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主要是一行的数据某些字段整成一个table,对此table进行操作即可
基础：select *, (select orderno) as new from bdorder , 可多次用select操作字段
扩展：可多次用select 取出数据，union成一个table 处理
举例：取出一行中最大的值
  
IF NOT OBJECT_ID('[tb]') IS NULL
    DROP TABLE [tb]
GO
CREATE TABLE [tb](
a SMALLINT,
b SMALLINT,
c SMALLINT,
d SMALLINT,
e SMALLINT,
f SMALLINT,
g SMALLINT 
)
INSERT [tb]
SELECT 1,2,3,4,5,2,3 UNION ALL
SELECT 4,5,6,7,7,2,0 UNION ALL
SELECT 4,9,6,7,7,9,6
GO
--SELECT * FROM [tb]
 
-->SQL查询如下:
 
SELECT *,
    (SELECT MAX(a)
     FROM(
       SELECT a UNION ALL       --用select 取出值union成table
       SELECT b UNION ALL
       SELECT c UNION ALL
       SELECT d UNION ALL
       SELECT e UNION ALL
       SELECT f UNION ALL
       SELECT g
       ) AS t   
    ) AS maxvalue
FROM tb