仅从 mnesia 中选择一列

发布于 2024-12-24 16:43:45 字数 166 浏览 1 评论 0原文

我怎样才能从 mnesia 中只选择一列？我可以使用以下代码选择 ets 表中的唯一列：

ets:match(AllData_TableId, {'_', '$1','_',','_'},3),

我需要类似的 mnesia。

谢谢。

原文

how can i select only one column from mnesia?
I can select onle column in ets table with this code:

ets:match(AllData_TableId, {'_', '$1','_',','_'},3),

I need something similar for mnesia.

Thank you.

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最偏执的依靠 2024-12-31 16:43:45

在此处找到的示例中：http://en.wikibooks.org/wiki/Erlang_Programming/ using_mnesia，看看作者如何使用函数mnesia:match_object/1，考虑在这里多读一下http://www.erlang.org/doc/man/mnesia.html#match_object -1

但是，建议我们设计 mnesia 数据库和/或表以避免使用此方法。这是因为它使 mnesia 遍历整个表来寻找匹配项。

你需要的是qlc

-include_lib("stdlib/include/qlc.hrl").

select(Q)->
    case mnesia:is_transaction() of
        false -> 
            F = fun(QH)-> qlc:e(QH) end,
            %% mnesia:transaction(F);
            mnesia:activity(transaction,F,[Q],mnesia_frag);
        true -> qlc:e(Q)
    end.

-record(book,{title,isbn,price,category}).

book_title({book,ISBN})->
    select(qlc:q([X#book.title || X <- mnesia:table(book),X#book.isbn == ISBN])).

In the examples found here: http://en.wikibooks.org/wiki/Erlang_Programming/Using_mnesia, look at how the author uses the function mnesia:match_object/1, and the consider reading it here more http://www.erlang.org/doc/man/mnesia.html#match_object-1

However, we are advised to design our mnesia databases and/or tables in a way to avoid the use of this method. This is because it makes mnesia traverse the entire table looking for a match.

What you need is qlc

-include_lib("stdlib/include/qlc.hrl").

select(Q)->
    case mnesia:is_transaction() of
        false -> 
            F = fun(QH)-> qlc:e(QH) end,
            %% mnesia:transaction(F);
            mnesia:activity(transaction,F,[Q],mnesia_frag);
        true -> qlc:e(Q)
    end.

-record(book,{title,isbn,price,category}).

book_title({book,ISBN})->
    select(qlc:q([X#book.title || X <- mnesia:table(book),X#book.isbn == ISBN])).

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红衣飘飘貌似仙 2024-12-31 16:43:45

您可以使用 mnesia:select 来实现：

Spec = [{#tablename{columnname = '$1', _ = '_'}, [], ['$1']}],
{atomic, Result} = mnesia:transaction(fun() -> mnesia:select(tablename, Spec) end),
Result.

You can use mnesia:select for that:

Spec = [{#tablename{columnname = '$1', _ = '_'}, [], ['$1']}],
{atomic, Result} = mnesia:transaction(fun() -> mnesia:select(tablename, Spec) end),
Result.

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