erlang学习: Mnesia Erlang数据库2

Mnesia数据库增加与查询学习

-module(test_mnesia).-record(shop, {item, quantity, cost}).
-record(cost, {name, price}).
-record(design, {info, plan}).
%% API
-export([insert/3,select/1,start/0]).
start() ->mnesia:start().insert(Name, Quantity, Cost) ->Row = #shop{item = Name, quantity = Quantity, cost = Cost},F = fun() ->mnesia:write(Row)end,mnesia:transaction(F).select() ->qlc:q([X || X <- mnesia:table(shop)]).

能够插入但是不能查询

需要对do函数进行实现，才能展示出数据库中的数据,do函数在书上20.4.3有相应实现

-module(test_mnesia).
-include_lib("stdlib/include/qlc.hrl").-record(shop, {item, quantity, cost}).
%% API
-export([insert/3,select/0,start/0,do_this_once/0]).
start() ->mnesia:start().insert(Name, Quantity, Cost) ->Row = #shop{item = Name, quantity = Quantity, cost = Cost},io:format("inserting ~p~n", [Row]),F = fun() ->mnesia:write(Row)end,mnesia:transaction(F).select() ->do(qlc:q([X || X <- mnesia:table(shop)])).do(Q) ->F  = fun() ->qlc:e(Q)end,{atomic,Val} = mnesia:transaction(F),Val.

实现do函数后的代码运行截图