Adding SQL entry

sql.html.markdown

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+---
+language: SQL
+filename: learnsql.sql
+contributors:
+- ["Bob DuCharme", "http://bobdc.com/"]
+lang: en-en
+---
+
+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. 
+
+
+```
+# Comments start with a pound sign. 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;
+```