Schema and Schematic Diagram (1 Viewer)

[Seraph]

Okay i am so confused by these two defintiions

A schema is the data definition for a DB . Organised plan of the entire DB showing how and where data is found, description of the data And RELATIONSHIPS .
Like Student

Then a Schematic Diagram is a graphical tool used to define relationships in a DB . E.g Entity Relationship Diagram


okay so this is a Schema
Entity..................Attribute
Students...............StudentID
.....................................FirstName
.....................................LastName
.....................................Address
.....................................Suburb
.....................................PostCode
.....................................PhoneNumber
Teachers.......................TeacherID
.....................................Teacher
.....................................PhoneNumber
.....................................Extension
Classes.........................ClassID
.....................................ClassName
.....................................TeacherID
.....................................Units
Students and Classes..StudentClassID
.....................................ClassID
.....................................StudentID


Right.... But hang on a sec... doesnt an ENTITY relationship Diagram which is a SCHEMATIC DIAGRAM show basically the same thing? (only with links...) ???? i dont get it .. what is the major difference here between the two.......
??

oh btw while im here i might as well ask this question as well.... why exactly do relational databases reduce the undesirable duplication of data more than flat file databases???? is it cause every entity has individual records in a relational database

 

[SamD]

In terms of database design, a schema is an ERD; they are one and the same.

The whole point of relational DBs is to reduce duplicate data. For example think about a flat file version of your above schema... The teachers names need to be entered for every class they teach, hang on, actually they'd need to be entered for every student in every one of their classes. Most teachers have 5 classes so their name would appear some 125 times (5 classes by 25 students/class). In a relatinal version the teacher's name appears once. Imagine Miss Smtih marries Mr Jones, in the flat file you have to edit 125 entries, in the relational version you edit just one!

HTH
Sam

 

[fatmuscle]

draw it in boxes
it's easier to see

 

[Seraph]

okay yea i understand about the Flat file and relational databases now

but with the Schema i think i get it ... The Schematic Diagram is a PART of the Schema

The actual Schema will consist of things like data Dictonaries , Schematic Diagrams , and so forth?

 

[lil_star]

ENTITY relationship diagram = ER diagram =SCHEMATIC diagram
Its just different names used to refer to the same diagram.....you draw all exactly like the ones shown in the ACCESS relationship diagram view!

 

[Seraph]

hmm well i just did a trial exam
I think it was Barker
i did a Schema like the one shown in my first post
however they had it as an Entity Relationship diagram
but i should still be able to obtain the marks right? if there one and teh same ?

and btw theres only ever one Primary key in a relational database right? so say i ahve PCID in one table and StaffID in another

i can choose to link 2 tables using any of these key's right?

 

[Beaker]

Originally posted by Seraph
hmm well i just did a trial exam
I think it was Barker
i did a Schema like the one shown in my first post
however they had it as an Entity Relationship diagram
but i should still be able to obtain the marks right? if there one and teh same ?

and btw theres only ever one Primary key in a relational database right? so say i ahve PCID in one table and StaffID in another

i can choose to link 2 tables using any of these key's right?

Hey is this exam downloadable or anything I'd love to get my hands on it..

 

[lil_star]

yeah it is ...i think on www.boredofstudies.org .....im unsure if like neap cssa has also demanded BoS to take em down:s

 

[Seraph]

noooooooooooooooo!!!!!!!!!

 

[lil_star]

??

 

[dayhawk]

ERD is a way of representing the schema.

 

[JayWalker]

Ok well ill just add my 2 cents worth a bit late...

Schema is that table that you attempted to draw that defines a record in the database.

A schematic diagram is that kewl lookin thing you see in access, showing relationships between fields in different tables

 

[Seraph]

lol i posted this a while ago


yea thats how i think of it now as well , the schematic diagram aka ERD is used very frequently when they ask us to NORMALISE