A brief introduction to relational databases, SQL and ActiveRecord. What's a SELECT, UPDATE, INSERT or DELETE query? What's an Index, a Foreign Key, a Primary Key?

1. Relational
Databases 101
Ombu Labs, August 2015

2. Think
spreadsheets

4. A database is
a set of
interconnected relations (tables)

5. Id Username Name
1 mauro-oto Mauro
2 alanhala Alan
3 ceci Cecilia
4 etagwerker Ernesto
usersTable Name
Columns
Rows
Unique Ids

6. Id Name
1 OmbuShop
2 InfractoresBA
3 Alpha
4 Beta
projects

7. Interconnected?

8. “A User has many Addresses.”
For example

9. Users Addresses
has many
has one

10. Id Address User Id
1 Peru 212 23
2 Moldes 1333 55
3 Gorriti 332 212
4 Carranza 321 98
addresses
Reference
Column
References

11. That is a “one to
many” relationship

12. “A project has many people, and a
person belongs to many projects.”
For example

13. Users Projects
has many
has many

14. Users Projects
has many has many
Users_Projects
has onehas one

15. That is a “many to
many” relationship

16. Users Partner
has one
has one

17. That is a “one to
one” relationship

18. Database Management
Systems

19. MySQL, PostgreSQL,
SQLServer, Oracle,
DB2, MariaDB, etc…

20. SQL

21. Structured Query
Language

22. A standardized way
to CRUD records

23. SQL Queries (SELECT)

24. SELECT * FROM users

25. Id Username Name
1 mauro-oto Mauro
2 alanhala Alan
3 ceci Cecilia
4 etagwerker Ernesto
users

26. SELECT id, name FROM users
WHERE name = ‘Ernesto’

27. Id Name
4 Ernesto
users

28. SELECT <field names
separated by commas>
FROM <table name>
WHERE <key> = <value>

29. SQL Updates (UPDATE)

30. UPDATE users SET name =
‘Ernesto T’ WHERE name =
‘Ernesto’

31. UPDATE <table_name>
SET <key> = <value>,
… WHERE <key> =
<value>

32. SQL Inserts (INSERT)

33. INSERT INTO users (username,
name) VALUES (‘pepe’, ‘Pedro P’)

34. INSERT INTO <table_name>
(<column_name_1, …) VALUES
(<value_1, …)

35. SQL Deletes (DELETE)

36. DELETE FROM users
WHERE username = ‘pepe’

37. DELETE FROM <table_name>
WHERE <key> = <value>

38. SQL Joins

39. “Find all the users with name
Ernesto who live in the Moldes St.”
For example

40. Users Addresses
has many
has one
username
name
address
zipcode

41. SELECT id, name FROM users
INNER JOIN addresses
ON users.id = addresses.user_id
WHERE addresses.address LIKE
‘Moldes%’ AND users.name =
‘Ernesto’

42. Constraints

43. Primary keys

44. Id Username Name
1 mauro-oto Mauro
2 alanhala Alan
3 ceci Cecilia
4 etagwerker Ernesto
users

45. users: primary key (id)

46. users_projects: primary
key (user_id, project_id)

47. Foreign keys

48. Id Address User Id
1 Peru 212 23
2 Moldes 1333 55
3 Gorriti 332 212
4 Carranza 321 98
addresses
Reference
Column
Foreign
Keys

49. Indexes

50. Id Username Name
1 mauro-oto Mauro
2 alanhala Alan
3 ceci Cecilia
4 etagwerker Ernesto
users
Unique Indexes

51. ActiveRecord Pattern

52. Users
(MySql)
user.rb
(Rails)
Table
Record/row
Column
Row value
Class
Object instance
Attribute name
Attribute

53. Thank you!

54. Questions?