Mnesia是一個分佈式數據庫管理系統，適合於電信和其它需要持續運行和具備軟實時特性的Erlang應用，越來越受關注和使用，但是目前Mnesia資料卻不多，很多都只有官方的用戶指南。下面的內容將着重說明 Mnesia 數據庫如何實現SQL查詢，實現select / insert / update / where / order by / join / limit / delete等SQL操作。
示例中表結構的定義：
%% 賬號表結構
-record( y_account,{ id, account, password }).
%% 資料表結構
-record( y_info, { id, nickname, birthday, sex }). 1、Create Table / Delete Table 操作
%%===============================================
%% create table y_account ( id int, account varchar(50),
%% password varchar(50), primary key(id)) ;
%%===============================================
%% 使用 mnesia:create_table
mnesia:create_table( y_account,[{attributes, record_info(fields, y_account)} , {type,set}, {disc_copies, [node()]} ]).
%%===============================================
%% drop table y_account;
%%===============================================
%% 使用 mnesia:delete_table
mnesia:delete_table(y_account) .
注：參數意義可以看文檔，{type,set} 表示id作爲主鍵，不允許id重複，如果改爲 {type,bag}，id可以重複，但整條記錄不能重複
2、Select 查詢
%%===============================================
%% select * from y_account
%%===============================================
%% 使用 mnesia:select
F = fun() ->
MatchHead = #y_account{ _ = '_' },
Guard = [],
Result = ['$_'],
mnesia:select(y_account, [{MatchHead, Guard, Result}])
end,
mnesia:transaction(F).
%% 使用 qlc
F = fun() ->
Q = qlc:q([E || E <- mnesia:table(y_account)]),
qlc:e(Q)
end,
mnesia:transaction(F).
查詢部分字段的記錄
%%===============================================
%% select id,account from y_account
%%===============================================
%% 使用 mnesia:select
F = fun() ->
MatchHead = #y_account{id = '$1', account = '$2', _ = '_' },
Guard = [],
Result = ['$$'],
mnesia:select(y_account, [{MatchHead, Guard, Result}])
end,
mnesia:transaction(F).
%% 使用 qlc
F = fun() ->
Q = qlc:q([[E#y_account.id, E#y_account.account] || E <- mnesia:table(y_account)]),
qlc:e(Q)
end,
mnesia:transaction(F).3、Insert / Update 操作
mnesia是根據主鍵去更新記錄的，如果主鍵不存在則插入
%%===============================================
%% insert into y_account (id,account,password) values(5,"xiaohong","123")
%% on duplicate key update account="xiaohong",password="123";
%%===============================================
%% 使用 mnesia:write
F = fun() ->
Acc = #y_account{id = 5, account="xiaohong", password="123"},
mnesia:write(Acc)
end,
mnesia:transaction(F).4、Where 查詢
%%===============================================
%% select account from y_account where id>5
%%===============================================
%% 使用 mnesia:select
F = fun() ->
MatchHead = #y_account{id = '$1', account = '$2', _ = '_' },
Guard = [{'>', '$1', 5}],
Result = ['$2'],
mnesia:select(y_account, [{MatchHead, Guard, Result}])
end,
mnesia:transaction(F).
%% 使用 qlc
F = fun() ->
Q = qlc:q([E#y_account.account || E <- mnesia:table(y_account), E#y_account.id>5]),
qlc:e(Q)
end,
mnesia:transaction(F).如果查找主鍵 key=X 的記錄，還可以這樣子查詢：
%%===============================================
%% select * from y_account where id=5
%%===============================================
F = fun() ->
mnesia:read({y_account,5})
end,
mnesia:transaction(F).如果查找非主鍵 field=X 的記錄，可以如下查詢：
%%===============================================
%% select * from y_account where account='xiaomin'
%%===============================================
F = fun() ->
MatchHead = #y_account{ id = '_', account = "xiaomin", password = '_' },
Guard = [],
Result = ['$_'],
mnesia:select(y_account, [{MatchHead, Guard, Result}])
end,
mnesia:transaction(F).5、Order By 查詢
%%===============================================
%% select * from y_account order by id asc
%%===============================================
%% 使用 qlc
F = fun() ->
Q = qlc:q([E || E <- mnesia:table(y_account)]),
qlc:e(qlc:keysort(2, Q, [{order, ascending}]))
end,
mnesia:transaction(F).
%% 使用 qlc 的第二種寫法
F = fun() ->
Q = qlc:q([E || E <- mnesia:table(y_account)]),
Order = fun(A, B) ->
B#y_account.id > A#y_account.id
end,
qlc:e(qlc:sort(Q, [{order, Order}]))
end,
mnesia:transaction(F).6、Join 關聯表查詢
%%===============================================
%% select y_info.* from y_account join y_info on (y_account.id = y_info.id)
%% where y_account.account = 'xiaomin'
%%===============================================
%% 使用 qlc
F = fun() ->
Q = qlc:q([Y || X <- mnesia:table(y_account),
X#y_account.account =:= "xiaomin",
Y <- mnesia:table(y_info),
X#y_account.id =:= Y#y_info.id
]),
qlc:e(Q)
end,
mnesia:transaction(F).7、Limit 查詢
%%===============================================
%% select * from y_account limit 2
%%===============================================
%% 使用 mnesia:select
F = fun() ->
MatchHead = #y_account{ _ = '_' },
mnesia:select(y_account, [{MatchHead, [], ['$_']}], 2, none)
end,
mnesia:transaction(F).
%% 使用 qlc
F = fun() ->
Q = qlc:q([E || E <- mnesia:table(y_account)]),
QC = qlc:cursor(Q),
qlc:next_answers(QC, 2)
end,
mnesia:transaction(F).8、Select count(*) 查詢
%%===============================================
%% select count(*) from y_account
%%===============================================
%% 使用 mnesia:table_info
F = fun() ->
mnesia:table_info(y_account, size)
end,
mnesia:transaction(F).9、Delete 查詢
%%===============================================
%% delete from y_account where id=5
%%===============================================
%% 使用 mnesia:delete
F = fun() ->
mnesia:delete({y_account, 5})
end,
mnesia:transaction(F).注：使用qlc模塊查詢，需要在文件頂部聲明“-include_lib("stdlib/include/qlc.hrl").”，否則編譯時會產生“Warning: qlc:q/1 called, but "qlc.hrl" not included”的警告。
實踐
方法3使用的是事務，也可以使用方法2的mnesia:dirty_select。
方法1：
F = fun () ->
MatchHead = #person{name='$1', sex=male, age='$2', _='_'},
Guard = [{'>', '$2', 30}],
Result = ['$1'],
L = mnesia:select(Tab,[{MatchHead, Guard, Result}])
end,
mnesia:transaction(F).
方法2：
MatchHead = #person{name='$1', sex=male, age='$2', _='_'}, % 也可包含等於條件 name=Name
Guard = [{'>', '$2', 30}], %條件 > == <
Result = ['$1'], %結果 '$n'返回變量$n；'$$'返回所有 $變量； '$_'返回整行；
mnesia:dirty_select(Tab,[{MatchHead, Guard, Result}]).
方法3：
F = fun() ->
MatchHead = #?ONLINE_TAB{type = '$1', user = '$2', _ = '_' },
Guard = [],
Result = ['$1'],
Types = mnesia:select(?ONLINE_TAB, [{MatchHead, Guard, Result}]),
io:format("topic list : ~p~n", [Types]),
%lists:foreach(fun(Type) -> plugin:start(Type) end, Types)
[plugin:start(Type) || Type <- Types]
end,
mnesia:transaction(F).