Week 3 notes completed!

CM3010 Databases and Advanced Data Techniques/Week 3/Week 3 Notes.md

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+# Relational Database
+A relational database implements the relational model.
+
+A **model** is a way of abstracting complex real-world data into structures we can understand, talk about, manipulate and represent.
+
+A **relation** is the definition of a table and the values stored in it as well as the rules on manipulating the rules.
+
+**Relational** refers to tables and the relation tables have between each other.
+
+## Relational Databases - some rules
+Everything is a relation:
+* All operations use the relational model
+* All data is represented and accessed as relations
+* Table and database structure is accessed and altered as relations
+
+The system is unaffected by its implementation:
+* If the hardware changes
+* If the operating system changes
+* If the disks are replaced
+* If data is distributed
+
+## Relational model ≠ Entity-relationship model
+
+An ER model helps model concepts, often as part of relational database design
+
+## Relational model ≠ SQL ≠ Most RDBMS implementations
+
+# Drawing a database
+**Entity**
+
+It's a thing we want to model and can be uniquely identified.
+
+**Attribute**
+
+Information that describes an aspect of an entity.
+
+**Relationship**
+
+A connection or dependency between two entities.
+
+## Entity-Relationship Diagrams
+
+__Entities__ are drawn in a box (quadrilateral).
+
+__Attributes__ are drawn inside an ellipse.
+
+__Relationships__ are drawn in a rhombus.
+
+# Speaking to databases
+## Basic SQL
+```SQL
+SELECT
+INSERT
+UPDATE
+DELETE
+```
+`SELECT PlanetName FROM Planets`
+SELECT keyword introduces/starts a query
+After the SELECT we specify the columns we want in the result (table).
+The FROM keyword tells us where to look.
+
+```SQL
+SELECT PlanetName FROM Planets WHERE DayLength > 200;
+```
+Here the constraint for the field DayLength is 200. We'll see the PlanetName only for the rows which comply with the constraint.
+
+```SQL
+CREATE TABLE Planets (
+    PlanetName CHAR(8),
+    DayLength INT,
+    YearLength INT,
+    PRIMARY KEY (PlanetName)
+);
+```
+We use this to create a table with 3 field, one with character values up to 8 in length and two fields with integer values
+
+In the relational model, every row has to be unique, this is done via the primary key in SQL.
+