你不应将其trigger用于存储计算值。但是，如果你正在学习，那么请使用下面的代码（请参阅内联注释）

CREATE OR REPLACE TRIGGER staff_trigger
  BEFORE INSERT OR UPDATE OR DELETE ON Staff
  FOR EACH ROW
  BEGIN
      IF INSERTING THEN -- used IF . CASE usage is not allowed as IF
        UPDATE branch SET branchTotalStaffNumber = branchTotalStaffNumber + 1
    Where branchno = :new.staffBranchNo; -- used :new to identify the record
    
      ELSIF UPDATING THEN 
        -- update the branch table only if staffBranchno is updated
        -- adding one staff to the new branch
        IF :new.staffBranchNo <> :old.staffBranchNo THEN
          UPDATE branch SET branchTotalStaffNumber = branchTotalStaffNumber + 1
    Where branchno = :new.staffBranchNo;
          -- removing one staff from old branch
          UPDATE branch SET branchTotalStaffNumber = branchTotalStaffNumber - 1
    Where branchno = :old.staffBranchNo;
        END IF;
      ELSE -- IF DELETING can be omitted
        -- removing one staff from branch for delete action 
        UPDATE branch SET branchTotalStaffNumber = branchTotalStaffNumber - 1
    Where branchno = :old.staffBranchNo;
        
      END IF;
END;
/

请注意，你还可以在其中使用OF子句和列名来创建触发器。仅当提到的列onOF子句的值更改时，它才会执行触发器。