STRUCTURED QUERY LANGUAGE (SQL)

Structured Query Language (SQL) is an ISO/IEC 9075 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 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 available on github 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.

BASIC USAGE OF 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';

-- an example which updates all the vat either with 20 or 10 according to its initial value
```sql
UPDATE products
SET vat = CASE
    WHEN vat = 18 THEN 20
    WHEN vat = 8 THEN 10
    ELSE vat
END;

– 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;

**Ref:** https://learnxinyminutes.com/docs/sql/