有2个表，test1、test2，结构如下：

SQL> desc test1;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 A                                                  CHAR(20)
 B                                                  VARCHAR2(20)
 
SQL> desc test2;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 C                                                  VARCHAR2(100)
 D                                                  CHAR(10)

SQL> select * from test1;
 
A                              B
------------------------------ --------------------
111111                         asdfas
 
SQL> select * from test2;
 
C                              D
------------------------------ ----------
111111                         asasdf
 

其中A字段和C字段均为'111111',对字段A和C进行连接，执行联合查询，如下：

SQL> select test1.a,test2.c from test1,test2 where test1.a=test2.c;
 
no rows selected

分别查看A字段和C字段长度，

SQL> select length(a) from test1;
 
 LENGTH(A)
----------
        20
 
SQL> select length(c) from test2;
 
 LENGTH(C)
----------
         6

原来是两个表的字段类型和长度不一致。用trim函数去掉A字段中的空格后，再次执行查询：

SQL> select test1.a,test2.c from test1,test2 where trim(test1.a)=test2.c;
 
A                              C
------------------------------ ------------------------------
111111                         111111

                                                         ----end----