Everything about Database JOINS and Relationships

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Today, we continue our journey into the world of RDBMS (relational database management systems) and SQL (Structured Query Language).
In this presentation, you will understand about some key definitions and then you will learn how to work with multiple tables that have relationships with each other.
First, we will go covering some core concepts and key definitions, and then will begin working with JOINs queries in SQL.

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Everything about Database JOINS and Relationships

  1. 1. https://www.facebook.com/Oxus20 oxus20@gmail.com Everything about Database JOINS and Relationships » Definitions » JOINS » INNER JOIN » LEFT JOIN » RIGHT JOIN » FULL JOIN » CROSS JOIN » SELF JOIN Abdul Rahman Sherzad
  2. 2. Database Definition » A database is a set of related data that has a regular structure and that is organized in such a way that a computer can easily find the desired information. » A database is a collection of information that is organized so that it can easily be accessed, managed, and updated. » A database is a collection logically related data. 2 https://www.facebook.com/Oxus20
  3. 3. DBMS Definition » A DBMS (Database Management System) is a set of software programs or a tools which helps the user to perform all related operations i.e. to store, access, and process data or facts into useful information. » A DBMS guarantees security, integrity, and privacy by providing a centralized control of database. 3 https://www.facebook.com/Oxus20
  4. 4. DBMS Examples » Free and Open Source ˃ MySQL ˃ PostgreSQL ˃ SQLite ˃ Firebird » Proprietary and Closed Source ˃ Microsoft SQL Server (MS SQL) ˃ Oracle ˃ Microsoft Access ˃ DB2 4 https://www.facebook.com/Oxus20
  5. 5. Application Program Definition » An application program (sometimes shortened to application) accesses the database by sending queries or requests to the DBMS via a GUI (Graphical User Interface). 5 https://www.facebook.com/Oxus20
  6. 6. Database System Definition » The database, the DBMS software, and the application program together are called a database system. ˃ Computerized Library Systems ˃ ATM (Automated Teller Machines) ˃ Flight Reservation Systems ˃ Computerized Human Resource Systems 6 https://www.facebook.com/Oxus20
  7. 7. Summary at a Glance 7 https://www.facebook.com/Oxus20 GUI / Web Interface MySQL, Oracle, MS SQL Facebook, YouTube, Library System • Data related to the videos • Data related to the users • Data related to the library
  8. 8. Relationship Definition » When creating a database, common sense dictates that we use separate tables for different types of entities to reduce and overcome redundancy. » We need to establish relationships between these separated tables to provide useful information. » A relationship exists between two database tables when one table has a foreign key that references the primary key of another table. 8 https://www.facebook.com/Oxus20
  9. 9. Types of Relationships » One to One Relationships ˃ Both tables can have only one record on either side of the relationship. » One to Many / Many to One Relationships ˃ The primary key table contains only one record that relates to none, one, or many records in the related table. » Many to Many Relationships ˃ Each record in both tables can relate to any number of records (or no records) in the other table. » Self Referencing Relationships ˃ This is used when a table needs to have a relationship with itself. 9 https://www.facebook.com/Oxus20
  10. 10. JOINS » When selecting data from multiple tables with relationships, we will be using the JOIN query. » INNER JOIN » Natural JOIN » Left (Outer) JOIN » Right (Outer) JOIN » Cross JOIN 10 https://www.facebook.com/Oxus20
  11. 11. JOIN in Live Examples Students Subjects code first_name last_name 20120 Abdul Rahman Sherzad 20121 Cristina Silva 20122 Bob Logan 20123 Ana Nava 20124 Sekila Manzikalla id subject_name 1 Web Development 2 Web Design 3 Concept of Programming 4 Fundamentals of Database Systems 5 Graphic Design 11 https://www.facebook.com/Oxus20
  12. 12. Visualizing the Relationships 12 https://www.facebook.com/Oxus20 The database includes a "many-to-many" relationship; each student can take many subjects, while each subject can of course chosen by many students. To represent this, there is students table, subjects table, and enrollments table to show the combinations of the students enrolled in subjects and the subjects which taken by the students.
  13. 13. Database Schema DROP SCHEMA IF EXISTS joins; CREATE SCHEMA IF NOT EXISTS joins DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci; USE joins; 13 https://www.facebook.com/Oxus20
  14. 14. Table Students Schema DROP TABLE IF EXISTS students; CREATE TABLE IF NOT EXISTS students ( code INT NOT NULL, first_name VARCHAR(45) NOT NULL, last_name VARCHAR(45) NOT NULL, PRIMARY KEY (code) ) ENGINE = InnoDB; 14 https://www.facebook.com/Oxus20
  15. 15. Table Students Data INSERT INTO students (code, first_name, last_name) VALUES (20120, 'Abdul Rahman', 'Sherzad'), (20121, 'Cristina', 'Silva'), (20122, 'Bob', 'Logan'), (20123, 'Ana', 'Nava'), (20124, 'Sekila', 'Manzikalla'); 15 https://www.facebook.com/Oxus20
  16. 16. Table Subjects Schema DROP TABLE IF EXISTS subjects; CREATE TABLE IF NOT EXISTS subjects ( id INT NOT NULL AUTO_INCREMENT, subject_name VARCHAR(45) NULL, PRIMARY KEY (id), UNIQUE INDEX (subject_name) ) ENGINE = InnoDB; 16 https://www.facebook.com/Oxus20
  17. 17. Table Subjects Data INSERT INTO subjects (id, subject_name) VALUES (1, 'Web Development'), (2, 'Web Design'), (3, 'Concept of Programming'), (4, 'Fundamentals of Database Systems'), (5, 'Graphic Design'); 17 https://www.facebook.com/Oxus20
  18. 18. Table Enrollments Schema DROP TABLE IF EXISTS enrollments; CREATE TABLE IF NOT EXISTS enrollments ( student_code INT NOT NULL, subject_id INT NOT NULL, PRIMARY KEY (student_code, subject_id), FOREIGN KEY (student_code) REFERENCES students (code) ON DELETE CASCADE ON UPDATE CASCADE, FOREIGN KEY (subject_id) REFERENCES subjects (id) ON DELETE CASCADE ON UPDATE CASCADE ) ENGINE = InnoDB; 18 https://www.facebook.com/Oxus20
  19. 19. Table Enrollments Data INSERT INTO enrollments (student_code, subject_id) VALUES (20120, 1), (20120, 2), (20121, 2), (20121, 3), (20122, 3), (20123, 3), (20122, 4), (20123, 4); 19 https://www.facebook.com/Oxus20
  20. 20. INNER JOIN (JOIN) » The most frequently used clause is INNER JOIN or just JOIN. » Fetching Matching Records From All the Tables » Let's say we want to see which students taken which subjects. 20 https://www.facebook.com/Oxus20
  21. 21. INNER JOIN (JOIN) SELECT code, first_name, last_name, subject_name FROM students INNER JOIN enrollments ON students.code = enrollments.student_code INNER JOIN subjects ON enrollments.subject_id = subjects.id; 21 https://www.facebook.com/Oxus20
  22. 22. Alternative I - INNER JOIN (JOIN) SELECT code, first_name, last_name, subject_name FROM students INNER JOIN enrollments INNER JOIN subjects ON students.code = enrollments.student_code AND enrollments.subject_id = subjects.id; 22 https://www.facebook.com/Oxus20
  23. 23. Alternative II – Just JOIN SELECT code, first_name, last_name, subject_name FROM students JOIN enrollments ON students.code = enrollments.student_code JOIN subjects ON enrollments.subject_id = subjects.id; 23 https://www.facebook.com/Oxus20
  24. 24. Alternative III – Where Clause SELECT code, first_name, last_name, subject_name FROM students, subjects, enrollments WHERE students.code = enrollments.student_code AND enrollments.subject_id = subjects.id; 24 https://www.facebook.com/Oxus20
  25. 25. OUTPUT 25 https://www.facebook.com/Oxus20
  26. 26. Alternative IV - Alias SELECT code AS 'Student Code', first_name AS 'First Name', last_name AS 'Last Name', subject_name AS 'Subject' FROM students AS stu INNER JOIN enrollments AS en ON stu.code = en.student_code INNER JOIN subjects AS sub ON en.subject_id = sub.id; 26 https://www.facebook.com/Oxus20
  27. 27. Alternative V – Alias Refined SELECT code 'Student Code', first_name 'First Name', last_name 'Last Name', subject_name 'Subject' FROM students stu INNER JOIN enrollments en ON stu.code = en.student_code INNER JOIN subjects sub ON en.subject_id = sub.id; 27 https://www.facebook.com/Oxus20
  28. 28. OUTPUT 28 https://www.facebook.com/Oxus20
  29. 29. RIGHT JOIN (RIGHT OUTER JOIN) » What if we require a list of all students and their subjects even if they are not enrolled on one? » A RIGHT JOIN produces a set of records which matches every entry in the right table (students) regardless of any matching entry in the left table (subjects) and / or (enrollments). 29 https://www.facebook.com/Oxus20
  30. 30. RIGHT JOIN (RIGHT OTHER JOIN) SELECT code, first_name, last_name, subject_name FROM subjects INNER JOIN enrollments ON subjects.id = enrollments.subject_id RIGHT JOIN students ON students.code = enrollments.student_code; 30 https://www.facebook.com/Oxus20
  31. 31. OUTPUT 31 https://www.facebook.com/Oxus20
  32. 32. LEFT JOIN (LEFT OUTER JOIN) » Let's change the scenario, perhaps we require a list of all subjects and students even if the subjects are not chosen by any students? » A LEFT JOIN produces a set of records which matches every entry in the left table (subjects) regardless of any matching entry in the right table (students) and / or enrollments. 32 https://www.facebook.com/Oxus20
  33. 33. LEFT JOIN (LEFT OUTER JOIN) SELECT subject_name, code, first_name, last_name FROM subjects LEFT JOIN ( students INNER JOIN enrollments ON students.code = enrollments.student_code ) ON subjects.id = enrollments.subject_id; 33 https://www.facebook.com/Oxus20
  34. 34. Alternative – RIGHT JOIN SELECT subject_name, code, first_name, last_name FROM students INNER JOIN enrollments ON students.code = enrollments.student_code RIGHT JOIN subjects ON subjects.id = enrollments.subject_id 34 https://www.facebook.com/Oxus20
  35. 35. OUTPUT 35 https://www.facebook.com/Oxus20
  36. 36. LEFT JOIN vs. RIGHT JOIN » LEFT (OUTER) JOIN and RIGHT (OUTER) JOIN works exactly the same. » ONLY the order of the tables are reversed! 36 https://www.facebook.com/Oxus20
  37. 37. FULL JOIN (or FULL OUTER JOIN) » The OUTER JOIN which returns all records in both tables regardless of any match. Where no match exists, the missing side will contain NULL. » OUTER JOIN is less useful than INNER, LEFT or RIGHT joins and it's not implemented in MySQL. » However, you can work around this restriction using the UNION of a LEFT and RIGHT JOIN. 37 https://www.facebook.com/Oxus20
  38. 38. FULL JOIN (or FULL OUTER JOIN) SELECT code, first_name, last_name, subject_name FROM subjects LEFT JOIN ( students INNER JOIN enrollments ON students.code = enrollments.student_code ) ON subjects.id = enrollments.subject_id UNION SELECT code, first_name, last_name, subject_name FROM subjects INNER JOIN enrollments ON subjects.id = enrollments.subject_id RIGHT JOIN students ON students.code = enrollments.student_code; 38 https://www.facebook.com/Oxus20
  39. 39. OUTPUT 39 https://www.facebook.com/Oxus20
  40. 40. Cross Join » This is the default type of JOIN query when no condition is specified. » The result is a so called "Cartesian Product" of the tables. » It means that each row from the first table is matched with each row of the second table. » Since each table had 5 rows, we ended up getting a result of 25 rows. 40 https://www.facebook.com/Oxus20
  41. 41. Cross Join SELECT code, first_name, last_name, subject_name FROM students CROSS JOIN subjects; 41 https://www.facebook.com/Oxus20
  42. 42. Cross Join - Alternative SELECT code, first_name, last_name, subject_name FROM Students, subjects; 42 https://www.facebook.com/Oxus20
  43. 43. OUTPUT 43 https://www.facebook.com/Oxus20
  44. 44. SELF JOIN » The SELF JOIN is used to join a table to itself as if the table were two tables; temporarily renaming at least one table in the SQL statement. » You can view SELF JOIN as two identical tables. But in normalization you cannot create two copies of the table so you just simulate having two tables with SELF JOIN. 44 https://www.facebook.com/Oxus20
  45. 45. SELF JOIN » Let's say you have a table named "users" with following structure: ˃ User ID ˃ User Name ˃ User's Manager's ID UserID UserName ManagerID 1 Abdul Rahman Sherzad 0 2 Ana Nava 1 3 Bob Logan 2 4 Cristina Silva 3 https://www.facebook.com/Oxus20 45
  46. 46. Table Users Schema CREATE TABLE IF NOT EXISTS users ( UserID int(11) NOT NULL AUTO_INCREMENT, UserName varchar(50) NOT NULL, ManagerID int(11) NOT NULL, PRIMARY KEY (UserID) ) ENGINE=InnoDB; 46 https://www.facebook.com/Oxus20
  47. 47. Table Users Data INSERT INTO users (UserID, UserName, ManagerID) VALUES (1, 'Abdul Rahman Sherzad', 0), (2, 'Ana Nava', 1), (3, 'Bob Logan', 2), (4, 'Cristina Silva', 3); 47 https://www.facebook.com/Oxus20
  48. 48. SLEF JOIN - Example SELECT u.UserID, u.UserName AS 'User Name', m.UserName AS 'Manager Name' FROM users u INNER JOIN users m ON u.ManagerID = m.UserID; 48 https://www.facebook.com/Oxus20
  49. 49. OUTPUT 49 https://www.facebook.com/Oxus20
  50. 50. SELF JOIN with LEFT JOIN SELECT u.UserID, u.UserName AS 'User Name', m.UserName AS 'Manager Name' FROM users u LEFT JOIN users m ON u.ManagerID = m.UserID ORDER BY u.UserID ASC; 50 https://www.facebook.com/Oxus20
  51. 51. OUTPUT 51 https://www.facebook.com/Oxus20
  52. 52. Conclusion » Thank you for reading this presentation. I hope you that it gives you a better understanding of JOINS and helps you write more efficient SQL queries as well as enjoyed it! » Please leave your comments and questions, and have a great day  52 https://www.facebook.com/Oxus20
  53. 53. END https://www.facebook.com/Oxus20 53

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