Introduction to SQL
To define what columns should be displayed in an SQL SELECT statement:

use USING to name the source table(s) and list the columns to be shown after SELECT.
use USING to name the source table(s) and list the columns to be shown after WHER
use SELECT to name the source table(s) and list the columns to be shown after USING.
use FROM to name the source table(s) and list the columns to be shown after SELECT.

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Introduction to SQL
If a query involves NOT, AND, OR with no parenthesis

The order of occurrence determines the order of evaluation.
NOT will be evaluated first; OR will be evaluated second; AND will be evaluated last.
NOT will be evaluated first; AND will be evaluated second; OR will be evaluated last.
AND will be evaluated first; OR will be evaluated second; NOT will be evaluated last.

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Introduction to SQL
Find the names of the countries whose condition is sunny.

SELECT country FROM location WHERE city NOT IN (SELECT city FROM weather WHERE condition = 'sunny');
SELECT country FROM location WHERE city IN (SELECT city FROM weather WHERE condition = sunny');
SELECT country FROM location WHERE condition = 'sunny';
SELECT country FROM location WHERE city UNION (SELECT city FROM weather WHERE condition = 'sunny');

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