Alright so I read from somewhere
Every table should have a primary key
But some of my tables don't seem to behave!
I'd also like to know whether the relations as I'm using are fine or I need to dissolve them further, I'm open to suggestions.
The relations are
Dealers(DealerId(PK),DealerName)
Order(DealerId(FK),OrderDate,TotalBill)
Sales(DealerId(FK),ItemType,OrderDate,Quantity,Price)
P.S. I can't make a table named Items(ItemCode,Type,Price) Because the price is variable for different dealers. And all the constraints i.e not null + check that I needed are dealt with already just didn't mention.
1. Are the relations dissolved well?
2. Should I care about setting primary keys in the tables that don't have it already?
Helpful responses appreciated.
In your case, you should add an auto increment integer field to Order and Sales and set that to be the primary key.
In Relational Database Theory, you can sometimes identify a sub-set of the fields to use as a primary key, as long as those columns are non-null and unique. However, (1) the order table cannot have a primary key from DealerID and OrderDate because a dealer could make two orders on the same date. Maybe even for the same amount, which would mean that no sub-set of fields is unique, and (2) even when familiar data can uniquely identify the data, an auto-increment integer can be a good key.
I also think that you want a foreign key from Sales to Order. You are probably using DealerId and OrderDate for joins, but this will not work correctly if a dealer makes two orders on the same date.
Finally, take advice like
Every table should have a primary key
with a grain of salt. Linking tables used for many-to-many relationships can work perfectly fine without a primary key, although a primary key can be an improvement, since it will make deleting records easier, and if you don't have a primary key on a linking table, I would still recommend a unique index on all the fields, in which case that can be the primary key.
Do you really need separate Sales Table ?
Dealers(DealerId(PK),DealerName)
Order(OrderId(PK), DealerId(FK),OrderDate, ItemType, Quantity,Price)
Also,
TotalBill (can be calculated) = Quantity * Price
About the question 1 you should answer this question:
A sale can be made without an order?
If yes, your DealerId(FK) in Sales is alright, assuming that a sale will only exist if a dealer made it.
If no, you should put an OrderId(FK) in Sales, instead of DealerId(FK). If a sale belongs to an order, this order belongs do a dealer, so you already have the relation from dealers to sales.
About the question 2, you should have primary keys on your tables, because this is the way you have to select, update and delete some specific item on your database. Remembering that a primary key is not always an auto increment column.
And about the Items table, if the price is variable to different dealers, so you have an M to N relationship between Dealers and Items, which means you could have an intermediate table like this example:
DealerItemPrices(DealerId(FK), ItemId(FK), Price)
And these two Foreign Keys should be Unique Composite Keys, in this way a Dealer Y can't have two distinct prices to the same item.
Hope it helps!
Related
Sometimes, there are certain tables in an application with only one column in each of them. Data of records within the respective columns are unique. Examples are: a table for country names, a table for product names (up to 60 characters long, say), a table for company codes (3 characters long and determined by the user), a table for address types (say, billing, delivery), etc.
For tables like these, as the records are unique and not null, the only column can be used as the primary key, technically speaking.
So my question is, is it good enough to use that column as the primary key for the table? Or, is it still desirable to add another column (country_id, product_id, company_id, addresstype_id) as the primary key for the table? Why?
Thanks in advance for any advice.
there is always a debate between using surrogate keys and composite keys as primary key. using composite primary keys always introduces some complexity to your database design so to your application.
think that you have another table which is needed to have direct relationship between your resulting table (billing table). For the composite key scenario you need to have 4 columns in your related table in order to connect with the billing table. On the other hand, if you use surrogate keys, you will have one identity column (simplicity) and you can create unique constraint on (country_id, product_id, company_id, addresstype_id)
but it is hard to say this approach is better then the other one because they both have Pros and Cons.
You can check This for more information
This may seem like a simple question, but I am stumped:
I have created a database about cars (in Oracle SQL developer). I have amongst other tables a table called: Manufacturer and a table called Parentcompany.
Since some manufacturers are owned by bigger corporations, I will also show them in my database.
The parentcompany table is the "parent table" and the Manufacturer table the "child table".
for both I have created columns, each having their own Primary Key.
For some reason, when I inserted the values for my columns, I was able to use the same value for the primary key of Manufacturer and Parentcompany
The column: ManufacturerID is primary Key of Manufacturer. The value for this is: 'MBE'
The column: ParentcompanyID is primary key of Parentcompany. The value for this is 'MBE'
Both have the same value. Do I have a problem with the thinking logic?
Or do I just not understand how primary keys work?
Does a primary key only need to be unique in a table, and not the database?
I would appreciate it if someone shed light on the situation.
A primary key is unique for each table.
Have a look at this tutorial: SQL - Primary key
A primary key is a field in a table which uniquely identifies each
row/record in a database table. Primary keys must contain unique
values. A primary key column cannot have NULL values.
A table can have only one primary key, which may consist of single or
multiple fields. When multiple fields are used as a primary key, they
are called a composite key.
If a table has a primary key defined on any field(s), then you cannot
have two records having the same value of that field(s).
Primary key is table-unique. You can use same value of PI for every separate table in DB. Actually that often happens as PI often incremental number representing ID of a row: 1,2,3,4...
For your case more common implementation would be to have hierarchical table called Company, which would have fields: company_name and parent_company_name. In case company has a parent, in field parent_company_name it would have some value from field company_name.
There are several reasons why the same value in two different PKs might work out with no problems. In your case, it seems to flow naturally from the semantics of the data.
A row in the Manufacturers table and a row in the ParentCompany table both appear to refer to the same thing, namely a company. In that case, giving a company the same id in both tables is not only possible, but actually useful. It represents a 1 to 1 correspondence between manufacturers and parent companies without adding extra columns to serve as FKs.
Thanks for the quick answers!
I think I know what to do now. I will create a general company table, in which all companies will be stored. Then I will create, as I go along specific company tables like Manufacturer and parent company that reference a certain company in the company table.
To clarify, the only column I would put into the sub-company tables is a column with a foreign key referencing a column of the company table, yes?
For the primary key, I was just confused, because I hear so much about the key needing to be unique, and can't have the same value as another. So then this condition only goes for tables, not the whole database. Thanks for the clarification!
I've read similar questions both on Google and stackoverflow, for example, this thread Should each and every table have a primary key? and I understand it's generally a good idea to have a primary key in every table.
I'm now trying to create a simple table, it stores end of day prices for a list of stocks, so it has three columns: stock ticker, date and price. Apparently none of these three columns are unique and to use the table I'll need to join on both date and stock ticker (I have a unique constrain on that). Of course, I can add another surrogate id column just for the sake of having a primary key, but I just want to check if this is an acceptable design or there're better ways to model the data I'm storing?
Many thanks.
I am revising a legacy multi-tenant application where the shopping cart function stores multiple vendors and multiple clients in the same database. Some clients of one vendor may be clients of a different vendor. Some vendors might actually be clients of another vendor.
I currently have a table for the super-type 'party' with primary key party_ID, a table for the subtype 'company' with primary key company_ID (references party_ID) and a table for the role of 'vendor' with primary key vendor_ID (references company_ID). I also have a junction table, 'client' with a composite primary key of vendor_ID and party_ID.
My question is how should the 'order' table reference the vendor and client tables? My first thought is that the table should have a composite primary key of vendor_ID, client_ID and order_ID (order_ID could be auto-increment across the table or sequential per vendor_ID + client_ID)
but this seemed a bit fishy as there were three attributes making up the key...
Does anyone have any insight into this topic? Most 'shopping carts' only deal with a single vendor, so the order table simply lists client_ID as a foreign key.
Thanks!
My question is how should the order table reference the vendor and
client tables? My first thought is that the table should have a
composite primary key of 'vendor_ID', 'client_ID' and 'order_ID' but
this seemed a bit fishy as there were three keys...
Composite primary key doesn't mean three keys. It means one key consisting of three columns.
But that's not the real issue.
An order is an accounting record; it must not change over time. Storing the ID numbers is risky unless you've built temporal tables, and I doubt you've done that. If a vendor changes its name today, its name no longer matches the name on earlier orders. You must not let that happen with accounting records.
Unless you mean something unusual by "order", I'd expect Order_id to be its primary key. There might be other constraints; there might even be other key constraints to prevent duplicate orders that differ only by Order_id. But I'd still expect Order_id to be the primary key of a table of orders.
If vendors and clients are subtypes, I'd expect any (high risk) id numbers you store to reference the id numbers in the subtype tables. In your case, you seem to have an additional table that identifies the clients of vendors; it contains the columns {vendor_id, client_id}. The foreign key references for that table should be obvious.
Your table of orders should have one foreign key reference to that table, not one foreign key to vendors and another foreign key to clients. So in the table of orders, foreign key (vendor_id, client_id) references vendor_clients (vendor_id, client_id). The table of vendor clients will need either a primary key constraint or a unique constraint on {vendor_id, client_id}.
But you shouldn't do that for accounting unless you're using temporal tables. Instead, you should probably store both the id numbers and the text.
I would start with something like this. I do admit that I still do not quite understand difference between company, vendor, and client in your question. As Catcall mentioned, in this model you are not allowed to delete Parties (People, Organizations); accounting records should be frozen -- usually by capturing current customer/supplier info in order table.
For your primary key, you'll want just order_id.
Really, the composite (and unique) key I would use would be [vendor_id, client_id, occurredAt] (where occurredAt is a timestamp) - assuming orders could only be placed once a millisecond. However, this is something of a wide key, and some systems don't appreciate those. You'll still need these columns, and probably indexed, however.
I'm designing a database and I'm not sure how to define one of the relationships. Here's the situation:
An invoice is created
If the product is not in stock then it needs to be manufactured and so a work order is created.
The relationship is one-to-one. However work orders are sometimes created for other purposes so the WorkOrder table will also be linked to other tables in a similar one-to-one relationship. Also, some Invoices won't have a work order at all. This means I can't define these relationships in the normal way by using the same primary key in both tables. Instead of doing this I've created a linking table and then set unique indexes on both fields to define the one-to-one relationship (see image).
(source: markevans.org)
.
Is this the best way?
Cheers
Mark
EDIT: I just realised that this design will allow a single work order to be linked to an invoice and also to one of the other tables I mentioned via 2 linking tables. I guess no solution is perfect.
Okay, this answer is SQL Server specific, but should be adaptable to other RDBMSs, with a little work. So far as I see, we have the following constraints:
An invoice may be associated with 0 or 1 Work Orders
A Work Order must be associated with an invoice or an ABC or a DEF
I'd design the WorkOrder table as follows:
CREATE TABLE WorkOrder (
WorkOrderID int IDENTITY(1,1) not null,
/* Other Columns */
InvoiceID int null,
ABCID int null,
DEFID int null,
/* Etc for other possible links */
constraint PK_WorkOrder PRIMARY KEY (WorkOrderID),
constraint FK_WorkOrder_Invoices FOREIGN KEY (InvoiceID) references Invoice (InvoiceID),
constraint FK_WorkOrder_ABC FOREIGN KEY (ABCID) references ABC (ABCID),
/* Etc for other FKs */
constraint CK_WorkOrders_SingleFK CHECK (
CASE WHEN InvoiceID is null THEN 0 ELSE 1 END +
CASE WHEN ABCID is null THEN 0 ELSE 1 END +
CASE WHEN DEFID is null THEN 0 ELSE 1 END
/* + other FK columns */
= 1
)
)
So, basically, this table is constrained to only FK to one other table, no matter how many PKs are defined. If necessary, a computed column could tell you the "Type" of item that this is linked to, based on which FK column is non-null, or the type and a single int column could be real columns, and InvoiceID, ABCID, etc could be computed columns.
The final thing to ensure is that an invoice only has 0 or 1 Work Orders. If your RDMBS ignores nulls in unique constraints, this is as simple as applying such a constraint to each FK column. For SQL Server, you need to use a filtered index (>=2008) or an indexed view (<=2005). I'll just show the filtered index:
CREATE UNIQUE INDEX IX_WorkItems_UniqueInvoices on
WorkItem (InvoiceID) where (InvoiceID is not null)
Another way to deal with keeping WorkOrders straight is to include a WorkOrder type column in WorkOrder (e.g. 'Invoice','ABC','DEF'), including a computed or column constrained by check constraint to contain the matching value in the link table, and introduce a second foreign key:
CREATE TABLE WorkOrder (
WorkOrderID int IDENTITY(1,1) not null,
Type varchar(10) not null,
constraint PK_WorkOrder PRIMARY KEY (WorkOrderID),
constraint UQ_WorkOrder_TypeCheck UNIQUE (WorkOrderID,Type),
constraint CK_WorkOrder_Types CHECK (Type in ('INVOICE','ABC','DEF'))
)
CREATE TABLE Invoice_WorkOrder (
InvoiceID int not null,
WorkOrderID int not null,
Type varchar(10) not null default 'INVOICE',
constraint PK_Invoice_WorkOrder PRIMARY KEY (InvoiceID),
constraint UQ_Invoice_WorkOrder_OrderIDs UNIQUE (WorkOrderID),
constraint FK_Invoice_WorkOrder_Invoice FOREIGN KEY (InvoiceID) references Invoice (InvoiceID),
constraint FK_Invoice_WorkOrder_WorkOrder FOREIGN KEY (WorkOrderID) references WorkOrder (WorkOrderID),
constraint FK_Invoice_WorkOrder_TypeCheck FOREIGN KEY (WorkOrderID,Type) references WorkOrder (WorkOrderID,Type),
constraint CK_Invoice_WorkOrder_Type CHECK (Type = 'INVOICE')
)
The only disadvantage to this model, although closer to your original proposal, is that you can have a work order that isn't actually linked to any other item (although it claims to be for an e.g INVOICE).
What you have looks to be a perfectly normal way to construct your tables.
If you think you might like to use only one link table between your WorkOrder table and whatever other tables that may have WorkOrders, you could use a link table like:
WorkOrders
OtherId (Could be InvoiceId, or an ID for SomethingElse that may have a WorkOrder)
OtherType (ENUM - something like 'Invoice', 'SomethingElse')
WorkOrderId
So the issue is that you can have invoices that don't have work orders and work orders that don't have invoices but the two need to be linked when there is a link. I would say based upon that description that your database diagram is pretty good. This would open you up to allowing more than a one-to-one relationship. This way down the road you can consider having two work orders for one invoice. You might also have one work order that handles two invoices. This opens you up to a lot of possibilities that you may not need now but that you might in the future.
I would recommend your current design. In the future, you may want to add more information about the link between invoice and work order. This middle table will allow you to add this information.
In the interest of fairness to the other side of the coin, you do need to consider speed/number of tables/etc. that this will cause. For example, you have now created a third table which increased your table count by 50% in this example. Look at the rest of your database. If you did this everywhere, you would probably have the most normalized database but it might not be the most performant because of all the joins that are necessary. Basically, this isn't a "one-size-fits-all" solution. Instead it is a design choice. Personally, I hate nullable foreign key fields. I find they don't give me the granularity I usually want with my database designs.
Your schema corresponds to a many-to-many link between the 2 tables. You are de facto opening here the possibility to have one work order for multiple invoices, and multiple work orders for one invoice. The model offers then possibilities far above the rules you are setting.
You could use a simpler schema, that will reflect the (0,1) relation between work orders and invoices, and the (0,1) relation between Invoices and Work orders:
a Work Order can be independant from
an invoice, or linked to one specific
invoice: it has a (0,1) relation to Invoice table
An invoice can have no work orders, or one work orders: it has a (0,1) relation to Work Orders Table
Such a relation can be translated by the following model and rules
Invoice
id_Invoice, Primary Key
WorkOrder
id_WorkOrder, Primary Key
id_Invoice, Foreign Key, Nulls accepted, unique value
With such a structure, it will be easy to add new 'dependants' to work orders table. If, for example, you want to open the possibility to launch work orders from restocking orders (where you want to have minimal quantities of some items in stock), you can then just add the corresponding field to the WorkOrder table:
id_RestockingOrder, ForeignKey, Nulls accepted, unique value
You'll be then able to 'see' from where your WorkOrder comes: an invoice, a restocking order, etc.
Seems it corresponds to your needs.
Edit:
as noted by #mark, SQL Server will not allow multiple null values, in contradiction with ANSI specs (check here for some more details), As we do not want to wait for SQL Server 2011 to have this rule implemented, there is a workaround here, where you can build a view excluding the null values and set a unique index on this view. I must admit that I did not like this solution ...
There is still the possibility to implement the 'unique if not null' rule in your code. It will still be simpler than implementing the many-to-many model (with the Invoice_WorkOrder table) you are proposing and manage all additional unicity rules that you'll need to implement.
There is no real need for the link table, just have them linked directly and allow for NULL in the reference field of the work order. Because a work order can be linked to multiple tables what I would do is add a reference id on every work order to every table that can link from it. So you would have:
Invoice
PK - ID
FK - WorkOrderID
SomeOtherTable
PK - ID
FK - WorkOrderID
WorkOrder
PK - ID
FK - InvoiceID (allow NULL)
FK - SomeOtherTableID (allow NULL)
To make sure a WorkOrder is linked to only one item, you have to use code to validate the row (or perhaps a stored procedure which I cannot come up with right now).
EDIT: PS, if you want to use a link table, give it a generic name and add all the linked tables with the same sort of construct I just described allowing for NULL's. In my eyes adding the extra table makes the schema larger than it needs to be, but if a work order contains a lot of big text fields it could increase performance slightly and reduce database size with all the indexes flying around. In anything but the largest applications, I would consider it over-normalization though, but that is a matter of style.