JOIN（= INNER JOIN）：返回匹配的結果，沒有匹配則沒結果；
LEFT JOIN（= LEFT OUTER JOIN）：返回匹配的與左表的所有數據；
RIGHT JOIN（= RIGHT OUTER JOIN）：返回匹配的與右表的所有數據；
FULL JOIN（= FULL OUTER JOIN）：返回兩個表的所有數據，匹配的在同一行。
在oracle中的測試結果：
AAA表數據
BBB表數據
CCC表數據
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;
