原文链接如下：
另外一个链接感觉也很好，是MySQL的，一起贴上来：
谢谢原作者的辛勤付出，贴过来如下：
有如下sql语句：
【案例1】
SELECT
userName
AS
a, userAddress
AS
b
FROM TestTable
WHERE
a
like
'%am%'
该语句执行是会报错，因为
别名只是对字段的一种引用，不能当作字段一样使用，如果真要把别名当字段一样使用，可以如下变通：
SELECT a, b
FROM(
SELECT
userName AS a, userAddress AS b FROM TestTable
) AS
tempTable
WHERE a
like
'%dd%'
如此，便可把别名当字段一样使用了
但是需要和足以紫色字体部分，该处要给内嵌的select查询指定一个别名，否则会报错
【案例2】
SELECT coutDay
AS
a, month1
AS
b,
(select 信用天数 from PayDays where 自定义值 = month1
)
AS
c, (
a+
c) AS
'total'
FROM TestTable
变通为：
SELECT a, b,
a + c AS
'total'
FROM(
SELECT coutDay AS a, month1 AS b, (select 信用天数 from PayDays where 自定义值 = month1) AS c FROM TestTable
) AS
tempTable
【案例3】
下面的sql语句中中，灰色部分重复了两次，第二个灰色部分有不能用第一个灰色区域的别名CountDays替换，一旦替换会出现语法错误。
SELECT
cCustCode As 客户代码,
cPayWay As 付款条件,
cPayCode As 付款方式,
CASE
WHEN cPayWay like '%月结%' THEN (datediff(day, dOutDate, dateadd(ms,-3,DATEADD(mm, DATEDIFF(m,0,dOutDate)+1, 0))) + (select 信用天数 from PayDays where 自定义值 = cPayWay))
ELSE (select 信用天数 from PayDays where 自定义值 = cPayWay)
END As CountDays,
Convert(Varchar(12),dOutDate, 102) As 出库日期,
Convert(Varchar(12),DateAdd(Day, Cast(
(
CASE
WHEN cPayWay like '%月结%' THEN (datediff(day, dOutDate, dateadd(ms,-3,DATEADD(mm, DATEDIFF(m,0,dOutDate)+1, 0))) + (select 信用天数 from PayDays where 自定义值 = cPayWay))
ELSE (select 信用天数 from PayDays where 自定义值 = cPayWay)
END) AS int), dOutDate), 102) AS 实际回款日期
FROM ODS.F_DISPATCHLIST_ALL
where (isum-isnull(nSumARAmt,0))>10e-8
我们把第二个灰色部分提出来，改为
SELECT 客户代码,付款条件, 付款方式,CountDays,出库日期,Convert(Varchar(12),DateAdd(Day, Cast(
CountDays)AS int), dOutDate), 102)
AS 实际回款日期
FROM(
SELECT
cCustCode As 客户代码,
cPayWay As 付款条件,
cPayCode As 付款方式,
CASE
WHEN cPayWay like '%月结%' THEN (datediff(day, dOutDate, dateadd(ms,-3,DATEADD(mm, DATEDIFF(m,0,dOutDate)+1, 0))) + (select 信用天数 from PayDays where 自定义值 = cPayWay))
ELSE (select 信用天数 from PayDays where 自定义值 = cPayWay)
END As
CountDays,
Convert(Varchar(12),dOutDate, 102) As 出库日期
FROM ODS.F_DISPATCHLIST_ALL
where (isum-isnull(nSumARAmt,0))>10e-8
)