PL / SQL触发器更新或删除

Question

I have two table where their output looks like:

Project Table
CODE         NAME                           
------------ -------------- 
101          Alpha                       
222          Beta                       
355          Gamma                       
973          Delta                   

Assignment Table
ID    NAME         PROJCODE       HOURS              
----- ------------ -------------- ------------ 
55055 Smith        101            20                     
55055 Smith        222            10                     
39002 Hammond      973            25                     
00001 Preston      355            5                      
10000 Logan        355            5                      
00777 Bond         222            20                        

If the last person from a project in the Assignment Table is updated/deleted (so then the projcode has no person assigned to it), I want to remove it from the Project Table. And I want to output it to DBMS_OUTPUT. I wrote the following trigger but am getting an error each time I try to test the trigger.

CREATE OR REPLACE TRIGGER project_removal_trigger AFTER
  DELETE OR
  UPDATE ON ASSIGNMENT FOR EACH ROW DECLARE PRINT project.code%type;
  BEGIN
    DELETE
    FROM PROJECT
    WHERE code NOT IN
      (SELECT Projcode FROM assignment GROUP BY projcode HAVING COUNT(name) > 0
      );
  END;

What am I doing wrong?

Answer 1

Trigger can't select from a table it's fired for. Otherwise you'll get ORA-04091: table XXXX is mutating, trigger/function may not see it. try not to put too much logic into triggers.