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+---
+language: SQL
+filename: learnsql.sql
+contributors:
+  - ["Bob DuCharme", "http://bobdc.com/"]
+---
+
+Structured Query Language (SQL) is an ISO standard language for creating and working with databases stored in a set of tables. Implementations usually add their own extensions to the language; [Comparison of different SQL implementations](http://troels.arvin.dk/db/rdbms/) is a good reference on product differences.
+
+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. 
+
+
+```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 
+-- it easier to distinguish them from database, table, and column names.
+
+-- Create and delete a database. Database and table names are case-sensitive.
+CREATE DATABASE someDatabase;
+DROP DATABASE someDatabase;
+
+-- List available databases.
+SHOW DATABASES;
+
+-- 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. 
+SELECT * FROM departments;
+
+-- 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. 
+SELECT * FROM departments LIMIT 5;
+
+-- Retrieve dept_name column values from the departments
+-- 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. 
+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. 
+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. 
+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 
+-- date to what date. Retrieve this information, but instead of the
+-- 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.)
+
+SELECT employees.first_name, employees.last_name,
+       titles.title, titles.from_date, titles.to_date
+FROM titles INNER JOIN employees ON
+       employees.emp_no = titles.emp_no LIMIT 10;
+
+-- List all the tables in all the databases. Implementations typically provide
+-- their own shortcut command to do this with the database currently in use.
+SELECT * FROM INFORMATION_SCHEMA.TABLES
+WHERE TABLE_TYPE='BASE TABLE';
+
+-- Create a table called tablename1, with the two columns shown, for
+-- the database currently in use. Lots of other options are available
+-- 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 INTO tablename1 VALUES('Richard','Mutt');
+
+-- In tablename1, change the fname value to 'John'
+-- for all rows that have an lname value of 'Mutt'. 
+UPDATE tablename1 SET fname='John' WHERE lname='Mutt';
+
+-- Delete rows from the tablename1 table
+-- where the lname value begins with 'M'.
+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. 
+DROP TABLE tablename1;
+```