I have a problem in designing a database for a "business". I have some database for different types of products( for example car parts, groceries and some other stuff). Now if I want to make a business, meaning that I want to sell these items, I am thinking that I am going to need a table like Products which will contain a product foreign key, a stock and a price. My question is how can I use foreign keys in my Products table from my existing tables? I was thinking of using a combination of foreign keys (i.e in the Products table I would have a column for each product type), but I don't think it's such a good idea. What if I would want to expand my business to other types of products? Any ideas?
Consider inheritance:
You can use a table products which will be supertype of the products you have and refer to this table though the foreign key. If you want more info about how to implement inheritance click here
Related
I'm having trouble to develop my database model.
I have the tables TOUR and TRIP (both with GUID primary key) and both have some children tables in common. Like DETAIL, COMMENT, MEDIA, PRICE and like that.
I'm thinking in create in those children tables some "ParentId" column to link with TOUR or TRIP, but I need to know which parent table is. In this way I should to create a column "ParentType" where 1 is TOUR and 2 is TRIP or maybe create a TourDetail and a TripDetail to link each one with other table?
I'm looking for the best practice to do that.
In terms of design, the SQL convention is that each relationship should have its own column in the referencing table. That's the concept if foreign key columns.
So you need to create two columns in each child table (DETAIL, COMMENT, MEDIA, PRICE), like TOUR_ID and TRIP_ID that will contain the primary key of the correspondig records in, respectively, tables TOUR and TRIP. Of course if a children refers to only one parent and not the other, only one column is necessary.
This will then allow you to build proper JOIN queries like :
SELECT
...
FROM TRIP
JOIN DETAIL on DETAIL.TRIP_ID = TRIP.ID
...
WHERE
...
Apologies for the newbie question.
The primary key of a table, such as Holiday, would be something like Holiday_ID. Holiday reference a get-away ticket that you can buy to go on a type of holiday, based on the ticket you buy.
Suppose I used Holiday_ID in a composite entity with Customer_ID to identify an instance of Holiday associated with customer, for whatever purpose.
However, suppose I also want to keep track of other information related to this instace: how much has the customer paid for the ticket, how much has the customer yet to pay for the ticket
I have two options:
a) I can create another composite entity. However, I am not sure if I can do that because I am not sure if you can use a particualr foreign key more than once
b) I can create a composite/associate entity, however, I am not sure if you can create a composite entity with more than two foreign keys?
To answer the technical parts of your question, once you create a composite unique or primary key, ONLY ONE record in the table can have the same values in the set of fields defined in that key. SO, no, you cannot reuse the holidayId key WITH THE SAME customer. You can use it with another, different customer if you wish.
Second, there is no limit to the number of attributes that can be included in a Unique or primary key. If you need, and if it's appropriate and conforms to the rules of normalization, the key can include all the attributes of the table.
Third, to answer your question below, Any column, or set of columns in a table can be defined as a Foreign Key, as long as it is also the primary key or unique key of some table in the database. And there can be any number of FKs defined in a table, they can even overlap. (you can have HolidayId as a FK, and also have HolidayID and CustomerId as a composite FK) the only restriction is that the FK must reference a Primary or Unique Key of some table in the database.(It can also be the same table the FK is in as well, as when you add a supervisorId to an employee Table that is a FK to the EMployeeId of the same employee table)
This example illustrates one of the problems of using surrogate keys without also using a natural key. to wit, what, exactly is a "Holiday"? Is Christmas 2016 the same "Holiday" as Christmas 2015? Is Christmas in Aruba the same holiday as Christmas in Hawaii?
and then, about the composite table to identify associations of customer with Holiday, is it the same association if the customer goes to Aruba on Christmas the next year, or a different instance? What does the row in the table represent if the customer wants 5 tickets?
The first thing that should be done in database design is a logical design which defines, as clearly and unambiguously as possible, in business terms, the meanings of the entities for each table in the database.
I have a products table link to a bill of materials table. See diagram
Each product can have more than one formula. For example:
Currently ProdIDNeed and ProdIDNeeded are my composite primary key, both fields also link as foreign keys to ProdID (Products). The only way I can think of preventing a key violation is to create a Formula field and have a 3 field composite primary key (ProdIDNeed, ProdIDNeeded, FormulaNumber).
However, I have to link the product_billmaterials table to a workorders table (bascially an order to make the product according to formula). Linking three fields to another table is a pain.
I guess I could also create a surrogate key on the product_billmaterials table which I am not too crazy about either.
Is there any other way I can organize this or must I choose one of the options I have thought of?
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 have a doubt about this. I have 2 tables
Ordered_products
ordered_product_id
product_id
quantity
price_charged
Ordered_items
ordered_item_id
item_id
unit_of_measure
box_type
Each ordered product can be composed by one or more items and one item has only one ordered product.
Which solution is better:
Sol 1: Just add as foreign key the ordered_product_id into ordered_items table. (http://cl.ly/image/0g3G2J231U0P)
Sol 2: Create a new table with ordered_product_id and ordered_items keys. (http://cl.ly/image/0Z1D1C1g0R3t)
Please give me any advice
Solution one is better. The only time you need seperate table is for many to many relationships or when that table needs to store information specific to the relationship, like when it starts and ends.
With a one to many relationship, just add a column to the many table to point to its parent. It makes joins easy and clear.
Solution 1. Don't create unnecessary tables.
Association tables are only used for many-to-many relations.