add SQL resource

sql.html.markdown

@@ -9,14 +9,14 @@ Structured Query Language (SQL) is an ISO standard language for creating and wor
 
 Implementations typically provide a command line prompt where you can enter the commands shown here interactively, and they also offer a way to execute a series of these commands stored in a script file.  (Showing that you’re done with the interactive prompt is a good example of something that isn’t standardized--most SQL implementations support the keywords QUIT, EXIT, or both.)
 
-Several of these sample commands assume that the [MySQL employee sample database](https://dev.mysql.com/doc/employee/en/) available on [github](https://github.com/datacharmer/test_db) has already been loaded. The github files are scripts of commands, similar to the relevant commands below, that create and populate tables of data about a fictional company’s employees. The syntax for running these scripts will depend on the SQL implementation you are using. A utility that you run from the operating system prompt is typical. 
+Several of these sample commands assume that the [MySQL employee sample database](https://dev.mysql.com/doc/employee/en/) available on [github](https://github.com/datacharmer/test_db) has already been loaded. The github files are scripts of commands, similar to the relevant commands below, that create and populate tables of data about a fictional company’s employees. The syntax for running these scripts will depend on the SQL implementation you are using. A utility that you run from the operating system prompt is typical.
 
 
 ```sql
 -- Comments start with two hyphens. End each command with a semicolon.
 
 -- SQL is not case-sensitive about keywords. The sample commands here
--- follow the convention of spelling them in upper-case because it makes 
+-- follow the convention of spelling them in upper-case because it makes
 -- it easier to distinguish them from database, table, and column names.
 
 -- Create and delete a database. Database and table names are case-sensitive.
@@ -26,47 +26,47 @@ DROP DATABASE someDatabase;
 -- List available databases.
 SHOW DATABASES;
 
--- Use a particular existing database. 
+-- Use a particular existing database.
 USE employees;
 
 -- Select all rows and columns from the current database's departments table.
--- Default activity is for the interpreter to scroll the results on your screen. 
+-- Default activity is for the interpreter to scroll the results on your screen.
 SELECT * FROM departments;
 
--- Retrieve all rows from the departments table, 
--- but only the dept_no and dept_name columns. 
+-- Retrieve all rows from the departments table,
+-- but only the dept_no and dept_name columns.
 -- Splitting up commands across lines is OK.
 SELECT dept_no,
        dept_name FROM departments;
 
--- Retrieve all departments columns, but just 5 rows. 
+-- Retrieve all departments columns, but just 5 rows.
 SELECT * FROM departments LIMIT 5;
 
 -- Retrieve dept_name column values from the departments
--- table where the dept_name value has the substring 'en'. 
+-- table where the dept_name value has the substring 'en'.
 SELECT dept_name FROM departments WHERE dept_name LIKE '%en%';
 
 -- Retrieve all columns from the departments table where the dept_name
--- column starts with an 'S' and has exactly 4 characters after it. 
+-- column starts with an 'S' and has exactly 4 characters after it.
 SELECT * FROM departments WHERE dept_name LIKE 'S____';
 
 -- Select title values from the titles table but don't show duplicates.
 SELECT DISTINCT title FROM titles;
 
--- Same as above, but sorted (case-sensitive) by the title values. 
+-- Same as above, but sorted (case-sensitive) by the title values.
 SELECT DISTINCT title FROM titles ORDER BY title;
 
 -- Show the number of rows in the departments table.
 SELECT COUNT(*) FROM departments;
 
 -- Show the number of rows in the departments table that
--- have 'en' as a substring of the dept_name value. 
+-- have 'en' as a substring of the dept_name value.
 SELECT COUNT(*) FROM departments WHERE dept_name LIKE '%en%';
 
--- A JOIN of information from multiple tables: the titles table shows 
--- who had what job titles, by their employee numbers, from what 
+-- A JOIN of information from multiple tables: the titles table shows
+-- who had what job titles, by their employee numbers, from what
 -- date to what date. Retrieve this information, but instead of the
--- employee number, use the employee number as a cross-reference to 
+-- employee number, use the employee number as a cross-reference to
 -- the employees table to get each employee's first and last name
 -- instead. (And only get 10 rows.)
 
@@ -85,12 +85,12 @@ WHERE TABLE_TYPE='BASE TABLE';
 -- for how you specify the columns, such as their datatypes.
 CREATE TABLE tablename1 (fname VARCHAR(20), lname VARCHAR(20));
 
--- Insert a row of data into the table tablename1. This assumes that the 
--- table has been defined to accept these values as appropriate for it. 
+-- Insert a row of data into the table tablename1. This assumes that the
+-- table has been defined to accept these values as appropriate for it.
 INSERT INTO tablename1 VALUES('Richard','Mutt');
 
 -- In tablename1, change the fname value to 'John'
--- for all rows that have an lname value of 'Mutt'. 
+-- for all rows that have an lname value of 'Mutt'.
 UPDATE tablename1 SET fname='John' WHERE lname='Mutt';
 
 -- Delete rows from the tablename1 table
@@ -100,6 +100,11 @@ DELETE FROM tablename1 WHERE lname like 'M%';
 -- Delete all rows from the tablename1 table, leaving the empty table.
 DELETE FROM tablename1;
 
--- Remove the entire tablename1 table. 
+-- Remove the entire tablename1 table.
 DROP TABLE tablename1;
 ```
+
+## Further Reading
+
+* [Codecademy - SQL](https://www.codecademy.com/learn/learn-sql) A good introduction to SQL in a "learn by doing it" format.
+* [Database System Concepts](https://www.db-book.com) book's Chapter 3 - Introduction to SQL has an in depth explanation of SQL concepts.