Week 1.md

Week 1 - Getting Started and Selecting & Retrieving Data with SQL

Table of Contents

• Data Models, Part 1: Thinking About Your Data
• Data Models, Part 2: The Evolution of Data Models
• Data Models, Part 3: Relational vs. Transactional Models
  • Relational vs Transactional model
  • Building blocks of data models
  • ER diagrams
  • Primary and foreign keys
  • ER diagram notations
    • Chen notation
    • Crow's foot notation
    • UML class diagram notation
• Retrieving Data with a SELECT Statement
  • Limiting results
• Creating Tables
  • Inserting data
    • Insert implicitly
    • Insert explicitly
• Creating Temporary Tables
• Adding Comments to SQL
  • Single line comments
  • Multi-line comments

Data Models, Part 1: Thinking About Your Data

video

It's important to think about requirements before implementing anything. Front-loading this work saves time later.

Data Models, Part 2: The Evolution of Data Models

video

• data modeling: the organising of multiple tables, and how they relate to each other
  • usually represents a business process
  • can help in understanding business processes
  • data models should always represent a real world problem as closely as possible

Data Models, Part 3: Relational vs. Transactional Models

video

Relational vs Transactional model

• relational:
  • allows for easy querying and data manipulation
  • logical and intuitive querying
• transactional:
  • used for storing data, and not necessarily for querying data
  • e.g. medical database of patient data
  • data usually needs to be extracted into a relational model to be made sense of

Building blocks of data models

1. entities
   • these are discrete 'things'
   • e.g. person, place, event, etc.
2. attributes
   • characteristics of entities
   • e.g. age, height, etc.
3. relationships
   • associations between entities
   • one-to-many - a customer's invoices
   • many-to-many - students assigned to classes
   • one-to-one - a manager for a store

ER diagrams

ER (entity-relationship) diagrams are composed of entity types, and specify relationships between those entities.

They have the following functions:

• show entity relationships
• show business processes
• are visual aids
• show links through primary keys

Primary and foreign keys

Primary and foreign keys are the mechanism by which tables are related to each other.

• primary key
  • a column, or set of columns, that uniquely identifies every row in a table
• foreign key
  • one or more columns that can be used together to identify a single row in another table

ER diagram notations

Chen notation

Uses 1 and M to signify relationships on entities:

Crow's foot notation

• --||-: 1
• --|ᗕ: many

UML class diagram notation

• 1.1: 1
• 1.*: many

Retrieving Data with a SELECT Statement

video

SELECT is used to retrieve data from tables, and FROM indicates where to retrieve the data from.

Select one field from a table:

SELECT my_field
FROM my_table;

Select multiple fields from a table:

SELECT
  my_field_a
  , my_field_b
  , my_field_c
FROM my_table;

Select all fields from a table using a wildcard:

SELECT *
FROM my_table;

Limiting results

To limit the number of results retrieved, use the LIMIT statement.

SELECT [fields]
FROM table_name
LIMIT quantity;

Syntax differs depending on the flavour of SQL.

Creating Tables

video

CREATE TABLE table_name (
  id        [dataType] PRIMARY KEY,
  field_a   [dataType] NOT NULL,
  field_b   [dataType] NOT NULL,
  field_c   [dataType] NULL,
)

id is the primary key for the table - how we will reference this field from other tables. Primary keys cannot accept NULL values.

field_a and field_b may not ever contain NULL. NULL is not the same as falsy values in programming languages. An empty string or 0 are not NULL. NULL indicates no presence of any value.

Fields that are defined as NOT NULL will return an error if not provided a value.

Inserting data

We use INSERT INTO and VALUES to define the table to write data to. There are two ways to insert data; implicitly without field names, or explicitly with field names.

Insert implicitly

Given a table shoes with fields for id, brand, color, price, and desc:

INSERT INTO shoes
VALUES (
  1, 'Gucci', 'Black', '25000', NULL
);

Insert explicitly

Given a table shoes with fields for id, brand, color, price, and desc:

INSERT INTO shoes
(id, brand, color, price, desc)
VALUES
(1, 'Gucci', 'Black', '25000', NULL);

This method is more flexible, as we're not relying on column order, and we can specify exactly which fields we want to write data to.

Creating Temporary Tables

video

Temporary tables are useful when we want to clone a table, or get a subset of a table without having to create an actual table.

Features of temporary tables:

• deleted once the session is over
• are faster to create than real tables
• are useful for complex queries that use subsets or joins

To create temporary tables, we use the TEMPORARY statement:

CREATE TEMPORARY TABLE sandals_temp AS (
  SELECT * from shoes
  WHERE shoe_type = 'sandals'
)

For MySQL there's no guarantee that a temporary table will be deleted. Dropping temporary tables once they are no longer required is considered good practice.

DROP TEMPORARY TABLE sandals_temp;

Although there's nothing preventing one from creating a temporary table with the same name as an existing table, it could lead to issues in deleting non-temporary data.

Adding Comments to SQL

Single line comments

Use -- to comment out single lines.

SELECT shoe_id
--, brand_id
, shoe_name
FROM shoes

MySQL allows for # to be used in addition to --.

Multi-line comments

Use /* */ to comment out multiple lines.

SELECT shoe_id
/*, brand_id
, shoe_name
*/
FROM shoes