Introduction to relational model theory

as everyone knows , Relational databases occupy a large share in the database market . Relational database has a solid theoretical foundation ---- Relational models and relational algebra .

The following is a brief summary of relational models and relational algebra , As the basis for subsequent understanding and learning of relational databases .

Relational databases deal with structured data , To be clear, it is to deal with data with fixed format , Table is its representative , So a relational database is a collection of tables （ Set theory ）

1. The basic structure

attribute ： A column in the table

Domain ： The value range of an attribute value in the table

Relationship ： A subset of Cartesian products over a series of fields （ surface ）

Tuples ： One line of the table

Tuple variables ： Variables representing tuples

database schema ： Design logic of database

Relationship model ： Normal is the definition of the database table ---DDL（ Such as mysql System table in information.schema,performance.schema）

Database instance ： A snapshot of database data at a given time （ Multi table and multi database ）

Relationship instance ： The value of the specific mode , That is, the value of the table at that time .

Main code ： Candidate codes that distinguish tuples in the same relationship

Hypercode ： A collection of one or more attributes , The combination of these attributes can make it uniquely identify a tuple in the relationship .

Candidate code ： Small value of super code .

Here, the specific database is the database key,primary key etc. .

The specification of code represents the constraints of the modeled thing in the real world .

Outer code ： A relational schema may contain the main code of another relational schema in its attributes （ Foreign keys , Referential integrity , Reference relationship ）

Pattern ： For the design of specific patterns and the relationship between patterns .

query language ： The language that users use to get information from a database . It is divided into process and non process

Process ： Operation must have results （ The practical application ）

Non procedural ： The operation only needs to describe the information , But there is no need to give the specific process of obtaining this information （ Theoretical deduction ）

In reality, it is often a combination of the two. For example, we delay materialization , That is to try to be non procedural first , Finally, process , Because the process involves the acquisition of data , Compare the consumption of resources .

2. Basic operations of relational algebra

choice （select）： Select the tuple that satisfies the specified predicate .（ Unary operation ）

Projection （project）： Return some attributes of the relationship as parameters （ duplicate removal ）（ Unary operation ）

Set operations ---- Know from the previous knowledge , Relational databases are collections , So the knowledge use of set theory applies to it

and 、 Bad

The cartesian product

renamed （ Unary operation ）

hand over

except

assignment （ Cannot increase the expressive ability of relational algebra , But it can make the expression of complex queries simple ）

Natural join （natural join）: Choose based on the equality of attributes that appear in both relationship patterns , Finally go to heavy -- Simply put, there are equal attributes in the two relationships, which are ties , Just combine the elements of these equal attributes and remove the duplication , The semantics are simple .

3. Extended relational algebra operation

Generalized projection ： Projection plus aggregation

Aggregation function ：

sum,min,max,avg,count etc.

External connection ：

The left outer join ： Based on the relationship on the left , To match with the tuple of the right relation , Mismatched additions null

Right connection ： Based on the relationship on the right ....

Full outer join ： All combinations of left-right relationships , Fill in the blank null

Null processing ---- Null value is an indeterminate , As long as it is calculated with a null value, it is null , Null value and null value are logically considered the same .

Database modification

Delete , Insert , to update （ Real database single operation, batch operation, etc ）