EntityFramework查询oracle数据库时报ora-12704: character set mismatch

1、这段linq，执行期间报ora-12704:character set mismatch错误。

 var query = from m in ctx.MENU
     where (m.SUPER_MENU_ID ?? "") == (parentMenuId ?? "")
     orderby m.SORT_ID descending
     select new { m.SORT_ID };

生成出来的sql如下：

 SELECT "Project1"."SORT_ID" AS "SORT_ID"
    FROM (

 SELECT "Extent1"."SORT_ID" AS "SORT_ID"
    FROM "BA"."MENU" "Extent1"
  WHERE ((CASE WHEN("Extent1"."SUPER_MENU_ID" IS NULL) THEN ''
                     ELSE "Extent1"."SUPER_MENU_ID" END) =
         (CASE WHEN(&p__linq__0 IS NULL) THEN ''
                     ELSE &p__linq__0 END))

 ) "Project1"
 ORDER BY "Project1"."SORT_ID" DESC

但是这条sql单独放到plsql里跑是OK的。

2、改成这样，让生成的sql去掉了里面的case when就OK了。

 parentMenuId = parentMenuId ?? "";
 var query = from m in ctx.MENU
       where m.SUPER_MENU_ID == parentMenuId
     orderby m.SORT_ID descending
       select new { m.SORT_ID };

生成的sql如下：

 SELECT "Project1"."SORT_ID" AS "SORT_ID" FROM ( 

 SELECT "Extent1"."SORT_ID" AS "SORT_ID"
    FROM "BA"."MENU" "Extent1"
 WHERE ("Extent1"."SUPER_MENU_ID" = :p__linq__0)

 ) "Project1"
 ORDER BY "Project1"."SORT_ID" DESC

3、目前的猜测是，ef生成出来的case when有问题，调整linq不生成case when即可。但奇怪的是，同样的sql在plsql里跑居然也是ok的，手工修改客户端的字符集也无法在plsql里重现这个问题，如下：

修改注册表里，HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\KEY_OraClient11g_home2\NLS_LANG，在plsql里客户端使用不同于服务端的字符集，但无法生成同样的错误。

修改前：

NLS_LANG = SIMPLIFIED CHINESE_CHINA.ZHS16GBK
Character Sets
Character size: 2 byte(s)
CharSetID: 852
NCharSetID: 2000
Unicode Support: True
NLS_LANG: SIMPLIFIED CHINESE_CHINA.ZHS16GBK
NLS_CHARACTERSET: ZHS16GBK
NLS_NCHAR_CHARACTERSET: AL16UTF16

修改后：

NLS_LANG = SIMPLIFIED CHINESE_CHINA.AL32UTF8
Character Sets
Character size: 2 byte(s)
CharSetID: 873
NCharSetID: 2000
Unicode Support: True
NLS_LANG: SIMPLIFIED CHINESE_CHINA.AL32UTF8
NLS_CHARACTERSET: ZHS16GBK
NLS_NCHAR_CHARACTERSET: AL16UTF16

4、updated on 2014.06.03，问题有了新的线索，下面这条linq仍然会报同样的错误：

R1 == null ? string.Empty : R1.Emp_ID

但把string.Empty改成""就OK，很怪：

R1 == null ? "" : R1.Emp_ID

5、这个问题只是暂时解决，仍然存疑中，待完善。主要参考这篇：

《ef4 part 8: database agnostic linq to entities》