mysql触发器中怎么从把一个表中的字段值赋给一个变量

mysql触发器中怎么从把一个表中的字段值赋给一个变量

delimiter //
CREATE TRIGGER jian_reportnum_num after delete ON report 
FOR EACH ROW 
BEGIN 
  set @new_prjid= OLD.prjid;

  set @new_num=num from reportnum where prjid=@new_prjid; 这段是错的 我不知道怎么写呢
  if (@new_num = 0) then
update reportnum set num=0 where prjid=@new_prjid;
  else
   
update reportnum set num=num-1 where prjid=@new_prjid;
  end if;
END;

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select  num into @new_num from reportnum where prjid=@new_prjid;

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谢了

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set @new_num= (select num from reportnum where prjid=@new_prjid);