Introduction to Relational Databases, SQL

Lecture Notes for CS 142
Fall 2010
John Ousterhout

• Readings for this topic:
  • If you would like to learn more about relational databases, a good textbook is Database Management Systems, Third Edition, by Ramakrishnan and Gehrke. Even better, take CS 145.

• Relational database: collection of tables (also called relations)
• Table:
  • Collection of rows (also called tuples or records).
  • Each row in a table contains a set of columns (also called fields or attributes).
  • Each column has a type:
    • String: VARCHAR(20)
    • Integer: INTEGER
    • Floating-point: FLOAT, DOUBLE
    • Date/time DATE, TIME, DATETIME
    • Several others...
  • Primary key: provides a unique identifier for each row (need not be present, but almost always is in practice).
  • Schema: the structure of the database, including for each table
    • The table name
    • The names and types of its columns
    • Various optional additional information (constraints, etc.)
    • The number of rows in a table is not part of the schema
• SQL: a language for creating and manipulating relational databases
  • Initially created at IBM as part of System-R
  • Implemented with modifications in numerous products: Oracle, Sybase, DB-2, SQL Server, MySQL
  • Vendor versions are not completely compatible
  • Has been (partially) standardized
  • Examples that follow use MySQL syntax
• Create a table for the students:

  CREATE TABLE students (
      id INT AUTO_INCREMENT,
      name VARCHAR(30),
      birth DATE,
      gpa FLOAT,
      grad INT,
      PRIMARY KEY(id));
  
• Add rows to the students table:

  INSERT INTO students(name, birth, gpa, grad)
       VALUES ('Anderson', '1987-10-22', 3.9, 2009);
  INSERT INTO students(name, birth, gpa, grad)
       VALUES ('Jones', '1990-4-16', 2.4, 2012);
  INSERT INTO students(name, birth, gpa, grad)
      VALUES ('Hernandez', '1989-8-12', 3.1, 2011);
  INSERT INTO students(name, birth, gpa, grad)
      VALUES ('Chen', '1990-2-4', 3.2, 2011);
  
• Delete row(s):

  DELETE FROM students WHERE name='Anderson';
  
• Delete table:

  DROP TABLE students;
  
• Queries: the strength of relational databases
  • Lots of ways to extract information
  • You specify what you want
  • The database system figures out how to get it efficiently
  • Refer to data by contents, not just name
  • Show entire contents of a table:

    SELECT * FROM students;
    +----+-----------+------------+------+------+
    | id | name      | birth      | gpa  | grad |
    +----+-----------+------------+------+------+
    |  1 | Anderson  | 1987-10-22 |  3.9 | 2009 |
    |  2 | Jones     | 1990-04-16 |  2.4 | 2012 |
    |  3 | Hernandez | 1989-08-12 |  3.1 | 2011 |
    |  4 | Chen      | 1990-02-04 |  3.2 | 2011 |
    +----+-----------+------------+------+------+
    

  • Show just a few columns from a table:

    SELECT name, gpa FROM students;
    +-----------+------+
    | name      | gpa  |
    +-----------+------+
    | Anderson  |  3.9 |
    | Jones     |  2.4 |
    | Hernandez |  3.1 |
    | Chen      |  3.2 |
    +-----------+------+
    

  • Filtering: only display a subset of the rows:

    SELECT name, gpa
        FROM students
        WHERE gpa > 3.0;
    +-----------+------+
    | name      | gpa  |
    +-----------+------+
    | Anderson  |  3.9 |
    | Hernandez |  3.1 |
    | Chen      |  3.2 |
    +-----------+------+
    

  • Sorting:

    SELECT gpa, name, grad
        FROM students
        WHERE gpa > 3.0
        ORDER BY gpa DESC;
    +------+-----------+------+
    | gpa  | name      | grad |
    +------+-----------+------+
    |  3.9 | Anderson  | 2009 |
    |  3.2 | Chen      | 2011 |
    |  3.1 | Hernandez | 2011 |
    +------+-----------+------+
    
• Updates:

  UPDATE students
      SET gpa = 2.6, grad = 2013
      WHERE id = 2;
  
• Joins: how to manage relationships between tables?
  • Join: a query that merges the contents of 2 or more tables, displays information from the results.
  • Can produce the equivalent of a linked list in a programming language, and many other effects.
• Join example: many-to-one relationship
  • Students have advisors; introduce new table describing faculty.

    +----+----------+-----------+
    | id | name     | title     |
    +----+----------+-----------+
    |  1 | Fujimura | assocprof |
    |  2 | Bolosky  | prof      |
    +----+----------+-----------+
    

  • Add new column advisor_id to the students table. This is a foreign key.

    +----+-----------+------------+------+------+------------+
    | id | name      | birth      | gpa  | grad | advisor_id |
    +----+-----------+------------+------+------+------------+
    |  1 | Anderson  | 1987-10-22 |  3.9 | 2009 |          2 |
    |  2 | Jones     | 1990-04-16 |  2.4 | 2012 |          1 |
    |  3 | Hernandez | 1989-08-12 |  3.1 | 2011 |          1 |
    |  4 | Chen      | 1990-02-04 |  3.2 | 2011 |          1 |
    +----+-----------+------------+------+------+------------+
    

  • Example query:

    SELECT s.name, s.gpa
        FROM students s, advisors p
        WHERE s.advisor_id = p.id AND p.student = 'Fujimura';
    +-----------+------+
    | name      | gpa  |
    +-----------+------+
    | Jones     |  2.4 |
    | Hernandez |  3.1 |
    | Chen      |  3.2 |
    +-----------+------+
    

  • A join creates the cross-product of 2 or more tables
    • Potentially very expensive!
    • In practice, optimized carefully by the database system.
• Join example: many-to-many relationship
  • Courses: students take many courses, courses have many students
  • Add new table describing courses:

    +----+--------+-----------------+-------------+
    | id | number | name            | quarter     |
    +----+--------+-----------------+-------------+
    |  1 | CS142  | Web stuff       | Winter 2009 |
    |  2 | ART101 | Finger painting | Fall 2008   |
    |  3 | ART101 | Finger painting | Winter 2009 |
    |  4 | PE204  | Mud wrestling   | Winter 2009 |
    +----+--------+-----------------+-------------+
    

  • Create a join table courses_students describing which students took which courses.

    +-----------+------------+
    | course_id | student_id |
    +-----------+------------+
    |         1 |          1 |
    |         3 |          1 |
    |         4 |          1 |
    |         1 |          2 |
    |         2 |          2 |
    |         1 |          3 |
    |         2 |          4 |
    |         4 |          4 |
    +-----------+------------+
    

  • Find all students who took a particular course:

    SELECT s.name, c.quarter
        FROM students s, courses c, courses_students cs
        WHERE c.id = cs.course_id AND s.id = cs.student_id
        AND c.number = 'ART101';
    +----------+-------------+
    | name     | quarter     |
    +----------+-------------+
    | Jones    | Fall 2008   |
    | Chen     | Fall 2008   |
    | Anderson | Winter 2009 |
    +----------+-------------+
    
• Additional database features:
  • Indexes: used to speed up searches (exact matches, ranges)
  • Transactions: used to group operations together to provide predictable behavior even when there are concurrent operations on the database.
  • Many other things.