Erlang 程式設計/使用 mnesia

使用 Mnesia

Mnesia 是用 Erlang 編寫的分散式資料庫，主要用於 Erlang 程式。簡單的查詢可以使用查詢列表推導（qlc）編寫。即時查詢可以決定不使用事務。

解釋程式: art.erl

該程式定義了一些類似於 SQL 的函式。我們希望構建一個名為“painting”的藝術品表，包含以下欄位：索引、藝術家和標題。

Table: painting
 
+---------------------------------------------+ 
| index | artist  | title                     |
+---------------------------------------------+  
| 1     | Dali    | The Ghost of Vermeer      |
| 2     | Dali    | The Persistance of Memory |
| 3     | Vermeer | Girl With Pearl Earring   |
+---------------------------------------------+

art:init(). 啟動資料庫並建立表：painting。
insert() 將資料放入表中
select() 從表中獲取資料

請注意，第二次呼叫 init() 時，資料庫將重啟，但我們會在嘗試重新建立表時收到錯誤（因為它已經存在）。

示例程式: art.erl

-module(art).
-compile(export_all).
-include_lib("stdlib/include/qlc.hrl"). 
 
-record(painting, {index, artist, title}).
  
init() ->
    mnesia:create_schema([node()]),
    mnesia:start(),
    mnesia:create_table(painting,
        [ {disc_copies, [node()] },
             {attributes,      
                record_info(fields,painting)} ]).
 
insert( Index, Artist, Title) ->
    Fun = fun() ->
         mnesia:write(
         #painting{ index=Index,
                   artist=Artist, 
                        title=Title    } )
               end,
         mnesia:transaction(Fun).
  
select( Index) ->
    Fun = 
        fun() ->
            mnesia:read({painting, Index})
        end,
    {atomic, [Row]}=mnesia:transaction(Fun),
    io:format(" ~p ~p ~n ", [Row#painting.artist, Row#painting.title] ).

select_some( Artist) ->
    Fun = 
        fun() ->
            mnesia:match_object({painting, '_', Artist, '_' } )
        end,
    {atomic, Results} = mnesia:transaction( Fun),
    Results.
 
select_all() -> 
    mnesia:transaction( 
    fun() ->
        qlc:eval( qlc:q(
            [ X || X <- mnesia:table(painting) ] 
        )) 
    end ).
  
select_search( Word ) -> 
    mnesia:transaction( 
    fun() ->
         qlc:eval( qlc:q(
              [ {F0,F1,F2,F3} || 
                   {F0,F1,F2,F3} <- 
                        mnesia:table(painting),
                        (string:str(F2, Word)>0) or  
                        (string:str(F3, Word)>0)
               ] )) 
    end ).

示例輸出: art.erl

%   Sample output:
 
%   6> c(art).
%   {ok,art}
  
%   7> art:init(). 
%   {atomic,ok}
 
%   6> art:insert(1,"Dali","The Ghost of Vermeer").
%   {atomic,ok}
  
%   7> art:select(1).
%   "Dali" "The Ghost of Vermeer"
%   ok
  
%   8> art:insert(2,"Dali","The Persistence of Memory").
%   {atomic,ok}
   
%   9> art:select(2).
%   "Dali" "The Persistence of Memory"
%   ok
   
%   10> art:select(1).
%   "Dali" "The Ghost of Vermeer"
%   ok 
 
%   25> art:insert(3,"Vermeer", "Girl With Pearl Earring").
%   {atomic,ok}
 
%   26> art:select_some("Dali").
%   [{painting,1,"Dali","The Ghost of Vermeer"},
%   {painting,2,"Dali","The Persistence of Memory"}]
 
%   27> art:select_all().
%   {atomic,[{painting,1,"Dali","The Ghost of Vermeer"},
%               {painting,2,"Dali","The Persistence of Memory"},
%               {painting,3,"Vermeer","Girl With Pearl Earring"}]}

%---to run a new session after restarting erlang---
 
%   2> art:init().
%   {aborted,{already_exists,painting}}

%   3> art:select_search("Vermeer").
%   {atomic,[{painting,1,"Dali","The Ghost of Vermeer"},
%         {painting,3,"Vermeer","Girl With Pearl Earring"}]}