CTFS Workshop Shameema Esufali Asian data coordinator and technical resource for the network [email protected].

CTFS Workshop

Shameema EsufaliAsian data coordinator and technical resource

for the network

[email protected]

CTFS WorkshopRelational database basics Why relational databases?

Why MySQL?

What about R?

Relational Theory

In order to work with MySQL it is necessary to understand the basics of relational theory.

i.e how and why data is stored and managed in a relational database.

The guiding principle behind a relational database is to store data once and only

once.

What is a Relation?

A table. Columns are fields (attributes) of data related to other fields on the

same row (tuple).

Primary Key

Identifies the row of a table without duplicates.

Tells you what the row contains Eg. If treeid is the primary key then the row

has information about that tree

Candidate Primary Key

Any attribute(s) which together would serve as the primary key.

Must uniquely identify a row of data. Each part of the key must be essential to unique

identification. No redundancy.

Foreign Key

A foreign key is a column in a table that matches the primary key column of another table. Its function is to link the basic data of two entities on demand, i.e. when two tables are joined using the common key.

First Normal Form

One piece of information per column. No repeated rows. Eliminate fused data eg Code1,Code2

Tag Species Code

1234 SHORME A

1234 SHORME BA

Tag Species Code

1234 SHORME A, BA

Wrong!

Right

Second Normal FormEach column depends on the entire primary key.

Tag Census Species Seedsize X Y DBH

1234 1 SHORTR Medium 11.3

15.4 12

Tag Species Seedsize X Y

1234 SHORTR Medium 11.3

15.4

Wrong

Right

Third Normal FormEach column depends ONLY on the primary key. i.e.

there are no transitive dependencies

Tag Species Seedsize X Y

1234 SHORTR Medium 11.3 15.4

Tag Species X Y

1234 SHORTR 11.3 15.4

Wrong

Right

Fourth Normal FormThe table must contain no more than one

multi-valued dependency

Tag DBH Code

1234 10 A

1234 11 A

1234 11 BA

Entity Relationship diagram (ERD)

Shows in a diagram how entities (tables) are related to one another.

One to One One to many Many to many

One to one Extension of number of attributes in a single

table Rarely required

Tree More tree attributes

One to Many

Most common

Requires two tables.

Linked by Foreign Key

Parent Child

Family Genus Species

Many to many

Need to break down to one to many

Requires three tables

Associative table provides common key

Measurement Code

Tree Code

Measurement

Reassembling data

Data was broken down into tables to preserve integrity

How can we put it together to derive information?

Use Structured Query Language (SQL) to JOIN tables using a common attribute

Joins Two tables may be

joined when they share at least one common attribute

The Primary key of the Parent table is stored in the Child table as a cross reference. This is called a Foreign Key.

Genus

1 Acacia 4

2 3

3 3

4 3

5 3

GenusID FamilyID

Acalypha

Adelia

Aegiphila

Alchornea

Species

1 1

2 2

3 2

4 3

5 4

6 5

7 5

SpeciesID GenusId

melanoceras

diversifolia

macrostachya

triloba

panamensis

costaricensis

latifolia

Primary Key in Parent

Foreign Key in Child

Table joined on Foreign Key GenusID

Species Genus1 1 ⇿ 1 Acacia 4

2 2 ⇿ 2 33 2 ⇿ 2 3

4 3 ⇿ 3 35 4 ⇿ 4 3

6 5 ⇿ 5 37 5 ⇿ 5 3

SpeciesID GenusId GenusID FamilyIDmelanoceras

diversifolia Acalyphamacrostachya Acalypha

triloba Adeliapanamensis Aegiphila

costaricensis Alchornealatifolia Alchornea

The Genus ID in the Species table is used to pick up information for the corresponding Genus. It looks for a row with the matching Primary Key

Extend to join many tables

With SQL you can join as many tables as you need to in order to get the set of information you need. Thus the previous example can be extended to include Family which is a parent table of Genus and/or extended in the another direction to include Tree which is a child of Species as long as there is a linking attribute. This attribute is called a Foreign Key.