1.是否select字段需要全部出现?

Oracle中出现在select列表中的字段或者出现在order by后面的字段，如果不是包含在分组函数中，那么该字段必须同时在group by子句中出现。
Mysql没有此限制
例:
Mysql中,以下SQL语句是成立的,可见这里的Group by

SELECT p.PDTNO, p.PDTNAME, sum(a.QUANTITY) AS applySumCount
FROM PRODUCT p 
JOIN APPLY a ON p.PDTNO = a.PDTNO 
GROUP BY PDTNO 
ORDER BY sum(a.QUANTITY) DESC;

Oracle中,这样的行为则会提示ORA-00979: 不是 GROUP BY 表达式
必须写成: GROUP BY p.PDTNO, p.PDTNAME 才可以

SELECT t1.pdtno, t1.pdtname, t1.applySumCount
FROM(
		SELECT p.PDTNO, p.PDTNAME, sum(a.QUANTITY) AS applySumCount
		FROM PRODUCT p
		JOIN APPLY a ON p.PDTNO = a.PDTNO 
		GROUP BY p.PDTNO
	)t1
ORDER BY t1.applySumCount DESC;

准确来说,这是SQL标准里面严格语法和宽松语法的区别
参考: 神奇的 SQL 之层级 → 为什么 GROUP BY 之后不能直接引用原表中的列

2.Mysql中group by可以使用别名，Oracle 中不可以

如下:
Mysql中:像这样在Group by中使用别名是可以的

SELECT p.PDTNO pdt, p.PDTNAME, sum(a.QUANTITY) AS applySumCount
FROM PRODUCT p 
JOIN APPLY a ON p.PDTNO = a.PDTNO 
GROUP BY pdt
ORDER BY sum(a.QUANTITY) DESC;

但是在ORacle中,使用别名会报错:ORA-00904: "PDN": 标识符无效

SELECT t1.pdtno, t1.pdtname, t1.applySumCount
FROM(
		SELECT p.PDTNO pdt, p.PDTNAME pdn, sum(a.QUANTITY) AS applySumCount
		FROM PRODUCT p
		JOIN APPLY a ON p.PDTNO = a.PDTNO 
		GROUP BY pdt, pdn
	)t1
ORDER BY t1.applySumCount DESC;