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?
Related
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.
Its just a quick question :
I have user table and it has fields name, address and books_bought. books_bought is foreign key and its value is some PK from other table. Now after 1st insert, I will fill out all of this fields , but after second INSERT I want only to add a additional books_bought, so that am creating array of books_bought values?
You're doing it the wrong way around - this is a one-to-many relationship i.e. many books bought to one user. You need to have the foreign key on the many side of the relationship, so instead of having a foreign key to books_bought on the users table, add a foreign key to users on the books_bought table.
If you have a books table and a users table, then this is a many-to-many relationship and you will need a link table to go between them to hold the foreign keys.
You should not have more than one book in the books_bought cell because it will violate the atomicity constraint fo the database tables. You have to have a separate entry for each book_bought. This would cause a lot of redundant information as name, address would be repeated for each book bought by a specific person.
To solve this, you have to split the table into something like this:
R1(primary_key , name , address) and R2(foreign_key , books_bought)
Here foreign_key refers to primary_key of R1
I am in a situation where i have to store key -> value pairs in a table which signifies users who have voted certain products.
UserId ProductID
1 2345
1 1786
6 657
2 1254
1 2187
As you can see that userId keeps on repeating and so can productId. I wanted to know what can be the best way to represent this data. Also is there a necessity of using primary key in here. I've searched a lot but am not able to find the exact specification about my problem. Any help would be appreciated. Thank you.
If you want to enforce that a given user can vote for a given product at most once, create a unique constraint over both columns:
ALTER TABLE mytable ADD UNIQUE INDEX (UserId, ProductID);
Although you can use these two columns together as a key, your app code is often simpler if you define a separate, typically auto increment, key column, but the decision to do this depends on which app code language/library you use.
If you have any tables that hold a foreign key reference to this table, and you intend to use referential integrity, those tables and the SQL used to define the relationship will also be simpler if you create a separate key column - you just end up carting multiple columns around instead of just one.
Let's assume there is a table, with theese rows:
-personID,
-personName,
-personInterests
There is also another table, which stores the interests:
-interestID
-interestName
One person can have multiple interests, so I put the serialize()-d or JSON representation of the interest array into the interest field. This is not a String, like "reading", buth rather an index of the interests table, which stores the possible interests. Something like multiple foreign keys in one field.
The best way would be to use foreign keys, but it is not possible to achieve multiple references in one field...
How do I run such a query, without REGEX or splitting the field's content by software? If putting indexes to one field is not the way to go, then how is it possible, to achieve a structure like this?
Storing multiple indexes or any references in one field is strictly not advised.
You have to create something that I call "rendezvous" table.
In your case it has:
- ID
- UserID (foreign key)
- InterestID (foreign key)
Every single person can have multiple interests, so when a person adds a new interest to himself, you just add a new row into this table, that will have a reference to the person and the desired interest with a foreign key NOT NULL.
On large-scale projects when there are too many variations available, it is advised, to not to give an ID row to this table, but rather set the two foreign keys also primary keys, so the duplication will be impossible and the table-index will be smaller, as well as in case of lookup, it will consume less from the expensive computing power.
So the best solution is this:
- UserID (foreign key AND primary key)
- InterestID (foreign key AND primary key)
I believe the only way you can implement this is to create a third table, which will actually get updated by a trigger (Similar to what Gabor Dani advised)
Table1
-personID,
-personName,
-personInterests
Table2
-interestID
-interestName
Table3
-personInterestID (AutoIncrement Field)
-personID
-interestID
Then you need to write a trigger which will do this a stored procedure may be needed because you will need to loop through all the values in the field.
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