# DDIA - chp 2

## Introduction

- Each abstraction layer hides the complexity of the layers below it by providing a clean data model.
- Focus on data storage and querying

## Relation model vs Document model

- Relation model
- Still dominant nowadays
- The goal is to hide that implementation detail behind a cleaner interface
- NoSQL
- Scalability
- Free and open sourced
- Specialized operations
- Dynamic and expressive data model

### The object-relational database mismatch

- Object programming issue (ORM)
- Example

    {
    "user_id": 251,
    "first_name": "Bill",
    "last_name": "Gates",
    "summary": "Co-chair of the Bill & Melinda Gates... Active blogger.",
    "region_id": "us:91",
    "industry_id": 131,
    "photo_url": "/p/7/000/253/05b/308dd6e.jpg",
    "positions": [
    {"job_title": "Co-chair", "organization": "Bill & Melinda Gates Foundation"},
    {"job_title": "Co-founder, Chairman", "organization": "Microsoft"}
    ],
    "education": [
    {"school_name": "Harvard University", "start": 1973, "end": 1975},
    {"school_name": "Lakeside School, Seattle", "start": null, "end": null}
    ],
    "contact_info": {
    "blog": "http://thegatesnotes.com",
    "twitter": "http://twitter.com/BillGates"
    }
    }

### Many-to-one and Many-to-many relationships

- Why Id instead of fixed readable text
- Consistent style and spelling across profiles
- Avoiding ambiguity
- Localization support
- Better search
- Example

### Are Document database repeating history

- History
- Hierarchical model → relation model → document model
- Hierarchical model
- document based tree structure
- Network model
- The only way of accessing a record was to follow a path from a root record along these chains of
links. This was called an access path
- Traversal of a linked list
- Relation model
- Query optimizer
- Comparison to document databases
- Different to hierarchical model: storing nested records within their parent record rather than in a separate
table.

## Relational Versus Document Databases Today

### Document database pros

- Good for document-like structured application
- Schema flexibility
- Example

    if (user && user.name && !user.first_name) {
    	// Documents written before Dec 8, 2013 don't have first_name
    	user.first_name = user.name.split(" ")[0];
    }

    ALTER TABLE users ADD COLUMN first_name text;
    UPDATE users SET first_name = split_part(name, ' ', 1); -- PostgreSQL
    UPDATE users SET first_name = substring_index(name, ' ', 1); -- MySQL

### Document database cons

- Cannot refer directly to a nested
item within a document
- many-to-many relationships: The poor support for joins
- Data locality for queries

### Convergence of document and relational databases

- Relational database support document data type
- Document database side, RethinkDB supports relational-like joins

## Query Languages for Data

### Imperative query

- Example

function getSharks() {
	var sharks = [];
	for (var i = 0; i < animals.length; i++) {
		if (animals[i].family === "Sharks") {
			sharks.push(animals[i]);
		}
	}
	return sharks;
}

### Declarative query

- Parallel execution
- Better optimization
- Example

SELECT * FROM animals WHERE family = 'Sharks';

### Declarative Queries on the Web

- Declarative

- Imperative query

## MapReduce Querying

- SQL version

- MongoDB’s MapReduce

- Result

- MongoDB aggregation pipeline

## Graph-Like Data Models

### Property Graphs

- Representing a property graph using a relational schema

### The Cypher Query Language

- Cypher query

- Cypher query to find people who emigrated from the US to Europe

- Declarative query optimization

### Graph Queries in SQL

- Example

### Triple-Stores and SPARQL

- Subset of the data in Figure 2-5, represented as Turtle triples

- Another form

- The semantic web
- The RDF data model

- The SPARQL query language

### The Foundation: Datalog

- A subset of the data in Figure 2-5, represented as Datalog facts

- The same query as Example 2-4, expressed in Datalog

## Summary

- Hierarchical data model
- Relation data model
- NoSQL database
- Document databases
- Graph databases
- Other domain specified database