User-defined Triggers

e.g. 1

--how many varieties of pizza each store sells
create or replace function
        nvarieties(_store_id integer) returns integer
as $$
declare
      nv integer;
begin
      select count(*) into nv
      from Stores t
              join Suburbs s on t.located_in = s.id
              join soldin  si on si.store = t.id
      where  t.id  =  _store_id;
      return nv;
end;
$$  language  'plpgsql';

select nvarieties(100);

-- a view to give #varieties per store
create view storeInfo(location, varieties)
as
select  s.name,  nvarieties(t.id)
from   Stores  t
            join  Suburbs s on t.located_in  =  s.id;

--  a more efficient  view to give #varieties  per store
create view storeInfo(location, varieties)
as
select s.name,t.varieties
from   Stores t
          join Suburbs s on t.located_in = s.id;

第二个View没有调用function（少了一些nvarieties里的sub-query）

--存放这个varieties有一些冗余，并且数据可能会过时
--add a new column to hold #varieties per store
alter table Stores add column varieties integer;
--sets this value up initially
--在空的column中加入数据
update Stores set varieties=nvarieties(id);

1.修改表结构alter table
alter table table_name add column column_name column_type;
2.更新表数据 update table
update table_name set column_name=value;

Trigger 引入

drop trigger if exists maintainVcount on SoldIn;

create trigger maintainVcount
after insert or delete on SoldIn
for each row
execute procedure  changeVarieties();

create or replace function
        changeVarieties()  returns  trigger
as $$
begin
      if  TG_OP = 'INSERT'  then
          update  Stores
          set         varieties  =  varieties + 1
          where     id = NEW.Store;
      elseif  TG_OP  =  'DELETE'  then
          update Stores
           set varieties = vartieies -1
           where id = OLD.store; 
  else
        raise exception ' changeVarieties() failed'
  end if;
  return null;
end;
$$ language = 'plpgsql'
 

exists 函数：sql built-in函数，用于判断表/sub-query中条目存在性
单独使用，如果无条目为False，否则为True
on SoldIn: trigger在表SoldIn的Event中 激活
After
slide上的解释：NEW contains current value and OLD contains the previous value of changed tuple
John Sherpherd: We are not
for each row: 使用each，自动在使用group by