||拼接列关联和直接关联区别

SQL> select * from emp;

     EMPNO ENAME      JOB		MGR HIREDATE	     SAL       COMM	DEPTNO
---------- ---------- ---------- ---------- --------- ---------- ---------- ----------
      7369 SMITH      CLERK	       7902 17-DEC-80	     800		    20
      7499 ALLEN      SALESMAN	       7698 20-FEB-81	    1600	300	    30
      7521 WARD       SALESMAN	       7698 22-FEB-81	    1250	500	    30
      7566 JONES      MANAGER	       7839 02-APR-81	    2975		    20
      7654 MARTIN     SALESMAN	       7698 28-SEP-81	    1250       1400	    30
      7698 BLAKE      MANAGER	       7839 01-MAY-81	    2850		    30
      7782 CLARK      MANAGER	       7839 09-JUN-81	    2450		    10
      7788 SCOTT      ANALYST	       7566 09-DEC-82	    3000		    20
      7839 KING       PRESIDENT 	    17-NOV-81	    5000		    10
      7844 TURNER     SALESMAN	       7698 08-SEP-81	    1500	  0	    30
      7876 ADAMS      CLERK	       7788 12-JAN-83	    1100		    20
      7900 JAMES      CLERK	       7698 03-DEC-81	     950		    30
      7902 FORD       ANALYST	       7566 03-DEC-81	    3000		    20
      7934 MILLER     CLERK	       7782 23-JAN-82	    1300		    10
      9999

15 rows selected.

SQL> select * from emp3;

ENAME	   JOB		    COMM
---------- ---------- ----------
SMITH	   CLERK
ADAMS	   CLERK
JAMES	   CLERK
MILLER	   CLERK

SQL> select * from emp where COMM is null;

     EMPNO ENAME      JOB		MGR HIREDATE	     SAL       COMM	DEPTNO
---------- ---------- ---------- ---------- --------- ---------- ---------- ----------
      7369 SMITH      CLERK	       7902 17-DEC-80	     800		    20
      7566 JONES      MANAGER	       7839 02-APR-81	    2975		    20
      7698 BLAKE      MANAGER	       7839 01-MAY-81	    2850		    30
      7782 CLARK      MANAGER	       7839 09-JUN-81	    2450		    10
      7788 SCOTT      ANALYST	       7566 09-DEC-82	    3000		    20
      7839 KING       PRESIDENT 	    17-NOV-81	    5000		    10
      7876 ADAMS      CLERK	       7788 12-JAN-83	    1100		    20
      7900 JAMES      CLERK	       7698 03-DEC-81	     950		    30
      7902 FORD       ANALYST	       7566 03-DEC-81	    3000		    20
      7934 MILLER     CLERK	       7782 23-JAN-82	    1300		    10
      9999

11 rows selected.


SQL> SELECT empno, ename, job, comm, deptno
  FROM emp
 WHERE (ename , job , comm) IN (SELECT ename, job , comm FROM emp3);   2    3  

no rows selected

为什么会返回空行呢?

因为COMM列有空值
SQL> SELECT empno, ename, job, comm, deptno
  FROM emp
 WHERE ename || job || comm IN (SELECT ename || job || comm FROM emp3);
  2    3  
     EMPNO ENAME      JOB	       COMM	DEPTNO
---------- ---------- ---------- ---------- ----------
      7369 SMITH      CLERK			    20
      7876 ADAMS      CLERK			    20
      7900 JAMES      CLERK			    30
      7934 MILLER     CLERK			    10
利用||拼接符号有空值会继续返回值

SQL> select ename || job || comm from emp where comm is null;

ENAME||JOB||COMM
------------------------------------------------------------
SMITHCLERK
JONESMANAGER
BLAKEMANAGER
CLARKMANAGER
SCOTTANALYST
KINGPRESIDENT
ADAMSCLERK
JAMESCLERK
FORDANALYST
MILLERCLERK


11 rows selected.

继续测试:

 SQL> create table c1(id char(10));

Table created.

SQL> create table c2(id char(10));

Table created.

SQL> insert into c1 values(null);

1 row created.

SQL>  insert into c2 values(null);

1 row created.

SQL> commit;

Commit complete.

SQL> select * from c1;

ID
----------


SQL> select * from c2;

ID
----------


SQL> select c1.* from c1 where c1.id in (select c2.id from c2);

no rows selected

SQL> select c1.* from c1 where c1.id||'a' in (select c2.id||'a' from c2);

ID
----------


SQL>