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 Mnesia 数据库如何实现SQL查询，实现select / insert / up

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 Mnesia 数据库如何实现SQL查询，实现select / insert / update / where / order by / join / limit / delete等SQL操作。

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、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：参数意义可以看文档，{type,set} 表示id作为主键，不允许id重复，如果改为 {type,bag}，id可以重复，但整条记录不能重复

2、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).

3、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).

4、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).

5、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).

6、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).

7、Limit 查询

%%===============================================
%% 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(*) 查询

%%===============================================
%% select count(*) from y_account
%%===============================================

%% 使用 mnesia:table_info
F = fun() ->
	mnesia:table_info(y_account, size)
end,
mnesia:transaction(F).

9、Delete 查询

%%===============================================
%% 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。

更新说明：

2013/11/20 补充了 mnesia:select 方式的 limit 查询