BD - 05 Diseño de Base de Datos Relacional

8/10/2019 BD - 05 Diseo de Base de Datos Relacional

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CPEL- CARRERASUNIVERSITARIAS

PARA PERSONAS CON EXPERIENCIA LABORAL

Base de datos

Prof. Carlos Takano

Semana 2


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Diseo de base de datos

relacional


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CPELCARRERAS UNIVERSITARIAS PARAPERSONAS CON EXPERIENCIA LABORAL

Fases en el diseo de base de datos

Elmasri, R., & Navathe, S. (2011). Fundamentals of database systems(6th ed., p. 201).Boston: Addison-Wesley.


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Modelo conceptual de datos


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Modelos

Un modelo es Una abstraccin, una descripcin simplificada y

aproximada de la realidadPara un propsito especfico, se enfoca en un

aspecto particular de inters

Una herramienta para comprender, aprender yevaluar opciones

Un vehculo de comunicacin entre los diferentesgrupos de inters


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Modelo conceptual de datos

Debe describir con precisin y claridad las

necesidades de informacin del negocio

Facilita la comprensin del negocio y laevaluacin de opciones

Proporciona una base slida para el

diseo lgico y fsico de base de datos


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Ejemplo diagrama Entidad-Relacin


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Ejemplo diagrama Entidad-Relacin

Departamento

Cdigo departamento

Nombre departamento

Ubicacin departamento

Empleado

Cdigo empleado

Nombre empleado

email empleado

Cdigo departamento

Cdigo puesto

Competencia

Cdigo competencia

Descripcin competencia

Puesto

Cdigo puesto

Descripcin puesto

Dependiente

Cdigo empleado

Nombre

Tipo relacin

Tipo relacin

Tipo relacin


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Notacin: Entidad

Asociado

DNI

NombresApellido paterno

Apellido materno

Direccin

email

Telfono

GneroFecha nacimiento


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Notacin: Relaciones


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Normalizacin


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Dependencia funcional

Reglas o restricciones que se debe

cumplir

Son el objetivo

Para un determinante, la funcin devuelve

un dependiente

Claves determinan funcionalmente los

atributos no clave


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Anomalas

Son los sntomas de un problema

Insercin

Actualizacin

Eliminacin


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Redundancia

Es la causa


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Normalizacin

Es la solucin

Descomponer una relacin en otras

Mantener la informacin acerca de una cosaen un solo lugar

Inconsistencia=violacin / infraccin deintegridad

EjemplosDireccin: texto vs. estructura

Supertipos y subtipos


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Ejemplos

Claves

Casos


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Diseo de esquemas relacionales Puede haber muchos diseos diferentes

Algunos mejores que otros

Cmo elegir? Ejemplo:

Todos los atributos en una sola relacin

Redundancia, cantidad de tuplas

Actualizacin -> inconsistencia Eliminacin -> prdida de informacin

Mostrar diseo normalizado


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Boyce-Codd normal form is stricter than fourth normal form.

Specifically if we make a big Venn diagram here of all the relational designs that satisfiedBoyce-Codd Normal Form, which by the way is very often abbreviated BCNF, then thatcontains all of the relations that satisfy fourth normal form, normally abbreviated 4NF.

So every relation that's in fourth normal form is also in Boyce-Codd normal form, but notvice versa.

You might be wondering what happened to first, second and third, normal forms.

So first normal form is pretty much just a specification that relations are real relations withatomic values in each cell.

Second normal form is specifying something about the way relations are structured withrespect to their keys.

Neither of those is discussed very much anymore.

Third normal form is a slight weakening of Boyce-Codd normal form and sometimes people

do like to talk about third normal form. So you can think of third normal form as a little bit of a even bigger circle here.

We're not going to cover third normal form in this video because Boyce-Codd normal formis the most common normal form used if we have functional dependencies only, and fourthnormal form if we have functional and multivalued dependencies.


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Dependencias funcionales

Let's say that our student, 123Ann, applies to 7 colleges.

Then there will be 7 tuples and there will be 7 instances where we know that a student withthe social security number 123 is named Ann.

Specifically, we're going to store for every student the name and social security numberpair once for each college that they apply to.

So now let me explain what a functional dependency is and then we'll see how functionaldependencies are used to recognize when we have a bad design like this one, and to seehow we can fix it.

A functional dependency, in this case from social security number to name, and we'resaying social security number functionally determines the student name says that the samesocial security number always has the same name.

In other words, every time we see 123, we're going to see Ann.

Now it doesn't necessarily go in the other direction.

It might not be that whenever we see Ann, it's 123, but whenever we see 123, it is Ann. And so what we'd like to do is store that relationship just one time.

One time say that for 123, the name is Ann.


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4NF

Now, let's similarly motivate the concept of multi-valuedependencies, and fourth normal form.

It is actually a little bit more complicated, but it follows the samerough outline.

Now let's look at a different portion of the information about applyingand let's suppose, for now, that we're just concerned about students,what colleges they're applying to, and what high schools they wentto.

We still have redundancy and update and deletion anomalies.

For example, a student who applies to Stanford is going to have that

fact captured once for every high school that they went to. A student who went to Palo Alto high School will have that fact

captured once for every college they apply to.


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4NF

In addition, we get a kind of multiplicative effect here.

Because let's say a student applies to C colleges and they went to Hhigh schools; I know students don't go to a lot of high schools butlet's suppose that this is one that had moved a lot.

In that case, we're going to have C times H, tuples. What we'd really like to have is something more on the order of C

plus H, because then we'd be capturing each piece of informationjust once.

Now the interesting thing is that the badness of this particular designis not addressed by Boyce-Codd Normal Form, in fact this relation is

in Boyce-Codd Normal Form, because it has no functionaldependencies.

It's not the case that every instance of a social security number isassociated with a single college name or a single high school.


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4NF

As we will see later, if there are no functional dependencies, then the relation isautomatically in Boyce-Codd Normal Form, but it's not in and fourth normal form.

So fourth normal form is associated with what are called multi-value dependencies.

When we specify a multi-value dependency as we've done here with the double arrow,what this is saying is that if we take a particular social security number in the relation, wewill have every combination of college names that are associated with that social securitynumber with every high school that's associated with that social security number.

We'll actually see that when we have this multi-value dependency, we automatically havethis one, too.

I know it seems a bit complicated, and we will formalize it completely, but for now now justthink about the English statement that multi-valued dependency is saying that we are goingto have every combination of those two attributes and values in those attributes for a givensocial security number.

In other words, those values are really independent of each other.

So if we have that situation, then what we should really do is store each college name andeach high school for each social security number one time, and that's what fourth normalform will do for us.


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4NF

Fourth normal form, similarly to Boyce-Codd normal form, says if we have a dependency,then the left hand side must be a key.

In this case, it's a multi-value dependency we're looking at, so it's really saying somethingdifferent but the basic idea is the same which is that we want only one tuple that has eachvalue that's appears on the left hand side of a multi-value dependency.

So let's see what would happen, in this example, if we use our multi-value dependencies todecompose the relation, based on the idea of fourth Normal Form.

Well it is the intuitive thing that happens.

We separate the information about the college names that a student applies to from theinformation about the high schools themselves, and then we'll see that that we only storeeach fact once and we do get the behavior of having C plus H tuples instead of having Ctimes H tuples.

Like with functional dependencies and Boyce-Codd Normal Form we'll be completelyformalizing all of this reasoning and the definitions in later videos.

A -> B, no significa que B -> A

Dependencias triviales y no triviales


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Data Definition Languaje (DDL)

Comando Sirve para

CREATE Crear tablas y otros objetos

ALTER Modificar tablas y otros objetos

DROP Eliminar tablas y otros objetos


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Diseo en desarrollo de software

Diseo, en general, es el plan para construir unasolucin

Hay ms de una manera de hacerlo Es un trabajo tcnico y creativo, no mecnico Implica exploracin, descubrimiento, invencin,

desarrollo evolutivo

Estrategias: Proceso previo a la construccin

Disear mientras se construye


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Diseo en desarrollo de

software El diseo de base de datos es parte del diseo de una aplicacin de

software

Diseo de base de datos es esttico, muestra la estructura mas noel comportamiento

Comprende: Modelo conceptual: investigar el dominio del problema para identificar

los conceptos principales del negocio

Diseo lgico: definir qu tablas se necesita, qu atributos tendr cadauna y sus respectivas claves primarias y otras reglas de integridad.Definir vistas. Independiente del sistema de gestin de base de datossobre el que se implementar. Tcnica: normalizacin

Diseo fsico: parmetros de almacenamiento, creacin de ndices yotros objetos para optimizar el rendimiento

BD - 05 Diseño de Base de Datos Relacional

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