我正在尝试在 Oracle 中创建一个 SQL 触发器，它会在每次插入或删除后计算表中的行，这些行满足具有特定值的特定行的条件。然后，应该用这个数字更新另一个表。
具体例子：我有一个employees表，当添加或删除一个员工时，我想统计该部门当前工作的员工人数，然后更新部门表以包含新的员工人数对于那个特定的部门。
有人有想法吗？我很难理解这个查询。
【问题讨论】：
正如詹姆斯已经评论的那样，您根本不应该这样做。如果必须，请参阅以下示例：
示例表：
SQL> create table employees
2 (deptno number,
3 empno number);
Table created.
SQL> create table emp_count
2 (deptno number,
3 cnt number);
Table created.
触发器，它负责您的操作（插入、更新或删除行）：
SQL> create or replace trigger trg_aiud_emp
2 after insert or update or delete on employees
3 for each row
4 begin
5 if inserting then
6 merge into emp_count c
7 using dual d
8 on (:new.deptno = c.deptno)
9 when matched then update set
10 c.cnt = c.cnt + 1
11 when not matched then insert (deptno, cnt)
12 values (:new.deptno, 1);
13 elsif deleting then
14 update emp_count c set
15 c.cnt = c.cnt - 1
16 where c.deptno = :old.deptno;
17 elsif updating then
18 if :new.deptno <> :old.deptno then
19 update emp_count c set
20 c.cnt = c.cnt - 1
21 where c.deptno = :old.deptno;
22
23 merge into emp_count c
24 using dual d
25 on (:new.deptno = c.deptno)
26 when matched then update set
27 c.cnt = c.cnt + 1
28 when not matched then insert (deptno, cnt)
29 values (:new.deptno, 1);
30 end if;
31 end if;
32 end;
33 /
Trigger created.
SQL>
一点测试：
SQL> insert into employees (deptno, empno) values (10, 7744);
1 row created.
SQL> select * From employees;
DEPTNO EMPNO
---------- ----------
10 7744
SQL> select * From emp_count;
DEPTNO CNT
---------- ----------
10 1
SQL> insert into employees (deptno, empno)
2 select 10, 8722 from dual union all
3 select 20, 1234 from dual;
2 rows created.
SQL> select * From employees;
DEPTNO EMPNO
---------- ----------
10 7744
10 8722
20 1234
SQL> select * From emp_count;
DEPTNO CNT
---------- ----------
10 2
20 1
SQL>
SQL> update employees set deptno = 30 where empno = 7744;
1 row updated.
SQL> select * From employees;
DEPTNO EMPNO
---------- ----------
30 7744
10 8722
20 1234
SQL> select * From emp_count;
DEPTNO CNT
---------- ----------
10 1
20 1
30 1
SQL> update employees set deptno = 10;
3 rows updated.
SQL> select * From employees;
DEPTNO EMPNO
---------- ----------
10 7744
10 8722
10 1234
SQL> select * From emp_count;
DEPTNO CNT
---------- ----------
10 3
20 0
30 0
SQL>
SQL> delete from employees where empno = 7744;
1 row deleted.
SQL> select * From employees;
DEPTNO EMPNO
---------- ----------
10 8722
10 1234
SQL> select * From emp_count;
DEPTNO CNT
---------- ----------
10 2
20 0
30 0
SQL> delete from employees;
2 rows deleted.
SQL> select * From employees;
no rows selected
SQL> select * From emp_count;
DEPTNO CNT
---------- ----------
10 0
20 0
30 0
SQL>
所有这些都没有什么大惊小怪的，因为您只需要一个简单、微小的视图：
SQL> create or replace view v_emp_count as
2 select deptno, count(*) cnt
3 from employees
4 group by deptno;
View created.
SQL> insert into employees (deptno, empno)
2 select 10, 7744 from dual union all
3 select 10, 8722 from dual union all
4 select 20, 1234 from dual;
3 rows created.
SQL> select * From employees;
DEPTNO EMPNO
---------- ----------
10 7744
10 8722
20 1234
SQL> select * From v_emp_count;
DEPTNO CNT
---------- ----------
20 1
10 2
SQL> update employees set deptno = 30 where empno = 7744;
1 row updated.
SQL> select * From employees;
DEPTNO EMPNO
---------- ----------
30 7744
10 8722
20 1234
SQL> select * From v_emp_count;
DEPTNO CNT
---------- ----------
30 1
20 1
10 1
结论：使用视图。
【讨论】：