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148 lines
4.7 KiB
Markdown
148 lines
4.7 KiB
Markdown
# Databases
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Organized collection of inter-related data that models some aspect of the real-world. Databases are the core component of most computer applications.
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A **DBMS** is the software that allows applications to store and analyze information in a database.
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A general-purpose DBMS is designed to allow the definition, creation, querying, update and administration of database Management System
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A **data model** is a collection of concepts for describing the data in a database. A **schema** is a description of a particular collection of data, using a given data model.
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## Relational Model
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It was proposed by Ted Codd in 1970. It's an abstraction to avoid high maintenance of the DBMS software:
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- store database in simple data structures-
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- access data through high level language.
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- physical storage left to the implementation.
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- store database in simple data structures-
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- access data through high level language.
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- physical storage left to the implementation.
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Concepts:
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- **Structure**: the definition of relations and their contents.
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- **Integrity**: ensures that the database's contents satisfy constraints.
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- **Manipulation**: how to access and modify a database's content.
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### Relations
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A **relation** (aka **table**)is an _unordered_ set that contains the relationship of the _attributes_ (aka _fields_) that represent entities.
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The **domain** of a relation is the set of possible values that the relation can contain.
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A **tuple** (aka **record**) is a set of attribute values in a relation. Values are (normally) atomic/scalar. The special value `NULL` is a member of every domain.
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### Primary Keys (PK)
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A relation's **primary key** uniquely identifies a single tuple. Some DBMSs automatically create an internal primary key if one is not provided.
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### Foreign Keys (FK)
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A **foreign key** specifies that an attribute from one relation has to map to a tuple in another relation.
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## Data Manipulation Language (DML)
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The **data manipulation language** describes how to _store_ and _retrieve_ information from a database.
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Kinds of DMLs:
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- **Procedural**: the query specifies the (high-level) strategy that the DBMS should use to find the desired result. (Relational Algebra)
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- **Non-Procedural**: the query specifies only what data is wanted and not how to find it. (Relational Calculus)
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## Relational Algebra
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Set of fundamental operations to retrieve and manipulate tuples in a relation. Each operator takes one or more relations as inputs and outputs a new relation. This allows to chain operations together to create more complex operations.
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Relational algebra describes the steps needed to obtain a particular result. The order of the steps does influence the performance of the complete operation.
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Fundamental Operators:
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- Selection (`σ`)
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- Projection (`π`)
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- Union (`U`)
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- Intersection (`∩`)
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- Difference (`-`)
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- Product (`x`)
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- Natural Join (`|X|`)
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Extra Operators:
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- Rename (`p`)
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- Assignment (`R ← S`)
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- Duplicate Elimination
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- Aggregation (`Y`)
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- Sorting (`τ`)
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- Division (`÷`)
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> **Note**: reactional algebra operates on sets. A set is an unordered list of unique values.
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### Select (`σ`)
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Choose a subset of tuples from a relation that satisfies a selection predicate. Predicates acts a filters to retain only tuples that fulfill its qualifying requirements. It's possible to combine multiple predicates using conjunctions/disjunctions.
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Syntax: `σ(R)`
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```sql
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SELECT * FROM R WHERE <predicate>;
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```
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### Projection (`π`)
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Generate a relation with tuples that contain only the specified attributes. Allows to rearrange attribute ordering and can manipulate values.
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Syntax: `π(R)`
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```sql
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SELECT (<tuple>) FROM R;
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```
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### Union (`U`)
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Generate a relation that contains all tuples that appear in either only one ore both input relations.
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Syntax: `(R U S)`
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```sql
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(SELECT * FROM R) UNION ALL (SELECT * FROM S);
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```
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### Intersection (`∩`)
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Generate a relation that contains only the tuples that appear in both of the input relations.
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Syntax: `(R ∩ S)`
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```sql
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(SELECT * FROM R) INTERSECT (SELECT * FROM S);
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```
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### Difference (`-`)
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Generate a relation that contains only the tuples that appear in the first and not the second of the input relations.
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Syntax: `(R - S)`
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```sql
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(SELECT * FROM R) EXCEPT (SELECT * FROM S);
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```
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#### Product (`x`)
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Generate a relation that contains all possible combinations of tuples from the input relations.
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Syntax: `(R x S)`
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```sql
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SELECT * FROM R CROSS JOIN S;
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SELECT * FROM R, S;
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```
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### Natural Join (`|X|`)
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Generate a relation that contains all tuples that are a combination of two tuples (one from each relation) with common value(S) for one or more attributes.
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> **Note**: the matching values must be on the same fields.
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Syntax: (`R |X| S`)
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```sql
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SELECT * FROM R NATURAL JOIN S;
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```
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