I am new to SQL Server 2008 database development.
Here I have a master table named ‘Student’ and a child table named ‘Address’. The common column between these tables is ‘Student ID’.
My doubts are:
Do we need to put ‘Address Id’ in the ‘Address’ table and make it primary key? Is it mandatory? ( I won’t be using this ‘Address Id’ in any of my reports )
Is Primary key column a must in any table?
Would you please help me on these.
Would you please also refer best links/tutorials for SQL Server 2008 database design practices (If you are aware of) which includes naming conventions, best practices, SQL optimizations etc. etc.
1) Yes, having an ADDRESS_ID column as the primary key of the ADDRESS table is a good idea.
But having the STUDENT_ID as a foreign key in the ADDRESS table is not a good idea. This means that an address record can only be associated to one student. Students can have roommates, so they'd have identical addresses. Which comes back to why it's a good idea to have the ADDRESS_ID column as a primary key, as it will indicate a unique address record.
Rather than have the STUDENT_ID column in the ADDRESS table, I'd have a corrollary/xref/lookup table between the STUDENT and ADDRESS tables:
STUDENT_ADDRESSES_XREF
STUDENT_ID, pk, fk to STUDENTS table
ADDRESS_ID, pk, fk to ADDRESS table
EFFECTIVE_DATE, date, not null
EXPIRY_DATE, date, not null
This uses a composite primary key, so that only one combination of the student & address exist. I added the dates in case there was a need to know when exactly, because someone could move back home/etc after all.
Most importantly, this works off the ADDRESS_ID column to allow for a single address to be associated to multiple people.
2) Yes, defining a primary key is frankly a must for any table.
In most databases, the act also creates an index - making searching more efficient. That's on top of the usual things like making sure a record is a unique entry...
Every table should have a way to uniquely and unambiguously identify a record. Make AddressID the primary key for the address table.
Without a primary key, the database will allow duplicate records; possibly creating join problems or trigger problems (if you implement them) down the road.
Related
I want to create a table structure to store customers and I am facing a challenge: for each customer I can have multiple names, one being the primary one and the others being the alternative names.
The initial take on the tables looks like this:
CREATE TABLE dbo.Customer (
CustomerId INT IDENTITY(1,1) NOT NULL --PK
-- other fields below )
CREATE TABLE dbo.CustomerName (
CustomerNameId INT IDENTITY(1,1) NOT NULL -- PK
,CustomerId INT -- FK to Customer
,CustomerName VARCHAR(30)
,IsPrimaryName BIT)
Though, the name of the customer is part of the Customer entity and I feel that it belongs to the Customer table.
Is there a better design for this situation?
Thank you
Personally, I would keep the Primary name in the Customer table and create an "AlternateNames" table with a zero-to-many relationship to Customer.
This is because presumably most of the time when you are returning customer data, you are only going to be interested in returning the Primary Name. And probably the main (if not only) reason you want the alternate names is for looking up customers when an alternate name has been supplied.
Unfortunately, this is too long for a comment.
Before figuring this out, more information is needed.
Is additional information needed for names? For instance, language or title or date created?
Are the names unique? Is the uniqueness within a customer or over all names?
Are the primary names unique?
Does every customer have to have a primary name?
How often does the primary name change to an alternate name? (As opposed to just having the name updated.)
When querying the data, will you know if the name is a primary or alternate name? (Or do they all need to be compared?)
Depending on the answer to this question, the appropriate data structure can have some tricky nuances. For instance, if you have a flag to identify the primary name, it can be tricky to ensure that exactly one row has this value set -- particularly when updating rows.
Note: If you update the question with the answers, I'll delete this.
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 am working on a sql query which is complex. I have the idea in informatica but I am finding it hard to convert it to sql. I have a table which is a temp table created by various joins and unions.for that table which contains active student information I have to find major ids. Some of the students have only primary major id and they work in only a specific area other doesn't have a primary id but they could have multple secondary major ids. Major here means subject or course. In informatica I will use a router to separate the students with primary id and non primary Id in two groups and then for one's with primary Id do something and for non primary do something else. Plz excuse if there is any typo.
I have 2 tables:
roomtypes[id(PK),name,maxAdults...]
features(example: Internet in room, satelite tv)
Can both id and name field be unique in the same table in mysql MYISAM?
If the above is posible, I am thinking of changing the table to:
features[id(PK),name,roomtypeID] ==> features[id(PK),name,roomtypeNAME]
...because it is helping me not to do extra querying in presentation for features because the front end users can't handle with IDs.
Of course, you can make one of them PRIMARY and one UNIQUE. Or both UNIQUE. Or one PRIMARY and four UNIQUEs, if you like
Yes, you can define UNIQUE constraints to columns other than the primary key in order to ensure the data is unique between rows. This means that the value can only exist in that column once - any attempts to add duplicates will result in a unique constraint violation error.
I am thinking of changing the FEATURES table to features[id(PK), name, roomtypeNAME] because it is helping me not to do extra querying in presentation for features because the front end users can't handle with IDs.
There's two problems:
A unique constraint on the ROOM_TYPE_NAME wouldn't work - you'll have multiple instances of a given room type, and a unique constraint is designed to stop that.
Because of not using a foreign key to the ROOM_TYPES table, you risk getting values like "Double", "double", "dOUBle"
I recommend sticking with your original design for sake of your data; your application is what translates a room type into its respective ROOM_TYPE record while the UI makes it presentable.
I would hope so otherwise MySQL is not compliant with the SQL standard. You can only have one primary key but you can mark other columns as unique.
In SQL, this is achieved with:
create table tbl (
colpk char(10) primary key,
coluniq char(10) unique,
colother char(10)
);
There are other ways to do it (particularly with multi-part keys) but this is a simple solution.
Yes you can.
Also keep in mind that MySQL allow NULL values in unique columns, whereas a column that is a primary key cannot have a NULL value.
1 RoomType may have many Features
1 Feature may be assigned to many RoomTypes
So what type of relationship do i have? M:N ?
You have there a many-to-many relationship, which has to be represented by an extra table.
That relationship table will have 2 fields: the PK of RoomTypes and the PK of Features.
The PK of the relationship table will be made of those 2 fields.
If that's usefull, you can add extra fields like the Quantity.
I would like to encourage you to read about database Normalization, which is he process of creating a correct design for a relational database. You can Google for that, or look eventually here (there are plenty of books/web pages on this)
Thanks again for very helpful answers.
1 roomType may have many features
1 feature may be assigned to many roomTypes
So what type of relationship do i have? M:N ?
If yes the solution I see is changing table structure to
roomTypes[id,...,featuresIDs]
features[id(PK),name,roomtypeIDs] multiple roomTypesIDs separated with comma?