I'm trying to design my database with very basic tables and I am confused on the CORRECT way to do it.
I've attached a picture of the main info, and I'm not quite sure how to link them. Meaning what should be a foreign key, or should some of these tables include of LIST<> of the other tables.
UPDATE TO TABLES
As per your requirements, You are right about the associative table
Client can have multiple accounts And Accounts can have multiple clients
Then, Many (Client) to Many (Account)
So, Create an associate table to break the many to many relationship first. Then join it that way
Account can have only one Manager
Which means One(Manager) to Many(Accounts)
So, add an attribute called ManagerID in Accounts
Account can have many traedetail
Which means One(Accounts) to Many(TradeDetails)
So, add an attribute called AccountID in TradeDetails
Depends on whether you are looking to have a normalized database or some other type of design paradigm. I recommend doing some reading on the concepts of database normalization and referential integrity.
What I would do is make tables that have a 1 to 1 relationship such as account/manager into a single table (unless you can think of a really good reason not to). Add Clientid as a foreign key to Account. Add AccountID as a foreign key to TradeDetail. You are basically setting up everything as 1 to many relationships where the table that has 1 record for the id has the field as a primary key and the table that has many has it as a foreign key.
So as you can see I have an Identifying 1 to many relationship in the tables above.
If I was to change this relationship to a Identifying 1 to 1 relationship, then the auto_leads table will still contain two composite primary keys from its parent leads table. In other words, nothing will change.
Does an identifying relationship have any meaning in the context of relational models? It doesnt appear to change its effect with respect to relationships.
Identifying relationship is an ER-modelling concept which arises because ER modelling assumes there is some semantic significance to having a primary key for each entity. Primary keys have no special role in relational database design and therefore the concept of an identifying relationship is usually of no great importance.
Consider the example of a table with two candidate keys, A and B. A is also a foreign key. According to ER-modelling convention if A is chosen as a primary key then the foreign key relationship is an identifying one. If A is an alternate key then the relationship is deemed to be non-identifying. Yet the form, function, integrity constraints and presumably the business meaning is exactly the same in both cases. The concept of identifying relationships is only as important as you want it to be.
I'm trying to understand a concept rather than fixing a piece of code that won't work.
I'll take a general example of a form (parent table) and a form field (child table). Logically, this would be an identifying relationship, since a form field cannot exist without a form.
This would make me think that in order to translate the logical relationship into the technical relationship, a simple NOT NULL for the form_id field in the form_field table would suffice. (See the left part of above screenshot.)
However, when I add an identifying relationship using MySQL Workbench, form_id is not only NOT NULL but also part of the primary key. (See the right part of above screenshot.) And when I add a non-identifying relationship, NOT NULL is still applied so logically it would actually be an identifying relationship as well.
I guess this confuses me a little, as well as the fact that until now I always simply used the id field as primary key.
So I understand the logical concept of identifying vs. non-identifying relationships, but I don't understand the technical part.
Why is it, as this answer states, 'the "right" way to make the foreign key part of the child's primary key'?
What is the benefit of these composite primary keys?
Logically, this would be an identifying relationship, since a form field cannot exist without a form.
No, identifying relationship is about identification, not existence.
Any X:Y relationship where X >= 1 guarantees existence of the left side, whether identifying or not. In your case, a 1:N relationship guarantees existence of form for any given form_field. You could make it identifying or non-identifying and it would still guarantee the same.
Remarks:
You would model an identifying relationship by making form_field.form_id part of a key. For example form_field PK could look like: {form_id, label}, which BTW would be quite beneficial for proper clustering of your data (InnoDB tables are always clustered).
Just making a PK: {id, form_id} would be incorrect, since this superkey is not a candidate key (i.e. it is not minimal - we could remove form_id from it and still retain the uniqueness).
You would model a 0..1:N relationship by making the form_field.form_id NULL-able (but then you wouldn't be able to make it identifying as well - see below).
There are two definitions of the "identifying relationship":
Strict definition: A relationship that migrates parent key into child primary key1.
Loose definition: A relationship that migrates parent key into child key.
In other words, the loose definition allows migration into alternate key as well (and not just primary).
Most tools2 seem to use the strict definition though, so if you mark the relationship as identifying, that will automatically make the migrated attributes part of the child PK, and none of the PK attributes can be NULL.
1 Which is then either completely comprised from migrated attributes, or is a combination of migrated attributes and some additional attributes.
2 ERwin and Visio do. I haven't used MySQL Workbench for modeling yet, but your description seems to suggest it behaves the same.
An identifying relationship is supposed to be one where the primary key includes foreign key attributes. That's why when you designate a relationship as identifying the posted foreign key is deemed to be part of the primary key.
The difference between an "identifying" relationship and a non-identifying one is purely informational or diagrammatic if the same key constraints and nullability constraints apply in each case. The concept is analogous to and a consequence of designating a "primary" key. If a table has more than one candidate key then all other things being equal it doesn't matter from a logical perspective which key is designated the primary one - the form, function and (presumably) the business meaning of the table is the same.
In your example however, the keys in the two tables are NOT the same. In the first case ID is unique in the form_field table while in the second case it apparently isn't. I expect that's not what you intended.
I need help in understanding the applicability of foreign keys when setting up constraints. I understand that the role of setting up foreign keys is to prevent orphaned data, but I have found a desire to put the foreign key in the child, which seems to break a pattern. Not sure if I am doing this right, and would like some advice if I have my constraints correctly.
Here is the design I have:
(1) I want all my "product"s to have a type of unit associate with the quantity. Units being like "Each", "Foot", "Gallon", etc, so between the quantity and the unit, you would have something like:
Quantity Unit
5 Gallons
I do not want to allow a bunch of crazy units, so I set this constraint up. This is pretty much by the book.
(2) I also believe that not all products will have an "Image", so I put the foreign key in the "ProductImage" table so I would not have "Product"s with a column with an empty row because I am also trying to "Normalize" the design.
The same issue with "FeeTypes" because not all "Product"s will have fees.
I feel guilt about breaking the pattern of putting the foreign key constraint in the child and not the parent. I just cannot wrap my head around "FeeType" being a parent. This conflict in logic is where I have the question.
Is my design correct, from a design perspective?
Am I still constraining the data properly?
Is there another "role" besides preventing orphaned data?
Thanks in advance.
There are three cases here (from the Product table's point of view):
Many-to-one relationship, e.g. many products having the same unit type - one unit type per product.In this case the foreign key must be in the Product table referencing the primary key UnitType.UnitTypeID.
One-to-many relationship, e.g. one product can have multiple images - one image can belong to only one productIn this case the foreign key must be in the ProductImages table, referencing Product.ProductID.
Many-to-many relationship, e.g. any product can have many categories - any category might describe many productsIn this case you will need a connection table that contain ProductID/CategoryID pairs, with columns being foreign keys referencing Product.ProductID and Category.CategoryID respectively.
So, the design of UnitType (case 1.) and ProductImage (case 2.) tables is OK, but FeeType should probably be case 1. and Category should be case 3.
BTW, it would be perfectly OK to have NULL in a foreign key column; it would not break the rules of normalization. So, for example, if some products do not have fees associated, you can have NULL in the Product.FeeTypeID column. But you will need to use an outer join in your queries to ensure that no products with no fees will not be excluded from the results.
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