name, Meal_Cost)
The customer information is already in the booking table. Is it necessary to create an order table? For the employee table, there is a relationship between other tables, however, this information is not required in my database...
Any ideas? Thanks
Definitely not. An ERD diagram is usually for the logical data model. The most obvious situation is when it contains a many-to-many relationship. Such a relationship would generally be represented using a junction/association table.
Under other circumstances, you might want to combine different but similar entities into a single table (for instance, to handle one-of relationships). Or, you may decide to represent some relationships using nested tables or similar mechanisms. There may be more arcane requirements that call for splitting tables as well.
For your particular model, I would expect one table per entity. You need additional junction/association tables to handle the employee relationships to order and meal.
Are there any hard and fast rules against creating a junction table out of a table's primary key? Say I have a table with a structure similar to:
In this instance there are a list of items that can be sold together, but should be marked as dangerous. Any one item can have multiple other items with which it is dangerous. All of the items are uniquely identified using their itemId. Is it OK to refer a table to itself in a many-to-many relationship? I've seen other examples on SO, but they weren't SQL specific really.
This is the correct design for your problem, as long as your combinations can only be two-item combos.
In database design a conceptual design that renders a relation in many-to-many form is converted into 2 one-to-many in physical design. Say for example a Student could take one or many courses and a course could have many students, so that's many to many. So, in actual design it would be a Student table, Course table then CourseTaken table that has both the primary key of Student and Course table thus creating 2 one to many relayionship. In your case altough the two tables are one and the same but you have the virtual third table to facilitate the 2 one to many relationship so that to me is still very viable approach.
I am learning about databases and SQL for the first time. In the text I'm reading (Oracle 11g: SQL by Joan Casteel), it says that "many-to-many relationships can't exist in a relational database." I understand that we are to avoid them, and I understand how to create a bridging entity to eliminate them, but I am trying to fully understand the statement "can't exist."
Is it actually physically impossible to have a many-to-many relationship represented?
Or is it just very inefficient since it leads to a lot of data duplication?
It seems to me to be the latter case, and the bridging entity minimizes the duplicated data. But maybe I'm missing something? I haven't found a concrete reason (or better yet an example) that explains why to avoid the many-to-many relationship, either in the text or anywhere else I've searched. I've been searching all day and only finding the same information repeated: "don't do it, and use a bridging entity instead." But I like to ask why. :-)
Thanks!
Think about a simple relationship like the one between Authors and Books. An author can write many books. A book could have many authors. Now, without a bridge table to resolve the many-to-many relationship, what would the alternative be? You'd have to add multiple Author_ID columns to the Books table, one for each author. But how many do you add? 2? 3? 10? However many you choose, you'll probably end up with a lot of sparse rows where many of the Author_ID values are NULL and there's a good chance that you'll run across a case where you need "just one more." So then you're either constantly modifying the schema to try to accommodate or you're imposing some artificial restriction ("no book can have more than 3 authors") to force things to fit.
A true many-to-many relationship involving two tables is impossible to create in a relational database. I believe that is what they refer to when they say that it can't exist. In order to implement a many to many you need an intermediary table with basically 3 fields, an ID, an id attached to the first table and an id atached to the second table.
The reason for not wanting many-to-many relationships, is like you said they are incredibly inefficient and managing all the records tied to each side of the relationship can be tough, for instance if you delete a record on one side what happens to the records in the relational table and the table on the other side? Cascading deletes is a slippery slope, at least in my opinion.
Normally (pun intended) you would use a link table to establish many-to-many
Like described by Joe Stefanelli, let's say you had Authors and Books
SELECT * from Author
SELECT * from Books
you would create a JOIN table called AuthorBooks
Then,
SELECT *
FROM Author a
JOIN AuthorBooks ab
on a.AuthorId = ab.AuthorId
JOIN Books b
on ab.BookId = b.BookId
hope that helps.
it says that "many-to-many relationships can't exist in a relational database."
I suspect the author is just being controversial. Technically, in the SQL language, there is no means to explicitly declare a M-M relationship. It is an emergent result of declaring multiple 1-M relations to the table. However, it is a common approach to achieve the result of a M-M relationship and it is absolutely used frequently in databases designed on relational database management systems.
I haven't found a concrete reason (or better yet an example) that explains why to avoid the many-to-many relationship,
They should be used where they are appropriate to be used would be a more accurate way of saying this. There are times, such as the books and authors example given by Joe Stafanelli, where any other solution would be inefficient and introduce other data integrity problems. However, M-M relationships are more complicated to use. They add more work on the part of the GUI designer. Thus, they should only be used where it makes sense to use them. If you are highly confident that one entity should never be associated with more than one of some other entity, then by all means restrict it to a 1-M. For example, if you were tracking the status of a shipment, each shipment can have only a single status at any given time. It would over complicate the design and not make logical sense to allow a shipment to have multiple statuses.
Of course they can (and do) exist. That sounds to me like a soapbox statement. They are required for a great many business applications.
Done properly, they are not inefficient and do not have duplicate data either.
Take a look at FaceBook. How many many-to-many relationships exist between friends and friends of friends? That is a well-defined business need.
The statement that "many-to-many relationships can't exist in a relational database." is patently false.
Many-to-many relationships are in fact very useful, and also common. For example, consider a contact management system which allows you to put people in groups. One person can be in many groups, and each group can have many members.
Representation of these relations requires an extra table--perhaps that's what your book is really saying? In the example I just gave, you'd have a Person table (id, name, address etc) and a Group table (id, group name, etc). Neither contains information about who's in which group; to do that you have a third table (call it PersonGroup) in which each record contains a Person ID and a Group ID--that record represents the relation between the person and the group.
Need to find the members of a group? Your query might look like this (for the group with ID=1):
SELECT Person.firstName, Person.lastName
FROM Person JOIN PersonGroup JOIN Group
ON (PersonGroup.GroupID = 1 AND PersonGroup.PersonID = Person.ID);
It is correct. The Many to Many relationship is broken down into several One to Many relationships. So essentially, NO many to many relationship exists!
Well, of course M-M relationship does exist in relational databases and they also have capability of handling at some level through bridging tables, however as the degree of M-M relationship increases it also increases complexity which results in slow R-W cycles and latency.
It is recommended to avoid such complex M-M relationships in a Relational Database. Graph Databases are the best alternative and good at handling Many to Many relationship between objects and that's why social networking sites uses Graph databases for handling M-M relationship between User and Friends, Users and Events etc.
Let's invent a fictional relationship (many to many relationship) between books and sales table. Suppose you are buying books and for each book you buy needs to generate an invoice number for that book. Suppose also that the invoice number for a book can represent multiple sales to the same customer (not in reality but let's assume). We have a many to many relationship between books and sales entities.
Now if that's the case, how can we get information about only 1 book given that we have purchased 3 books since all books would in theory have the same invoice number? That introduces the main problem of using a many to many relationship I guess. Now if we add a bridging entity between Books and sales such that each book sold have only 1 invoice number, no matter how many books are purchases we can still correctly identify each books.
In a many-to-many relationship there is obvious redundancy as well as insert, update and delete anomaly which should be eliminated by converting it to 2 one-to-many relationship via a bridge table.
M:N relationships should not exist in database design. They are extremely inefficient and do not make for functional databases. Two tables (entities) with a many-to-many relationship (aircraft, airport; teacher, student) cannot both be children of each other, there would be no where to put foreign keys without an intersecting table. aircraft-> flight <- airport; teacher <- class -> student.
An intersection table provides a place for an entity that is dependent on two other tables, for example, a grade needs both a class and a student, a flight needs both an aircraft and an airport. Many-to-many relationships conceal data. Intersection tables reveal this data and create one-to-many relationships that can be more easily understood and worked with. So, the question arises, what table should the flight be in--aircraft or airport. Neither, they should be foreign keys in the intersection table, Flight.
What is the real difference between one-to-many and many-to-one relationship? It is only reversed, kind of?
I can't find any 'good-and-easy-to-understand' tutorial about this topic other than this one: SQL for Beginners: Part 3 - Database Relationships
Yes, it is vice versa. It depends on which side of the relationship the entity is present on.
For example, if one department can employ several employees then department to employee is a one-to-many relationship (1 department employs many employees), while employee to department relationship is many-to-one (many employees work in one department).
More info on the relationship types:
Database Relationships - IBM DB2 documentation
From this page about Database Terminology
Most relations between tables are one-to-many.
Example:
One area can be the habitat of many readers.
One reader can have many subscriptions.
One newspaper can have many subscriptions.
A Many to One relation is the same as one-to-many, but from a different viewpoint.
Many readers live in one area.
Many subscriptions can be of one and the same reader.
Many subscriptions are for one and the same newspaper.
What is the real difference between one-to-many and many-to-one relationship?
There are conceptual differences between these terms that should help you visualize the data and also possible differences in the generated schema that should be fully understood. Mostly the difference is one of perspective though.
In a one-to-many relationship, the local table has one row that may be associated with many rows in another table. In the example from SQL for beginners, one Customer may be associated to many Orders.
In the opposite many-to-one relationship, the local table may have many rows that are associated with one row in another table. In our example, many Orders may be associated to one Customer. This conceptual difference is important for mental representation.
In addition, the schema which supports the relationship may be represented differently in the Customer and Order tables. For example, if the customer has columns id and name:
id,name
1,Bill Smith
2,Jim Kenshaw
Then for a Order to be associated with a Customer, many SQL implementations add to the Order table a column which stores the id of the associated Customer (in this schema customer_id:
id,date,amount,customer_id
10,20160620,12.34,1
11,20160620,7.58,1
12,20160621,158.01,2
In the above data rows, if we look at the customer_id id column, we see that Bill Smith (customer-id #1) has 2 orders associated with him: one for $12.34 and one for $7.58. Jim Kenshaw (customer-id #2) has only 1 order for $158.01.
What is important to realize is that typically the one-to-many relationship doesn't actually add any columns to the table that is the "one". The Customer has no extra columns which describe the relationship with Order. In fact the Customer might also have a one-to-many relationship with ShippingAddress and SalesCall tables and yet have no additional columns added to the Customer table.
However, for a many-to-one relationship to be described, often an id column is added to the "many" table which is a foreign-key to the "one" table -- in this case a customer_id column is added to the Order. To associated order #10 for $12.34 to Bill Smith, we assign the customer_id column to Bill Smith's id 1.
However, it is also possible for there to be another table that describes the Customer and Order relationship, so that no additional fields need to be added to the Order table. Instead of adding a customer_id field to the Order table, there could be Customer_Order table that contains keys for both the Customer and Order.
customer_id,order_id
1,10
1,11
2,12
In this case, the one-to-many and many-to-one is all conceptual since there are no schema changes between them. Which mechanism depends on your schema and SQL implementation.
Hope this helps.
SQL
In SQL, there is only one kind of relationship, it is called a Reference. (Your front end may do helpful or confusing things [such as in some of the Answers], but that is a different story.)
A Foreign Key in one table (the referencing table)
References
a Primary Key in another table (the referenced table)
In SQL terms, Bar references Foo
Not the other way around
CREATE TABLE Foo (
Foo CHAR(10) NOT NULL, -- primary key
Name CHAR(30) NOT NULL
CONSTRAINT PK -- constraint name
PRIMARY KEY (Foo) -- pk
)
CREATE TABLE Bar (
Bar CHAR(10) NOT NULL, -- primary key
Foo CHAR(10) NOT NULL, -- foreign key to Foo
Name CHAR(30) NOT NULL
CONSTRAINT PK -- constraint name
PRIMARY KEY (Bar), -- pk
CONSTRAINT Foo_HasMany_Bars -- constraint name
FOREIGN KEY (Foo) -- fk in (this) referencing table
REFERENCES Foo(Foo) -- pk in referenced table
)
Since Foo.Foo is a Primary Key, it is unique, there is only one row for any given value of Foo
Since Bar.Foo is a Reference, a Foreign Key, and there is no unique index on it, there can be many rows for any given value of Foo
Therefore the relation Foo::Bar is one-to-many
Now you can perceive (look at) the relation the other way around, Bar::Foo is many-to-one
But do not let that confuse you: for any one Bar row, there is just one Foo row that it References
In SQL, that is all we have. That is all that is necessary.
What is the real difference between one to many and many to one relationship?
There is only one relation, therefore there is no difference. Perception (from one "end" or the other "end") or reading it backwards, does not change the relation.
Cardinality
Cardinality is declared first in the data model, which means Logical and Physical (the intent), and then in the implementation (the intent realised).
One to zero-to-many
In SQL that (the above) is all that is required.
One to one-to-many
You need a Transaction to enforce the one in the Referencing table.
One to zero-to-one
You need in Bar:
CONSTRAINT AK -- constraint name
UNIQUE (Foo) -- unique column, which makes it an Alternate Key
One to one
You need a Transaction to enforce the one in the Referencing table.
Many-to-Many
There is no such thing at the Physical level (recall, there is only one type of relation in SQL).
At the early Logical levels during the modelling exercise, it is convenient to draw such a relation. Before the model gets close to implementation, it had better be elevated to using only things that can exist. Such a relation is resolved by implementing an Associative Table at the physical [DDL] level.
There is no difference. It's just a matter of language and preference as to which way round you state the relationship.
Answer to your first question is : both are similar,
Answer to your second question is: one-to-many --> a MAN(MAN table) may have more than one wife(WOMEN table) many-to-one --> more than one women have married one MAN.
Now if you want to relate this relation with two tables MAN and WOMEN, one MAN table row may have many relations with rows in the WOMEN table. hope it clear.
One-to-Many and Many-to-One are similar in Multiplicity but not Aspect (i.e. Directionality).
The mapping of Associations between entity classes and the Relationships between tables. There are two categories of Relationships:
Multiplicity (ER term: cardinality)
One-to-one relationships (abbreviated 1:1): Example Husband and Wife
One-to-Many relationships (abbreviated 1:N): Example Mother and Children
Many-to-Many relationships (abbreviated M:N): Example Student and Subject
Directionality : Not affect on mapping but makes difference on how we can access data.
Uni-directional relationships: A relationship field or property that refers to the other entity.
Bi-directional relationships: Each entity has a relationship field or property that refers to the other entity.
This is an excellent question, according to my experience, in ERD diagrams and relational databases direction is implied. In RDBMS you always define Many-To->One (trivial case One-To->One) relationships. The Many side of the relationship, a.k.a children, references the One side, a.k.a parent and you implement this with a Foreign Key constraint. Technically speaking you have to access an index, fetch the Primary Key record of the One side and then visit this record to get more information.
You cannot do this the other way around unless we are speaking about Object-Relational DBMS such as Postgres, Intersystems Cache, etc. These DBMS allow you to define a bi-directional relationship between the two entities (tables). In that case accessing records the other way around, i.e. One--To-->Many is achieved by using an array of references (children). In ORMs you have classes that reference each other the same way we described here.
WARNING: Most RDBMS in the IT market are NOT relational database management systems in the strict sense, think about null values, duplicate records etc, many of these allowed features break the definition of what a Relation is.
There's no practical difference. Just use the relationship which makes the most sense given the way you see your problem as Devendra illustrated.
One-to-many and Many-to-one relationship is talking about the same logical relationship, eg an Owner may have many Homes, but a Home can only have one Owner.
So in this example Owner is the One, and Homes are the Many.
Each Home always has an owner_id (eg the Foreign Key) as an extra column.
The difference in implementation between these two, is which table defines the relationship.
In One-to-Many, the Owner is where the relationship is defined. Eg, owner1.homes lists all the homes with owner1's owner_id
In Many-to-One, the Home is where the relationship is defined. Eg, home1.owner lists owner1's owner_id.
I dont actually know in what instance you would implement the many-to-one arrangement, because it seems a bit redundant as you already know the owner_id. Perhaps its related to cleanness of deletions and changes.
---One to Many--- A Parent can have two or more children.
---Many to one--- Those 3 children can have a single Parent
Both are similar. This can be used according to the need. If you want to find children for a particular parent, then you can go with One-To-Many. Or else, want to find parents for twins, you may go with Many-To-One.
The easiest explanation I can give for this relationship is by piggybacking on evendra D. Chavan'sanswer.
Using the department and employee relationship
A department can have multiple employees, so from the employee side, it's one-to-many relationship, and from the department side it's many-to-one relationship
But if an employee can also belong to more than one department, we can also say from the employee side it's now many as opposed to one, so the relationship becomes many-to-many
In order words, a simple understanding would be, we can state that a relationship is many-to-many if one-to-many can be viewed from both sides
that is if;
one employee can belong to many departments (one-to-many)
one department can have many employees (one-to-many)
I am new to SQL and only have experience using SQLAlchemy. The documentation on relationships in SQLAlchemy does a good job explaining this, in my opinion.
You may find some clarity by reading this part
Also, I had to come up with my own example to think through this. I'll try to explain without writing a bunch of code for simplicity.
table Vehicle
column (name)
table Manufacturer
column (name)
A Vehicle can only have One manufacturer (Ford, Tesla, BMW etc.)
Manufacturers can make many Vehicles
Ford
Ford makes Mustang
Ford makes F-150
Ford makes Focus
Tesla
Tesla makes Model S
Tesla makes Model X
Tesla makes Roadster
When looking at the database rows you will want to decide if you want a column that references the other side of the relationship. This is where the SQLAlchemy documentation brings in the difference between backref vs. back_populates. I understand that is the difference between having a column in the table to reference the other side of the relationship or not having a column to reference the other side.
I hope this helps, and even more so, I hope I am accurate in the way I learned and understand this.
I have read most of the answer. The problem is not the relationship here at all. If you look at One to Many or Many to One conceptually, it is just a reversible relationship. HOWEVER, while implementing the concept in your software or application it differs a lot.
In case of Many to One, we often desire the table that has Many aspect to be written first and we desire it to associate with the table containing One aspect. If you convert Many to One concept into One to Many, you will have hard time writing the One aspect table first in your code. Since, the relationship is defined while you engineer the database, Many aspect table will seek for the One aspect table data for integrity. So if you are planning to do it by using foreign key -> unique key or foreign key -> primary key, Many to One implementation will be different even if you consider it as a One to Many.
I personally make associations without using actual relationship concepts in many cases. There is no such boundaries as to use Database concept to form relationship every time. Just make sure that your database integrity is maintained as you want, it is indexed properly for your search needs and is decently normalized.
one-to-many has parent class contains n number of childrens so it is a collection mapping.
many-to-one has n number of childrens contains one parent so it is a object mapping
I'm reading Pro JPA 2. The book talks begins by talking about ORM in the first few pages.
It talks about mapping a single Java class named Employee with the following instance variables - id,name,startDate, salary.
It then goes on to the issue of how this class can be represented in a relational database and suggests the following scheme.
table A: emp
id - primary key
startDate
table B: emp_sal
id - primary key in this table, which is also a foreign key referencing the 'id' column in table A.
It thus seems to suggest that persisting an Employee instance to the database would require operations on two(multiple) tables.
Should the Employee class have an instance variable 'salary' in the first place?
I think it should possibly belong to a separate class (Class salary maybe?) representing salary and thus the example doesn't seem very intuitive.
What am I missing here?
First, the author explains that there are multiples ways to represent a class in a database: sometimes the mapping of a class to a table is straightforward, sometimes you don't have a direct correspondence between attributes and columns, sometimes a single class is represented by multiples tables:
In scenario (C), the EMP table has
been split so that the salary
information is stored in a separate
EMP_SAL table. This allows the
database administrator to restrict
SELECT access on salary information to
those users who genuinely require it.
With such a mapping, even a single
store operation for the Employee class
now requires inserts or updates to two
different tables.
So even storing the data from a single class in a database can be a challenging exercise.
Then, he describes how relationships are different. At the object level model, you traverse objects via their relations. At the relational model level, you use foreign keys and joins (sometimes via a join table that doesn't even exist at the object model level).
Inheritance is another "problem" and can be "simulated" in various ways at the relational model level: you can map an entire hierarchy into a single table, you can map each concrete class to its own table, you can map each class to its own table.
In other words, there is no direct and unique correspondence between an object model and a relational model. Both rely on different paradigms and the fit is not perfect. The difference between both is known as the impedance mismatch, which is something ORM have to deal with (allowing the mapping between an object model and the many possible representations in a relation model). And this is what the whole section you're reading is about. This is also what you missed :)