A relational database is a collection of relations
- everything is a table
- every row in a table has the same number of columns
- relations are implicit: no pointers
Structure of Data
A relation is a two-dimension table
For example, Consider a Movie relation:
| Title |
Year |
Length |
Genre
|
| Gone with the Wind |
1939 |
231 |
Drama
|
| Star Wars |
1977 |
124 |
Sci-Fi
|
Columns
- The columns of a relation are named by attributes
- E.g.: Title, Year, etc.
- They describe the meaning of entities in columns
The name of a relation with its attributes is called schema
- e.g.: Movies(Title, Year, Length, Genre)
- Note that the attributes form a set, not a list
- A database consists of one or more relations, each with a schema
- And the set of all schemata within one database is called Relational DB Schema
Rows of a relation (other than the header row - which describes the attributes) are called tuples
- ("Gone with the Wind", 1939, Drama)
- We don't include the attribute names, so the order is important here
We usually associate a domain with each attribute - a particular elementary type
- E.g.: Movie(Title: String, Year: Integer, Length: Integer, Genre: String)
Operations on Data
- Usually the operations are Relational Algebra expressions
- SQL is usually transformed to Relational Algebra for processing
Constraint
One fundamental constraint in this model is key constraint
A set of attributes form a key for a relation if there are no two tuples with the same values in for the attributes of the key
We indicate that the attributes form a key by underlining them:
- e.g.: Movies(Title, Year, Length, Genre)
Modeling Relational Databases
Query Processing
- Main Article: Query Processing
How to translate a SQL query into physical query plan
Sources