From 4def7beeab6d82eb256e9266b4be884c644dd595 Mon Sep 17 00:00:00 2001 From: Bob DuCharme Date: Mon, 29 Oct 2018 18:16:47 -0400 Subject: Adding SQL entry --- sql.html.markdown | 106 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 106 insertions(+) create mode 100644 sql.html.markdown (limited to 'sql.html.markdown') diff --git a/sql.html.markdown b/sql.html.markdown new file mode 100644 index 00000000..0dc07eb5 --- /dev/null +++ b/sql.html.markdown @@ -0,0 +1,106 @@ +--- +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; +``` -- cgit v1.2.3