Queries Introduction

One of the core purposes of the SQL language is to retrieve information stored in a database. This is commonly referred to as querying.

SELECT:

SELECT is used every time you want to query data from a database and * means all columns.
Suppose we are only interested in two of the columns. We can select individual columns by their names (separated by a comma):

SELECT column1, column2 
FROM table_name;

AS:

SELECT name AS 'Titles'
FROM movies;

AS is a keyword in SQL that allows you to rename a column or table using an alias. 
The new name can be anything you want as long as you put it inside of single quotes. Here we renamed the name column as Titles.

DISTINCT:

DISTINCT is used to return unique values in the output. It filters out all duplicate values in the specified column(s).
For instance,
SELECT tools 
FROM inventory;

might produce:
tools
Hammer
Nails
Nails
Nails

By adding DISTINCT before the column name:
SELECT DISTINCT tools 
FROM inventory;

the result would now be:
tools
Hammer
Nails

WHERE:
We can restrict our query results using the WHERE clause in order to obtain only the information we want.

Following this format, the statement below filters the result set to only include top rated movies (IMDb ratings greater than 8):
SELECT *
FROM movies
WHERE imdb_rating > 8;

The WHERE clause filters the result set to only include rows where the following condition is true.

imdb_rating > 8 is the condition. Here, only rows with a value greater than 8 in the imdb_rating column will be returned.

The > is an operator. Operators create a condition that can be evaluated as either true or false.

Comparison operators used with the WHERE clause are:

= equal to
!= not equal to
> greater than
< less than
>= greater than or equal to
<= less than or equal to