Say I run an online business where you can order products from my website, and I have a database with two tables:
Table order with the fields order_number, customer_ID, address
Table customer with the fields customer_ID, first_name, last_name
To get a full, detailed 'report' of the order, I would perform a LEFT JOIN to concatenate data from the order table to include the customer's first and last names along with their address and order number.
My question is, how are these tables related? Are they at all? What would an entity relationship diagram look like? Separately they don't seem to interact and act more like lookup tables for each other.
An order would always have a customer, no? So it is not a left but inner join.
What links them is the customer_id. So your SQL is simply:
select o.order_number, o.customer_ID, o.address,
c.first_name, c.last_name
from orders o
inner join customer c on o.customer_ID = c.customer_ID;
Entity relationship:
Order Customer
Customer_Id 0...N >---+ 1 Customer_Id
... ...
This EF relation is from MS SQL Server's sample database Northwind. In that sample database, just like yours, there are Customers and Orders. Customers and Orders tables are related via the CustomerId fields in both tables (it is the primary key in Customers, and foreign key in Orders table). When you model that as an Entity relation than you have the above diagram. Customer entity have an "Orders" navigation property (via customerId) that points to a particular Customer's Orders. And Order entity have a navigation property that points to its Customer (again via CustomerId). The relation is 1 to 0 or many (1 - *), meaning a Customer might have 0 or more Orders.
When you do the join from Customer's side, you use a LEFT join "if you want to see all Customers regardless they have Order(s) or not" - 0 or more Order(s). If you want to see only those with Order(s) then you use an inner join.
When you do the join from Orders' side, then an Order must have a Customer so it can't be a LEFT join. It is an INNER join.
You can check the relation from both sides using the connecting CustomerId field.
You wouldn't have a separate table for "OrderId, CustomerId" as it is not many-to-many relation (it would be pure redundancy and would create normalization anomalies).
Hope it is more clear now.
In the entity-relationship model, we don't relate tables. Instead, we relate entities which are represented by their key values. In your example, the customer entity (represented by customer_ID) has a one-to-many relationship with the order entity (represented by order_number) and this relationship is recorded in the order table (where order_number is associated with customer_ID). If we were to strictly follow the ER model, we would record (order_number, customer_ID) (a relationship relation) in a separate table from (order_number, address) (an entity relation).
If there's a foreign key constraint on order.customer_ID referencing customer.customer_ID, that's a subset relation that ensures that every order's customer is a known customer. Thus, the relation between the tables and the relation between the entities is not the same thing.
The relational model allows us to relate (join) tables in any way we need. The obvious join for your example would be on the shared domain customer_ID, but I could just as easily find orders which contain a customer's last_name in its delivery address.
An ER diagram for your example could look like this:
My question is, how are these tables related? Are they at all? What
would an entity relationship diagram look like? Separately they don't
seem to interact and act more like lookup tables for each other.
When dealing with data modeling in an ER-Diagram, more in the conceptual model, we should never start thinking of tables, primary keys, foreign keys and stuff as this is for the later model to the conceptual model, the logical model.
By modeling entities in ER-Diagram, we must always recognize them by their attributes/properties. The entities become concrete when we can find properties in them. I feel a lack of context here, so I'll proceed with a guess:
If you need to persist products in your database, and you need to
save the attributes/properties of them, then we have a Products
entity.
If you need to persist clients/users in your database along with
their properties/attributes, then we have a Customers entity.
According to what you said, customers are able to purchase products. So, you have to relate these two entities in any way. With this in mind, stop and think about the following:
You need to persist purchases/orders.
Purchases/orders show which users bought which products.
By linking products and customers, what would we get? The purchasing/orders event, right?
We would then have two entities relating to each other forming the purchasing (or "orders", you name it) event. This event is formed by the need for the present business rules (your rules). You as modeler you need to persist the orders, and you know that products and customers relate in some way, so you can create a relationship between the two entities representing the orders event.
As has been well discussed in other answers, there is the need of separation of attributes here. If the field "address" is where a user lives, not where the product will be delivered, then this attribute must exist in the Customers entity.
If this field/attribute/property is the final location of delivery, it should remain in the relationship orders created between the two entities, customers and products.
Let's talk now about cardinality. If a customer can purchase/order more than one product, and the same product can be purchased by more than one user, then we have a N-N relationship here. I believe this is your case.
Based on everything that was said, we find the following conceptual model:
By decomposing this model, and developing the logic model, we would have:
Now, for what reason we end up with this kind of model?
Relations N-N allow the existence of properties/attributes (if needed), so we can have attributes/properties in the relation Orders, resulting in an ORDERS table with the fields shown. This table represents the purchases/orders made by users/customers.
And for what reason the existence of "Products from orders"?
We need to say what products were purchased in which purchases/orders, especially showing how many of the same type are acquired. In N-N relationships, some properties induces the appearance of new relations which become tables later. In such situations, it is the discretion of the modeler.
In "Products from orders" entity we have a composite primary key, represented by foreign keys.
With this type of model, you can see:
Which users made purchases/orders.
Which purchases/orders were made by which users.
Which products belong to which purchases/orders.
Which products have been acquired by which users.
How many products of a type were purchased/ordered.
Using the date field, you can find out how many purchases were made in periods:
Weeks.
Months.
Years.
Quarters.
Etc.
If you are interested, see also:
E-R diagram confusion
If you have any questions, please comment and I will answer.I hope I've helped a bit.
Cheers.
"Related" is a vague and confusing term. This is because "relationship" gets used in two different ways: as "association" (between values) and as "foreign key" (between tables).
You do not need to know anything about how tables are "related" by foreign keys or common columns in order to query. What matters is how values in a query's result rows are related/associated by the query. The important relationships/associations are (represented by) the tables.
From a relational perspective, there would be a foreign key from Order referencing Customer on customer_id.
A Chen-style ER diagram would have Order and Customer entity types (boxes) and an Orders relationship type (diamond) with participations (lines) from Orders to Order and to Customer. It would have a table for each type. This is an example of perfectly sensible relational design that the ER model, with its artificial and perverse distinction between entities and relationships, cannot capture. People will use a database design like yours to implement such an ER design, though. Even though the ER design isn't, then, the design.
--
When using a database, values are used to identify entities or as property names and magnitudes. Each base table holds the rows of values that participate in some relationship/association given by the DBA. Each query result holds the rows of values that participate in a relation/association that is a combination of conditions and the relationships/associations of the base tables it mentions. All you need to know to query is the relationships/associations of tables.
Order -- order ORDER_NUMBER is by customer CUSTOMER_ID to address ADDRESS
Customer -- customer CUSTOMER_ID is named FIRST_NAME LAST NAME
Order JOIN (Customer WHERE FIRST_NAME='Eddie')
-- order ORDER_NUMBER is by customer CUSTOMER_ID to address ADDRESS
AND customer CUSTOMER_ID is named FIRST_NAME LAST NAME
AND FIRST_NAME='Eddie'
Sometimes values for a list of columns in a row of one table must also appear as values for a list of columns in another table or that table. This is because if the listed values and others satisfy one table's relationship/association then the listed values and yet others will satisfy the other table's relationship/association.
/* IF there exist ORDER_ID & ADDRESS satisfying
order ORDER_NUMBER is by customer CUSTOMER_ID to address ADDRESS
THEN there exist FIRST_NAME & LAST_NAME satisfying
customer CUSTOMER_ID is named FIRST_NAME LAST_NAME
*/
FOREIGN KEY Order(customer_id) REFERENCES Customer(customer_id)
This means that those particular tables and column lists satisfy the (one and only) relationship/association "foreign key in this database". (And a database will have a meta-data table for its foreign key relationship/association.)
We say that "there is a foreign key" from the first table's list of columns to the second table's list of columns. There's only one foreign key relationship/association for the database. When there is a foreign key its tables and column lists satisfy this database's foreign key relationship. But a foreign key isn't a relationship/association. People call foreign keys "relationships", but they are not.
But foreign keys don't matter to querying! (Ditto for any other constraint.) A query's result holds rows whose values (entity & property info) are related/associated by that query's relationship/association, built from conditions and base table relationships/associations.
Related
I am trying to simplify an application's database. In that database I have two tables let's say Patient and MedicalRecord. I know that two tables are said to be in One-to-One relationship iff that any given row from Table-A can have at most one row ine Table-B(It means there can be zero matchings).
But in my case, it is not at most, it is exactly. i.e., Every row in Patient should have exactly one row in MedicalRecord(no patient exist without a medical record).
Patient table has all personal details of the patient with his id as PK.
MedicalRecord talbe has details like his blood-group, haemoglobin, bp etc with his id as both PK and FK to the Patient.
My Question is, can I merge those two tables and create one table like,
PatientDetails : personal_details and blood-group, haemoglobin, bp etc
"bp" = "Blood pressure"? Then you must not combine the tables. Instead, it is 1:many -- each patient can have many sets of readings. It is very important to record and plot trends in the readings.
Put only truly constant values in the Patient -- name, birthdate (not age; compute that), sex, race (some races are more prone to certain diseases than others), not height/weight. Etc.
Sure, a patient may have a name change (marriage, legal action, etc), but that is an exception that does not affect the schema design, except to force you to use patient_id, not patient_name as a unique key.
Every patient must have a MedicalRecord? That is "business logic"; test it in the application; do not depend (in this case) on anything in the Database.
Both tables would have patient_id. Patients would have it as the PRIMARY KEY; MedicalRecord would haveINDEXed`. That's all it takes to have 1:many.
In situations where the tables are really 1:1 (optionally 1:0/1), I do recommend merging the table. (There are exceptions.)
If two tables have the same set of subrow values for a shared set of columns that is a superkey in both (SQL PRIMARY KEY or UNIQUE) then you can replace the two tables by their natural join. ("Natural join" is probably what you mean by "merge" but that is not a defined technical term.) Each original table will equal the projection of the join on that original's columns.
(1:1 means total on both sides, it does not mean 1:0-or-1, although most writing about cardinalities is sloppy & unclear.)
i have entity customer and entity order, a customer can have 0 or more orders but an order can only have 1 customer.
I already tried a lot of things like making the foreign key unchecked at NN but I cant get a foreign key order at customer
EDIT: Using mySQL ERD workbench
Your customer table will have a unique customer_id column. In the MySQL world we often use autoincrementing primary keys for this kind of id column.
Your order table will have a customer_id column that's a foreign key to customer.customer_id.
This allows the order table to have any number of rows relating to a particular customer_id: none, one, or many. The foreign key relationship, when enforced--checked--simply prevents an order from having a customer_id value that references no valid customer.
Classic data design tools with their distinction between logical and physical design can drive ya nuts when you're trying to do easy stuff like this.
Pro tip if you name your id columns the same way everywhere they're used, data design tools tend to work better, especially when "reverse-engineering" your tables. That's why I suggested column names like customer.customer_id and order.customer_id rather than customer.id and order.customer_id.
I am designing a database for my app. In which I want to do mapping between multiple tables. Now situation is like There is one user table. User can have generate multiple orders. So I was thinking if I can put json obejct of order_id in a column in order table. Or I can create a user_id column in order table & repeat user_id for same user. So which is a better way of doing it?
PS: What is the standard way of doing it?
You should just have user_id in your order table, then make queries like
select * from orders where user_id = *some_user_id*
A user can place multiple orders which in turn can have multiple line items. Each line item can have n quantity of a specific product. So when product comes in picture, then it becomes many to many relationship between user and product because a user can place order for many products and a product can be ordered by many users. So my suggestion is -
Create a User table with UserID
Create a PurchaseOrder table with OrderID, UserID and LineItemID
Create a LineItem table with OrderID, ProductID and LineItemID
Create a SKU table with ProductID
A user can place multiple orders.
Based on this you should maintain three different tables as given below:
User (user_id,...)
Order (order_id,...)
UserOrder (user_id,order_id,...)
Only the primary keys in the above tables are focused
Storing comma separated list or json object will worsen the design. And this is strongly discouraged.
EDIT:
As #NevilleK suggested, the above design is typically used for many-to-many relationships. For one-to-many relationship you can create a foreign key constraint in orders table where user_id should refer to the user_id in the User table.
But still you can adopt the above design for one-to-many relationship since many-to-many qualifies for one-to-many too.
The best way is to have different table for your
User table - which hosts the user information
Transaction table - which will have order_id against each user_id.
Transaction table will carry all the transaction details with user_id. If you create a JSON object, how will you map the USER to the transaction. So at the time of retrieving the json information you will have to map it to the user table anyway. I suggest you the use the above said method, which will help you maintain and scale your application much easily.
My UNF is
database(
manager_id,
manager_name,
{supplier_id,
supplier_name,
{order_id,
order_quantity}}
{purchase_id,
purchase_date}
Here manager_name, supplier_id, order_id and purchase_id are primary key.
During normalization there will be 1 table called purchase. Is it necessary to make manager_name as a foreign key?
How can I normalize these database?
This is a part of my college project on database. Normalization is really confusing.
First consider splitting things out by things that naturally go together. In this case you have manager information, supplier information, order information and purchase information. I personally would want to know the difference between an order and a purchase because that is not clear to me.
So you have at least four tables for those separate pieces of information (although depending on the other fields you might need, suppliers and managers could be in the same table with an additional field such as person_type to distinguish them, in this case you would want a lookup table to grab the valid person type values from). Then you need to see how these things relate to each other. Are they in a one to one relationship or a one-to many or a many to many relationship? In a one-to one relationship, you need the FK to also have a unique constraint of index to maintain the uniqueness. In a many to many you will need an additional junction table that contains both ids.
Otherwise in the simplest case the child table of purchase would have FKs to the manager, supplier. and order tables.
Manager name should under no circumstances be a primary key. Many people have the same name. Use Manager ID as the key because it is unique where name is not. In general I prefer to separate out the names into First, middle and last so that you can sort on last name easily. However in some cultures this doesn't work so well.
I'm working on a very simple DB.
Imagine I've table customer and table seller.
The customer is able to request a quotation for some products
There will be a simple form that allow to customers to select products and submit the quotation.
Now, should I create table : "Quotation" and store all quotations (with id_quotation..etc)?
Thank you all
Without knowing all of the business rules that govern the requirements of this database, perhaps the following design will help to answer your question and explain a few concepts in the process.
In database terms, an entity is a person, place, or thing about which we want to collect and store data. From your description we can already see two entities: seller and customer. This is important since the entities we identify conceptually become database tables in their own right.
The seller table should contain data applicable only to sellers. Thus, the qualities (attributes) about sellers that we want to store become columns in our seller table. Each row (record) in the seller table represents an individual seller. Each individual seller is uniquely identified in the seller table with a unique value stored in it's primary key column, which we can name seller_id.
A simplified version of such a table could look like this:
In a similar manner, the customer table should contain data only applicable to customers. The qualities (attributes) about customers that we wish to store become the columns in the customer table. Each row (record) in the customer table represents an individual customer. Each individual customer is uniquely identified in that table with a unique value in it's primary key column, which we can declare as customer_id.
A simplified version of this table:
I'm guessing the business rules state that any customer is able to request any number of products, from any seller, any number of times...since surely any seller would want as many sales and customers as possible!
How can we express and record the interactions (relationship) between seller and customer?
This is done with a new kind of entity: a composite entity. It becomes a new table, having it's own primary key, and contains seller_id and customer_id as foreign keys. The foreign keys in this table connect (relate) the seller table to the customer table.
We can name this new table quotation (if that is your preferred name). Each row of this table is intended to capture and record each and every individual transaction between a customer and a seller. The columns (attributes) of this table are the data that apply to a transaction between a customer and seller, such as amount or date of sale.
A very simplified version of this composite entity:
Note that the foreign key values that exist in this table must already exist in their respective tables as a primary key value. That is, a foreign key value cannot be entered into this table unless it exists already as a primary key value in it's own table. This is important, and it is called referential integrity - it ensures that there is no record of a customer purchasing from a non-existent seller, etc.
In the example above we can see that Builder B requested a quotation from Acme Construction in the amount of $3500.00. They then requested another quotation at another time for the amount of $1800.00. What else does it reveal? All existing customers have ordered something. Acme Lumber has not made a sale at all (yet), etc.
A design such as this enables the database to store any number of transactions between sellers and customers. Likewise, it supports the addition of any number of new customers and sellers, even if they have not sold or purchased anything yet. Queries can be run that reveal which sellers have sold the most or least, and so on.
Good luck with your studies!