33
votes
Accepted
Why the term "relation(al)"?
First of all, I highly recommend the scientific paper in which Dr. Edgar Frank Codd published the relational framework to the general public in 1970, i.e., A Relational Model of Data for Large Shared ...
- 8,540
17
votes
Accepted
Is a table without a primary key normalized?
If a relation does not have any candidate key (and the primary key is just one of the candidate keys), then it can have duplicate rows, so in fact it is not a relation! (since relations are always ...
- 4,080
16
votes
Why the term "relation(al)"?
The interesting thing behind 'relational database' is, that it does not (primarily) refer to the relations between tables, as you might expect, but it refers to the relation of multiple properties (...
- 1,456
14
votes
Accepted
Integrity constraints in a relational database - should we overlook them?
If, as stated in your post, the intention is to create a relational database (RDB for brevity) and, therefore, it is expected that it functions as such, the short answer is:
No, you should not ...
- 8,540
13
votes
Accepted
Unused columns in WHERE clause
Both queries are ridiculously complicated. Use
DELETE FROM "Foo"
WHERE "CreatedAt" < current_timestamp;
- 56.4k
11
votes
Does using composite keys violate 2NF?
Your definition of 2NF is not quite correct.
2NF is when a relation is in 1NF and it has no partial dependencies, meaning there are no predicates (columns) that depend on only part of a multi-part ...
- 12.6k
11
votes
Accepted
Why Cassandra is not considered a relational database?
Cassandra is not an RDBMS because it does not support the relational data model.
The fundamental assumption of the relational model is that all data is
represented as mathematical n-ary relations, an ...
- 47.6k
10
votes
Why the term "relation(al)"?
The term "relational" comes from mathematics and has nothing to do with relationships between entities. I'm not a mathematician (whereas Codd had a PhD in Mathematics) and so won't elaborate, but ...
- 3,558
9
votes
Integrity constraints in a relational database - should we overlook them?
The basic premise of your developers is absolutely wrong. Foreign keys will impact slightly the performance of the DML of your system. They are not used at all in queries thus have no effect on their ...
- 2,159
9
votes
Does the concept of candidate key exist only in theory?
As far as I know, no SQL database management system (DBMS) supplies the CANDIDATE KEY keyword as such, but (as I consider that you are suggesting in the question) that does not mean that the notion (...
- 8,540
9
votes
Accepted
Why is it considered that a set unnormalizes a database absolutely?
The reference for this post is an amazing book called Database System Concepts 6th Edition which I recommend you read.
In the book, page 328, it states:
A domain is atomic if elements of the ...
- 1,223
8
votes
First Normal Form: Definitive Definition
Preliminary
The definition of normal form (which from the presentation of “Further Normalization of the Data Base Relational Model” in 1971 is known as first normal form) and the definition of the ...
- 8,540
8
votes
Accepted
RDBMS: Proper way to store data - comma separated variables or in different fields or tables?
Why storing data as a string a problem :
Storing multiple datums as (in this case comma separated) strings is bad practice because:
First reason:
It breaches Codd's second rule (called the "...
- 30.6k
7
votes
Accepted
Database design: Normalizing a "(many-to-many)-to-many" relationship
Option 1
*This doesn't seem like a great idea to me, because it complicates the SQL to select all properties applied to a feature…
It does not necessarily complicate query SQL (see conclusion ...
- 40.2k
7
votes
Why the term "relation(al)"?
It's an intuitive name when you think of them with natural keys. You can think
of a cell value as representing an entity.
Relation: Employee
|--------+------------+--------|
| name | job | ...
- 289
7
votes
Accepted
Is combining independent tables called normalisation?
No, the task you are being asked to fulfill —arranging (a) columns that belong in two or more distinct base tables in (b) a single base table— is not called normalisation (nor normalization for those ...
- 8,540
7
votes
Accepted
How does normalization fix the three types of update anomalies?
1NF is basically just "don't keep too much data in a single column", so I think that 2NF and 3NF are the primary fix for all 3 database anomalies, since both 2NF and 3NF involve breaking out items ...
- 10k
7
votes
Is saving disk space still considered an advantage of normalization?
Normalization was never about saving disk space. In his 1971 normalization paper E.F.Codd wrote: "It is important to remember that we are not making a case for or against any physical storage ...
- 3,797
7
votes
How do I identify the SQL server brand via SQL queries
Go by elimination. Failing select @@version will eliminate both SQL Server and MySQL; failing select version() will eliminate both Postgres and MySQL; etc.
Examples of such queries have been helpfully ...
- 26.9k
6
votes
Accepted
What are object-relational databases, and why is this model necessary in spatial databases?
The relational model and the object-oriented paradigm
The state-of-the-art relational model, originated in 1970 by Dr. E. F. Codd, is applied science concerning the field of database administration. ...
- 8,540
6
votes
Accepted
Proof that Inner Join is Commutative
An inner join is the subset of rows from the cartesian product where a certain condition is true. Although the cartesian product is not commutative (nor associative), it is with regard to relational ...
- 13.6k
6
votes
Why the term "relation(al)"?
You have already accepted a very long answer that has to say a lot about databases, but let me answer the question you actually asked:
Why the term "relational".
Because a table is a concrete ...
- 351
6
votes
Accepted
How are joins Commutative and Associative?
You've made an incorrect assumption.
If three's a relationship between A and B, and between B and C but no relationship between A and C, joining A and C doesn't give you no rows. It actually gives ...
- 11.7k
6
votes
Accepted
Do my relational algebra operations produce the desired results?
This is indeed the error:
R4 := R2 ∩ R3
If we try to get the intermediate results of your solution, we get:
R1:
bid bname color sid date
--- --------- ----- --- -...
- 98.6k
6
votes
Accepted
What normalization rules does this table break
This is a terrible pattern, but it doesn't actually break any normalization rules. The reason is that it's actually a change in what you are modeling. Instead of your database modeling, say, ...
- 47.6k
6
votes
Accepted
Incorrect result when using 'case when count()=0' & 'where'
just move where conditions to the join part:
select U.Mail, count(P.ID) as PostCount,
case
when count(P.ID) = 0 then 'none'
when count(P.ID) <= 2 then 'few'
when count(P.ID)...
- 2,088
6
votes
One and only one relationship - ER diagram to relational schema
You have several possibilities, each of them with advantages and disadvantages.
Solution 1
Define a single table:
AB(a b c d e f)
with two candidate keys, b and e. You chose one of them as primary ...
- 4,080
5
votes
Why the term "relation(al)"?
Relational databases is based on the relational model of E.F.Codd. The relational algebra describes methods of how to query data.
A relation is simply a subset of the crossproduct of some sets (...
- 7,752
5
votes
Accepted
Difference between functional dependency and join dependency
Join Dependencies can be considered a generalization of Multivalued Dependencies, following the fact that a Multivalued Dependency X →→ Y in a relation R can be seen as another way of writing a binary ...
- 4,080
5
votes
Accepted
Does a computed column break 3NF (third normal form)?
Computed columns are not a construct of the relational model, so it doesn't make sense to try and evaluate them using relational model terms such as 3NF.
Computed columns are a proprietary extension ...
- 4,098
Only top scored, non community-wiki answers of a minimum length are eligible