erlang mnesia 数据库实现SQL查询

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：

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%% 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

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%%===============================================
%% 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

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%%===============================================
%% 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

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%%===============================================
%% 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

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%%===============================================
%% 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

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%%===============================================
%% 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).