Mnesia是一个分布式
数据库管理系统，适合于电信和其它需要持续运行和具备软实时特性的Erlang应用，越来越受关注和使用，但是目前Mnesia资料却不多，很多都只有官方的用户指南。下面的内容将着重说明 如何做 Mnesia 数据库查询。
示例中表结构的定义：
%% 账号表结构
-record( y_account,{ id, account, password }).
%% 资料表结构
-record( y_info, { id, nickname, birthday, sex }).
1、Select 查询
查询全部记录
%%=============================================== %% 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). |
查询部分字段的记录
%%=============================================== %% 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). |
2、Insert / Update 操作
mnesia是根据主键去更新记录的，如果主键不存在则插入
%%=============================================== %% 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). |
3、Where 查询
%%=============================================== %% 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 = ['$2'], mnesia:select(y_account, [{MatchHead, Guard, Result}]) end, mnesia:transaction(F). %% 使用 qlc F = fun() -> Q = qlc:q([E#y_account.account || E <- mnesia:table(y_account), E#y_account.id>5]), qlc:e(Q) end, mnesia:transaction(F). |
如果查找主键 key=X 的记录，还可以这样子查询：
%%=============================================== %% select * from y_account where id=5 %%=============================================== F = fun() -> mnesia:read({y_account,5}) end, mnesia:transaction(F). |
如果查找非主键 field=X 的记录，可以如下查询：
%%=============================================== %% select * from y_account where account='xiaomin' %%=============================================== F = fun() -> MatchHead = #y_account{ id = '_', account = "xiaomin", password = '_' }, Guard = [], Result = ['$_'], mnesia:select(y_account, [{MatchHead, Guard, Result}]) end, mnesia:transaction(F). |
4、Order By 查询
%%=============================================== %% 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 的第二种写法 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). |
5、Join 关联表查询
%%=============================================== %% 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). |
6、Limit 查询
%%=============================================== %% select * from y_account limit 2 %%=============================================== %% 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). |
注：使用qlc模块查询，需要在文件顶部声明“-include_lib("stdlib/include/qlc.hrl").”，否则编译时会产生“Warning: qlc:q/1 called, but "qlc.hrl" not included”的警告。