JOIN（= INNER JOIN）：返回匹配的结果，没有匹配则没结果；
LEFT JOIN（= LEFT OUTER JOIN）：返回匹配的与左表的所有数据；
RIGHT JOIN（= RIGHT OUTER JOIN）：返回匹配的与右表的所有数据；
FULL JOIN（= FULL OUTER JOIN）：返回两个表的所有数据，匹配的在同一行。
在oracle中的测试结果：
AAA表数据 BBB表数据
CCC表数据
SELECT * FROM AAA A JOIN BBB B ON A.ID = B.ID;
SELECT * FROM AAA A JOIN CCC B ON A.ID = B.ID;
SELECT * FROM AAA A LEFT JOIN BBB B ON A.ID = B.ID;
SELECT * FROM AAA A LEFT JOIN CCC B ON A.ID = B.ID;
SELECT * FROM AAA A RIGHT JOIN BBB B ON A.ID = B.ID;
SELECT * FROM AAA A RIGHT JOIN CCC B ON A.ID = B.ID;
SELECT * FROM AAA A FULL JOIN BBB B ON A.ID = B.ID;
SELECT * FROM AAA A FULL JOIN CCC B ON A.ID = B.ID;