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>