利用merge优化

create table emp_test as select * from emp;

create table dept_test as select * from dept;

ALTER TABLE EMP_test ADD dname VARCHAR2(50) DEFAULT 'UFO';

SQL> set linesize 200
SQL> select empno,ename,deptno,dname from emp_test;  

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------------------------------------------
      7369 SMITH	      20 UFO
      7499 ALLEN	      30 UFO
      7521 WARD 	      30 UFO
      7566 JONES	      20 UFO
      7654 MARTIN	      30 UFO
      7698 BLAKE	      30 UFO
      7782 CLARK	      10 UFO
      7788 SCOTT	      20 UFO
      7839 KING 	      10 UFO
      7844 TURNER	      30 UFO
      7876 ADAMS	      20 UFO

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------------------------------------------
      7900 JAMES	      30 UFO
      7902 FORD 	      20 UFO
      7934 MILLER	      10 UFO

14 rows selected.

SQL> select a.empno, a.ename, a.deptno, b.dname
  from emp_test a, dept_test b
 where a.deptno = b.deptno(+)  2    3  ;

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------
      7934 MILLER	      10 ACCOUNTING
      7839 KING 	      10 ACCOUNTING
      7782 CLARK	      10 ACCOUNTING
      7902 FORD 	      20 RESEARCH
      7876 ADAMS	      20 RESEARCH
      7788 SCOTT	      20 RESEARCH
      7566 JONES	      20 RESEARCH
      7369 SMITH	      20 RESEARCH
      7900 JAMES	      30 SALES
      7844 TURNER	      30 SALES
      7698 BLAKE	      30 SALES

USQL> 
UPDATE emp_test emp
   SET emp.dname =
       (SELECT dept.dname
          FROM dept_test dept
         WHERE dept.dname IN ('SALES', 'RESEARCH')
           AND dept.deptno = emp.deptno);SQL>   2    3    4    5    6  

14 rows updated.

SQL> 
select empno,ename,deptno,dname from emp_test;  SQL> 

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------------------------------------------
      7369 SMITH	      20 RESEARCH
      7499 ALLEN	      30 SALES
      7521 WARD 	      30 SALES
      7566 JONES	      20 RESEARCH
      7654 MARTIN	      30 SALES
      7698 BLAKE	      30 SALES
      7782 CLARK	      10
      7788 SCOTT	      20 RESEARCH
      7839 KING 	      10
      7844 TURNER	      30 SALES
      7876 ADAMS	      20 RESEARCH

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------------------------------------------
      7900 JAMES	      30 SALES
      7902 FORD 	      20 RESEARCH
      7934 MILLER	      10

14 rows selected.

SQL> select a.empno, a.ename, a.deptno, b.dname
  from emp_test a, dept_test b
 where a.deptno = b.deptno(+)  2    3  ;

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------
      7934 MILLER	      10 ACCOUNTING
      7839 KING 	      10 ACCOUNTING
      7782 CLARK	      10 ACCOUNTING
      7902 FORD 	      20 RESEARCH
      7876 ADAMS	      20 RESEARCH
      7788 SCOTT	      20 RESEARCH
      7566 JONES	      20 RESEARCH
      7369 SMITH	      20 RESEARCH
      7900 JAMES	      30 SALES
      7844 TURNER	      30 SALES
      7698 BLAKE	      30 SALES

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------
      7654 MARTIN	      30 SALES
      7521 WARD 	      30 SALES
      7499 ALLEN	      30 SALES

14 rows selected.

SQL> UPDATE emp_test emp
   SET emp.dname = (SELECT dept.dname
                      FROM dept_test dept
                     WHERE dept.dname IN ('SALES', 'RESEARCH')
                       AND dept.deptno = emp.deptno)
 WHERE EXISTS (SELECT dept.dname
          FROM dept_test dept
         WHERE dept.dname IN ('SALES', 'RESEARCH')
           AND dept.deptno = emp.deptno);  2    3    4    5    6    7    8    9  

11 rows updated.

SQL> select empno,ename,deptno,dname from emp_test; 

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------------------------------------------
      7369 SMITH	      20 RESEARCH
      7499 ALLEN	      30 SALES
      7521 WARD 	      30 SALES
      7566 JONES	      20 RESEARCH
      7654 MARTIN	      30 SALES
      7698 BLAKE	      30 SALES
      7782 CLARK	      10 UFO
      7788 SCOTT	      20 RESEARCH
      7839 KING 	      10 UFO
      7844 TURNER	      30 SALES
      7876 ADAMS	      20 RESEARCH

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------------------------------------------
      7900 JAMES	      30 SALES
      7902 FORD 	      20 RESEARCH
      7934 MILLER	      10 UFO

14 rows selected.


     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------
      7654 MARTIN	      30 SALES
      7521 WARD 	      30 SALES
      7499 ALLEN	      30 SALES

14 rows selected.

SQL> SELECT dept.dname
          FROM dept_test dept,emp_test emp
         WHERE dept.dname IN ('SALES', 'RESEARCH')
           AND dept.deptno = emp.deptno
           and emp.deptno=10  2    3    4    5  ;

no rows selected

那么deptno=10对应的记录,找不到的部门会设置为空


改为merge：
SQL> merge into emp_test emp
using (SELECT dept.dname,dept.deptno
         FROM dept_test dept
        WHERE dept.dname IN ('SALES', 'RESEARCH')) dept 
on (dept.deptno = emp.deptno)
WHEN MATCHED THEN
  update set emp.dname = dept.dname  2    3    4    5    6    7  ;

11 rows merged.

SQL>  select empno,ename,deptno,dname from emp_test;

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------------------------------------------
      7369 SMITH	      20 RESEARCH
      7499 ALLEN	      30 SALES
      7521 WARD 	      30 SALES
      7566 JONES	      20 RESEARCH
      7654 MARTIN	      30 SALES
      7698 BLAKE	      30 SALES
      7782 CLARK	      10 UFO
      7788 SCOTT	      20 RESEARCH
      7839 KING 	      10 UFO
      7844 TURNER	      30 SALES
      7876 ADAMS	      20 RESEARCH

     EMPNO ENAME	  DEPTNO DNAME
---------- ---------- ---------- --------------------------------------------------
      7900 JAMES	      30 SALES
      7902 FORD 	      20 RESEARCH
      7934 MILLER	      10 UFO

14 rows selected.