||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>