Introduction to SQL
NULL is

not a value
the same as 0 for integer and blank for character
the same as blank for character
the same as 0 for integer

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

SELECT country FROM location WHERE city UNION (SELECT city FROM weather WHERE condition = 'sunny');
SELECT country FROM location WHERE condition = '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');

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