mnesia练习及基本操作

Mnesia基本用法

查看表结构

查看mnesia表的结构:

mnesia:info().

查看此表的基本信息:

mnesia:table_info(<tableName>, all).

Mnesia初使化

mnesia:stop(), mnesia:create_schema([node()]), mnesia:start().

创建表

mnesia:create_table(<tableName>, [{attributes, record_info(fields,<tableName>)}, {disc_copies, [node()]}]).

读表记录

读出表的所有key列表:

mnesia:dirty_all_keys(<tableName>).

根据key读表记录:

mnesia:dirty_read(<tableName>, <Key>).

写表记录

mnesia:dirty_write(<tableName>, {<tableName>, <ColumnValue1>, <ColumnValue2>, ...}).

删除表记录

mnesia:dirty_delete(<tableName>, <Key>).

其他实用操作

把表<table>的存储方式改成disc_copies:

mnesia:change_table_copy_type(dictionary, node(), disc_copies).

检测是否能ping通结点:

net_adm:ping('b@localhost').

查看内存使用情况:

memory().

ets表与mnesia ram表的区别： mnesia ram就是基于ets的；但mnesia ram支持事务，ets不支持.


-module(test_mnesia).

-compile(export_all).

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

%% 定义记录结构

-record(shop,{item,quantity,cost}).

-record(cost,{name,price}).

-record(design,{id,plan}).

start() ->

    mnesia:start(),

    %% 等待表的加载

    mnesia:wait_for_tables([shop,cost,design],20000).

%% 初始化mnesia表结构

init() ->

    mnesia:create_schema([node()]),

    mnesia:start(),

    %% 表创建 mnesia:create_table(TableName,[Args])

    %% {type,Type}  set,ordered_set,bag 表类型

    %% {ram_copies,NodeList} NodeList每个节点都有内存备份　默认为这个{ram_copies,[node()]}

    %% {disc_copies,NodeList} NodeList每个节点都有内存备份和磁盘备份

    %% {disc_only_copies,NodeList} NodeList每个节点有磁盘备份

    %% {attributes,AtomList} 要保存的列名称 一般和record有关　record_info(fields,RecordName)

    mnesia:create_table(shop,[{attributes,record_info(fields,shop)}]), %% 创建shop表

    mnesia:create_table(cost,[{attributes,record_info(fields,cost)}]),

    mnesia:create_table(design,[{attributes,record_info(fields,design)}]),

    mnesia:stop().

%% 加载测试数据

reset_tables() ->

    mnesia:clear_table(shop),

    mnesia:clear_table(cost),

    F = fun() ->

            lists:foreach(fun mnesia:write/1,example_tables())

    end,

    mnesia:transaction(F).

%% 测试数据

example_tables() ->

    [

        %% shop table

        {shop,apple,20,2.3},

        {shop,orange,100,3.8},

        {shop,pear,200,3.6},

        {shop,banana,420,4.5},

        {shop,potato,2456,1.2},

        %% cost table

        {cost,apple,1.5},

        {cost,orange,2.4},

        {cost,pear,2.2},

        {cost,banana,1.6},

        {cost,potato,0.6}

    ].

%%== 查询 =============================================================

do(Q) ->

    F = fun() -> qlc:e(Q) end,

    {atomic,Val} = mnesia:transaction(F),

    Val.

%% SELECT * FROM shop

%% 选取所有列

demo(select_shop) ->

    do(qlc:q([X || X <- mnesia:table(shop)]));

%% SELECT  item,quantity FROM shop

%% 选取指定列

demo(select_some) ->

    do(qlc:q([{X#shop.item, X#shop.quantity} || X <- mnesia:table(shop)]));

%% SELECT * FROM shop WHERE shop.quantity < 250

%% 选取指定条件的数据

demo(where) ->

    do(qlc:q([X || X <- mnesia:table(shop),

                X#shop.quantity < 250

            ]));

%% 关联查询

%% SELECT shop.* FROM shop,cost wHERE shop.item = cost.name AND cost.price < 2 AND shop.quantity < 250

demo(join) ->

    do(qlc:q([X || X <- mnesia:table(shop),

                   X#shop.quantity < 250,

                   Y <- mnesia:table(cost),

                   X#shop.item =:= Y#cost.name,

                   Y#cost.price < 2

            ])).

%% == 数据操作 ===============================================

%% 增加一行

add_shop_item(Name,Quantity,Cost) ->

    Row = #shop{item = Name,quantity = Quantity, cost = Cost},

    F = fun() ->

            mnesia:write(Row)

    end,

    mnesia:transaction(F).

%% 删除一行

remove_shop_item(Item) ->

    Oid = {shop,Item},

    F = fun() ->

            mnesia:delete(Oid)

    end,

    mnesia:transaction(F).

%% 取消一个事务

former(Nwant) ->

    F = fun() ->

            %% find the num of apples

            [Apple] = mnesia:read({shop,apple}),

            Napples = Apple#shop.quantity,

            %% update the database

            NewApple = Apple#shop{quantity = Napples + 2 * Nwant},

            mnesia:write(NewApple),

            %% find the num of oranges

            [Orange] = mnesia:read({shop,orange}),

            Noranges = Orange#shop.quantity,

            if

                Noranges >= Nwant ->

                    %% update the database

                    Num = Noranges - Nwant,

                    NewOrange = Orange#shop{quantity = Num},

                    mnesia:write(NewOrange);

                true ->

                    %% no enough oranges 取消事务

                    mnesia:abort(oranges)

            end

    end,

    mnesia:transaction(F).

%% 保存复杂数据

add_plans() ->

    D1 = #design{

        id = {joe,1},

        plan = {circle,10}

    },

    D2 = #design{

        id = fred,

        plan = {rectangle,[10,5]}

    },

    F = fun() ->

            mnesia:write(D1),

            mnesia:write(D2)

    end,

    mnesia:transaction(F).

%% 获复杂数据

get_plans(PlanId) ->

    F = fun() -> mnesia:read({design,PlanId}) end,

    mnesia:transaction(F).

Eshell V5.8.4  (abort with ^G)

1> c(test_mnesia).

{ok,test_mnesia}

2> test_mnesia:init().

stopped

=INFO REPORT==== 16-May-2012::17:24:29 ===

    application: mnesia

    exited: stopped

    type: temporary

3> test_mnesia:start().

ok

4> test_mnesia:reset_tables().

{atomic,ok}

5> test_mnesia:demo(select_shop).

[{shop,potato,2456,1.2},

 {shop,orange,100,3.8},

 {shop,apple,20,2.3},

 {shop,pear,200,3.6},

 {shop,banana,420,4.5}]

6> test_mnesia:demo(select_some).

[{potato,2456},

 {orange,100},

 {apple,20},

 {pear,200},

 {banana,420}]

7> test_mnesia:demo(where).

[{shop,orange,100,3.8},

 {shop,apple,20,2.3},

 {shop,pear,200,3.6}]

8> test_mnesia:demo(join).

[{shop,apple,20,2.3}]

9> test_mnesia:add_shop_item(apple,236,2.8).

{atomic,ok}

10> test_mnesia:demo(select_shop).

[{shop,potato,2456,1.2},

 {shop,orange,100,3.8},

 {shop,apple,236,2.8},

 {shop,pear,200,3.6},

 {shop,banana,420,4.5}]

11> test_mnesia:add_shop_item(egg,236,2.8).

{atomic,ok}

12> test_mnesia:demo(select_shop).

[{shop,potato,2456,1.2},

 {shop,egg,236,2.8},

 {shop,orange,100,3.8},

 {shop,apple,236,2.8},

 {shop,pear,200,3.6},

 {shop,banana,420,4.5}]

13> test_mnesia:delete_shop_item(egg).

** exception error: undefined function test_mnesia:delete_shop_item/1

14> test_mnesia:remove_shop_item(egg).

{atomic,ok}

15> test_mnesia:demo(select_shop).

[{shop,potato,2456,1.2},

 {shop,orange,100,3.8},

 {shop,apple,236,2.8},

 {shop,pear,200,3.6},

 {shop,banana,420,4.5}]

16> test_mnesia:former(50).

{atomic,ok}

17> test_mnesia:demo(select_shop).

[{shop,potato,2456,1.2},

 {shop,orange,50,3.8},

 {shop,apple,336,2.8},

 {shop,pear,200,3.6},

 {shop,banana,420,4.5}]

18> test_mnesia:former(100).

{aborted,oranges}

19> test_mnesia:demo(select_shop).

[{shop,potato,2456,1.2},

 {shop,orange,50,3.8},

 {shop,apple,336,2.8},

 {shop,pear,200,3.6},

 {shop,banana,420,4.5}]

20> test_mnesia:add_plans().

{atomic,ok}

21> test_mnesia:get_plans(a).

{atomic,[]}

22> test_mnesia:get_plans(fred).

{atomic,[{design,fred,{rectangle,[10,5]}}]}

23> test_mnesia:get_plans(joe).

{atomic,[]}

24>

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