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POSTGRESQL-FROM隐式连接

SELECT声明一个或者多个源表。如果声明了多个源表， 那么结果就是所有源表的笛卡儿积(交叉连接)。但是通常会添加一些条件（通过WHERE）， 把返回行限制成笛卡儿积的一个小的子集。

inner join 相当于隐式的表达。

sql
SELECT a.city FROM airports a EXCEPT SELECT arr.city 
	FROM airports dep, airports arr, flights f 
	WHERE dep.city = 'Москва' 
	AND f.departure_airport = dep.airport_code 
	AND f.arrival_airport = arr.airport_code;
SELECT a.city FROM airports a EXCEPT SELECT arr.city 
	FROM airports dep, airports arr, flights f 
	WHERE dep.city = 'Москва' 
	AND f.departure_airport = dep.airport_code 
	AND f.arrival_airport = arr.airport_code;
sql
SELECT arr.city FROM airports dep 
	INNER JOIN airports arr ON f.arrival_airport = arr.airport_code 
	INNER JOIN flights f ON f.departure_airport = dep.airport_code 
	WHERE dep.city = 'Москва'
SELECT arr.city FROM airports dep 
	INNER JOIN airports arr ON f.arrival_airport = arr.airport_code 
	INNER JOIN flights f ON f.departure_airport = dep.airport_code 
	WHERE dep.city = 'Москва'