mirror of
https://github.com/adambard/learnxinyminutes-docs.git
synced 2024-11-23 06:03:07 +03:00
f7b5f742b3
Capitalization on final instance of "LIKE" was inconsistent with the standard proposed at the beginning of the article
111 lines
5.1 KiB
Markdown
111 lines
5.1 KiB
Markdown
---
|
||
language: SQL
|
||
filename: learnsql.sql
|
||
contributors:
|
||
- ["Bob DuCharme", "http://bobdc.com/"]
|
||
---
|
||
|
||
Structured Query Language (SQL) is an [ISO/IEC 9075](https://www.iso.org/standard/63555.html) 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;
|
||
```
|
||
|
||
## 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.
|