linq用lambda表达式 left join 自连接怎么写

.问题解决
通过以上分析，将''改为NULL，问题得到有效解决或者把谓词写到每个子SQL里面。当然，只要是非''的都是可以的，比如' '(空格）。

dingjun123@ORADB> SELECT *
2 FROM (SELECT t1.subobject_name, t1.object_name, t1.object_type
3 FROM t1
4 UNION ALL
5 SELECT NULL, t2.object_name, t2.object_type
6 FROM t2
7 UNION ALL
8 SELECT t3.subobject_name, NULL, t3.object_type FROM t3) t
9 WHERE (t.subobject_name = 'T' OR t.object_name = 'T');

256 rows selected.
Elapsed: 00:00:00.72

Execution Plan
----------------------------------------------------------
Plan hash value: 3324794093
---------------------------------------------------------------------