Mnesia用户手册:二,Mnesia快速上手
本章介绍了Mnesia:
1)启动一个Erlang session并制定Mnesia数据库的目录
2)初始化数据库结构
3)启动Mnesia并创建必要的表
1,初次启动Mnesia
以下是Mnesia系统启动的一个简单展示:
上面的例子里:
1)启动erl时参数-mnesia dir '"/tmp/funky"'指定了Mnesia存储数据的目录,windows下可以是erl -mnesia dir 'D:/erl/code'
2)mnesia:create_schema([node()])在本地节点上初始化一个空的schema
3)DBMS通过mnesia:start()启动
4)通过mnesia:create_table(funky, [])来创建表funky
5)mnesia:info()根据数据库的状态来显示信息
2,一个例子
Mnesia数据库组织为一个表的集合,一个表也包含一些属性,如location和persistence
在这个例子中:
1)启动一个Erlang系统,指定数据库位置目录
2)初始化一个新的schema,使用一个属性来指定数据库在那些节点上操作
3)启动Mnesia本身
4)创建数据库表
例子数据库
在这个数据库例子里,我们将创建如下数据库和关系,称为Company数据库
Company ER图:
数据库模型如下:
1)有三个实体:employee,project,department
2)这些实体间有三个关系:
i)一个department由一个employee管理,manager关系
ii)一个employee在一个department工作,at_dep关系
iii)每个employee对多个project工作,in_proj关系
定义结构和内容
我们首先将record定义输入到一个company.hrl文件,该文件定义了如下结构:
该结构在我们的数据库定义了6个表
在Mnesia里,mnesia:create_table(Name, ArgList)用来创建表
Name是表名,当前的Mnesia不要求表名和record名一样
例如,employee表的创建为mnesia:create_table(employee, [{attributes, record_info(fields, employee)}])
record_info(fields, RecordName)表达式由Erlang预处理程序处理为一个包含一个record的不同fields的list
程序
以下启动一个Mnesia并初始化company数据库的schema
以下程序模块创建前面定义的表:
程序的解释
以下命令用来初始化Company数据库:
1)% erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"',-mnesia dir Dir指定了数据库目录的位置
2)mnesia:create_schema([nocde()])初始化一个新的schema
3)mnesia:start()启动Mnesia
4)mnesia:create_table(Name, ArgList)用来创建必要的数据库表
下面继续:
company:init/0创建我们的表,有两个表类型是bag
bag表示one-to-many,set表示one-to-one
mnesia:info()现在显示数据库有7个本地表,其中6个我们定义的表,另一个是schema
6个事务被提交,因为创建表时6个事务被成功运行
插入一个employee record到数据库、一个at_dep record和一些in_proj record的例子:
insert_emp/3的参数为:
1)Emp是employee record
2)DeptId是department的id
3)ProjNames是project的name的list
insert_emp/3 -> function创建一个函数式对象,函数式对象由Fun来表示
mnesia:transaction(Fun)表示Fun位于一个事务中:
1)Fun要么完全成功要么完全失败
2)操作同样数据record的代码可以并行运行,不同的进程不会相互干扰
该方法可以这样使用:
初始化数据库内容
employee表的数据对应的tuple展现: {employee, 104732, klacke, 7, male, 98108, {221, 015}}
at_dep表的数据对应的tuple展现: {at_dep, klacke, 'B/SFR'}
in_proj表的数据对应的tuple展现: {in_proj, klacke, erlang, klacke, otp, klacke, mnesia}
Mnesia表由Mnesia record展现,如tuple{boss, klacke, bjarne}是一个record,第一个元素为表名,第二个为key
object identifier(oid)是指{Tab, Key}这个tuple,第一个元素为表名,第二个为key
一个oid可以对应0、1或多个record,这取决于表类型是set或bag
我们可以插入{boss, klacke, bjarne}record到数据库,Mnesia不强迫非得有klacke和bjarne这两个employeee的数据
添加记录和关系到数据库
employee
dept
project
manager
at_dep
in_proj
写查询语句
从DBMS里获取数据的方法为mnesia:read/3或mnesia:read/1:
由于我们希望在增加salary之后使用mnesia:write/1来更新record,所以我们在从table读数据时获得一个写lock(read方法的第三个参数)
有时候我们需要搜索多个表才能获取数据,这种查询比直接的mnesia:read开销要大很多
有两种方式来写数据库查询:
1) Mnesia方法
2) QLC
Mnesia方法
从数据库获取女性employee的名字:
select必须运行在transaction等activity里面,所有我们需要构造一个方法:
select表达式匹配employee表里所有的记录中sex为female的记录
该方法可以从shell里直接调用:
使用QLC
使用QLC可能比使用Mnesia方法开销更大,但是它提供了一个很好的语法
使用QLC list comprehension来访问Mnesia表时必须运行在一个transaction里:
调用QLC写的方法:
list comprehension表达式:
1)[括号表示“构建list”
2)||表示“例如”,<-表示“从哪里取”
上面的list comprehension表示:构建list E#employee.name,E来自employee表,并且sex属性等于female
raise female salary的方法:
1)启动一个Erlang session并制定Mnesia数据库的目录
2)初始化数据库结构
3)启动Mnesia并创建必要的表
1,初次启动Mnesia
以下是Mnesia系统启动的一个简单展示:
- unix> erl -mnesia dir '"/tmp/funky"'
- Erlang (BEAM) emulator version 4.9
- Eshell V4.9 (abort with ^G)
- 1>
- 1> mnesia:create_schema([node()]).
- ok
- 2> mnesia:start().
- ok
- 3> mnesia:create_table(funky, []).
- {atomic,ok}
- 4> mnesia:info().
- ---> Processes holding locks <---
- ---> Processes waiting for locks <---
- ---> Pending (remote) transactions <---
- ---> Active (local) transactions <---
- ---> Uncertain transactions <---
- ---> Active tables <---
- funky : with 0 records occupying 269 words of mem
- schema : with 2 records occupying 353 words of mem
- ===> System info in version "1.0", debug level = none <===
- opt disc. Directory "/tmp/funky" is used.
- use fall-back at restart = false
- running db nodes = [nonode@nohost]
- stopped db nodes = []
- remote = []
- ram copies = [funky]
- disc copies = [schema]
- disc only copies = []
- [fnonode@nohost,disc copiesg] = [schema]
- [fnonode@nohost,ram copiesg] = [funky]
- 1 transactions committed, 0 aborted, 0 restarted, 1 logged to disc
- 0 held locks, 0 in queue; 0 local transactions, 0 remote
- 0 transactions waits for other nodes: []
- ok
unix> erl -mnesia dir '"/tmp/funky"' Erlang (BEAM) emulator version 4.9 Eshell V4.9 (abort with ^G) 1> 1> mnesia:create_schema([node()]). ok 2> mnesia:start(). ok 3> mnesia:create_table(funky, []). {atomic,ok} 4> mnesia:info(). ---> Processes holding locks <--- ---> Processes waiting for locks <--- ---> Pending (remote) transactions <--- ---> Active (local) transactions <--- ---> Uncertain transactions <--- ---> Active tables <--- funky : with 0 records occupying 269 words of mem schema : with 2 records occupying 353 words of mem ===> System info in version "1.0", debug level = none <=== opt disc. Directory "/tmp/funky" is used. use fall-back at restart = false running db nodes = [nonode@nohost] stopped db nodes = [] remote = [] ram copies = [funky] disc copies = [schema] disc only copies = [] [fnonode@nohost,disc copiesg] = [schema] [fnonode@nohost,ram copiesg] = [funky] 1 transactions committed, 0 aborted, 0 restarted, 1 logged to disc 0 held locks, 0 in queue; 0 local transactions, 0 remote 0 transactions waits for other nodes: [] ok
上面的例子里:
1)启动erl时参数-mnesia dir '"/tmp/funky"'指定了Mnesia存储数据的目录,windows下可以是erl -mnesia dir 'D:/erl/code'
2)mnesia:create_schema([node()])在本地节点上初始化一个空的schema
3)DBMS通过mnesia:start()启动
4)通过mnesia:create_table(funky, [])来创建表funky
5)mnesia:info()根据数据库的状态来显示信息
2,一个例子
Mnesia数据库组织为一个表的集合,一个表也包含一些属性,如location和persistence
在这个例子中:
1)启动一个Erlang系统,指定数据库位置目录
2)初始化一个新的schema,使用一个属性来指定数据库在那些节点上操作
3)启动Mnesia本身
4)创建数据库表
例子数据库
在这个数据库例子里,我们将创建如下数据库和关系,称为Company数据库
Company ER图:
- id name emp_no name salary sex phone room_no name number
- \ / \ / \ /
- Dept - Manager - Employee - In_proj - Project
- \ /
- At_dep
id name emp_no name salary sex phone room_no name number \ / \ / \ / Dept - Manager - Employee - In_proj - Project \ / At_dep
数据库模型如下:
1)有三个实体:employee,project,department
2)这些实体间有三个关系:
i)一个department由一个employee管理,manager关系
ii)一个employee在一个department工作,at_dep关系
iii)每个employee对多个project工作,in_proj关系
定义结构和内容
我们首先将record定义输入到一个company.hrl文件,该文件定义了如下结构:
- -record(employee, {emp_no,
- name,
- salary,
- sex,
- phone,
- room_no}).
- -record(dept, {id,
- name}).
- -record(project, {name,
- number}).
- -record(manager, {emp,
- dept}).
- -record(at_dep, {emp,
- dept_id}).
- -record(in_proj, {emp,
- proj_name}).
-record(employee, {emp_no, name, salary, sex, phone, room_no}). -record(dept, {id, name}). -record(project, {name, number}). -record(manager, {emp, dept}). -record(at_dep, {emp, dept_id}). -record(in_proj, {emp, proj_name}).
该结构在我们的数据库定义了6个表
在Mnesia里,mnesia:create_table(Name, ArgList)用来创建表
Name是表名,当前的Mnesia不要求表名和record名一样
例如,employee表的创建为mnesia:create_table(employee, [{attributes, record_info(fields, employee)}])
record_info(fields, RecordName)表达式由Erlang预处理程序处理为一个包含一个record的不同fields的list
程序
以下启动一个Mnesia并初始化company数据库的schema
- % erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"'
- Erlang (BEAM) emulator version 4.9
- Eshell V4.9 (abort with ^G)
- 1> mnesia:create_schema([node()]).
- ok
- 2) mnesia:start().
- ok
% erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"' Erlang (BEAM) emulator version 4.9 Eshell V4.9 (abort with ^G) 1> mnesia:create_schema([node()]). ok 2) mnesia:start(). ok
以下程序模块创建前面定义的表:
- -include_lib("stdlib/include/qlc.hrl").
- -include("company.hrl").
- init() ->
- mnesia:create_table(employee, [{attributes, record_info(fields, employee)}]),
- mnesia:create_table(dept, [{attributes, record_info(fields, dept)}]),
- mnesia:create_table(project, [{attributes, record_info(fields, project)}]),
- mnesia:create_table(manager, [{type, bag}, {attributes, record_info(fields, manager)}]),
- mnesia:create_table(at_dep, [{type, set}, {attributes, record_info(fields, at_dep)}]),
- mnesia:create_table(in_proj, [{type, bag}, {attributes, record_info(fields, in_proj)}]),
-include_lib("stdlib/include/qlc.hrl"). -include("company.hrl"). init() -> mnesia:create_table(employee, [{attributes, record_info(fields, employee)}]), mnesia:create_table(dept, [{attributes, record_info(fields, dept)}]), mnesia:create_table(project, [{attributes, record_info(fields, project)}]), mnesia:create_table(manager, [{type, bag}, {attributes, record_info(fields, manager)}]), mnesia:create_table(at_dep, [{type, set}, {attributes, record_info(fields, at_dep)}]), mnesia:create_table(in_proj, [{type, bag}, {attributes, record_info(fields, in_proj)}]),
程序的解释
以下命令用来初始化Company数据库:
1)% erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"',-mnesia dir Dir指定了数据库目录的位置
2)mnesia:create_schema([nocde()])初始化一个新的schema
3)mnesia:start()启动Mnesia
4)mnesia:create_table(Name, ArgList)用来创建必要的数据库表
下面继续:
- 3> company:init().
- {atomic,ok}
- 4> mnesia:info().
- nsactions <---
- ---> Active tables <---
- in proj : with 0 records occuping 269 words of mem
- at dep : with 0 records occuping 269 words of mem
- manager : with 0 records occuping 269 words of mem
- project : with 0 records occuping 269 words of mem
- dept : with 0 records occuping 269 words of mem
- employee : with 0 records occuping 269 words of mem
- schema : with 7 records occuping 571 words of mem
- ===> System info in version "1.0", debug level = none <===
- opt disc. Directory "/ldisc/scratch/Mnesia.Company" is used.
- use fall-back at restart = false
- running db nodes = [nonode@nohost]
- stopped db nodes = []
- remote = []
- ram copies =
- [at dep,dept,employee,in proj,manager,project]
- disc copies = [schema]
- disc only copies = []
- [fnonode@nohost,disc copiesg] = [schema]
- [fnonode@nohost,ram copiesg] =
- [employee,dept,project,manager,at dep,in proj]
- 6 transactions committed, 0 aborted, 0 restarted, 6 logged to disc
- 0 held locks, 0 in queue; 0 local transactions, 0 remote
- 0 transactions waits for other nodes: []
- ok
3> company:init(). {atomic,ok} 4> mnesia:info(). nsactions <--- ---> Active tables <--- in proj : with 0 records occuping 269 words of mem at dep : with 0 records occuping 269 words of mem manager : with 0 records occuping 269 words of mem project : with 0 records occuping 269 words of mem dept : with 0 records occuping 269 words of mem employee : with 0 records occuping 269 words of mem schema : with 7 records occuping 571 words of mem ===> System info in version "1.0", debug level = none <=== opt disc. Directory "/ldisc/scratch/Mnesia.Company" is used. use fall-back at restart = false running db nodes = [nonode@nohost] stopped db nodes = [] remote = [] ram copies = [at dep,dept,employee,in proj,manager,project] disc copies = [schema] disc only copies = [] [fnonode@nohost,disc copiesg] = [schema] [fnonode@nohost,ram copiesg] = [employee,dept,project,manager,at dep,in proj] 6 transactions committed, 0 aborted, 0 restarted, 6 logged to disc 0 held locks, 0 in queue; 0 local transactions, 0 remote 0 transactions waits for other nodes: [] ok
company:init/0创建我们的表,有两个表类型是bag
bag表示one-to-many,set表示one-to-one
mnesia:info()现在显示数据库有7个本地表,其中6个我们定义的表,另一个是schema
6个事务被提交,因为创建表时6个事务被成功运行
插入一个employee record到数据库、一个at_dep record和一些in_proj record的例子:
- insert_emp(Emp, DeptId, ProjNames) ->
- Ename = Emp#employee.name,
- Fun = fun() ->
- mnesia:write(Emp),
- AtDep = #at_dep{emp = Ename, dept_id = DeptId},
- mnesia:write(AtDep),
- mk_projs(Ename, ProjNames)
- end,
- mnesia:transaction(Fun).
- mk_projs(Ename, [ProjName|Tail]) ->
- mnesia:write(#in_proj{emp = Ename, proj_name = ProjName}),
- mk_projs(Ename, Tail);
- mk_projs(_, []) -> ok.
insert_emp(Emp, DeptId, ProjNames) -> Ename = Emp#employee.name, Fun = fun() -> mnesia:write(Emp), AtDep = #at_dep{emp = Ename, dept_id = DeptId}, mnesia:write(AtDep), mk_projs(Ename, ProjNames) end, mnesia:transaction(Fun). mk_projs(Ename, [ProjName|Tail]) -> mnesia:write(#in_proj{emp = Ename, proj_name = ProjName}), mk_projs(Ename, Tail); mk_projs(_, []) -> ok.
insert_emp/3的参数为:
1)Emp是employee record
2)DeptId是department的id
3)ProjNames是project的name的list
insert_emp/3 -> function创建一个函数式对象,函数式对象由Fun来表示
mnesia:transaction(Fun)表示Fun位于一个事务中:
1)Fun要么完全成功要么完全失败
2)操作同样数据record的代码可以并行运行,不同的进程不会相互干扰
该方法可以这样使用:
- Me = #employee{emp_no= 104732,
- name = klacke,
- salary = 7,
- sex = male,
- phone = 98108,
- room_no = {221, 015}},
- insert_emp(Me, 'B/SFR', [erlang, mnesia, otp]).
Me = #employee{emp_no= 104732, name = klacke, salary = 7, sex = male, phone = 98108, room_no = {221, 015}}, insert_emp(Me, 'B/SFR', [erlang, mnesia, otp]).
初始化数据库内容
employee表的数据对应的tuple展现: {employee, 104732, klacke, 7, male, 98108, {221, 015}}
at_dep表的数据对应的tuple展现: {at_dep, klacke, 'B/SFR'}
in_proj表的数据对应的tuple展现: {in_proj, klacke, erlang, klacke, otp, klacke, mnesia}
Mnesia表由Mnesia record展现,如tuple{boss, klacke, bjarne}是一个record,第一个元素为表名,第二个为key
object identifier(oid)是指{Tab, Key}这个tuple,第一个元素为表名,第二个为key
一个oid可以对应0、1或多个record,这取决于表类型是set或bag
我们可以插入{boss, klacke, bjarne}record到数据库,Mnesia不强迫非得有klacke和bjarne这两个employeee的数据
添加记录和关系到数据库
employee
- {employee, 104465, "Johnson Torbjorn", 1, male, 99184, {242,038}}.
- {employee, 107912, "Carlsson Tuula", 2, female,94556, {242,056}}.
- {employee, 114872, "Dacker Bjarne", 3, male, 99415, {221,035}}.
- {employee, 104531, "Nilsson Hans", 3, male, 99495, {222,026}}.
- {employee, 104659, "Tornkvist Torbjorn", 2, male, 99514, {222,022}}.
- {employee, 104732, "Wikstrom Claes", 2, male, 99586, {221,015}}.
- {employee, 117716, "Fedoriw Anna", 1, female,99143, {221,031}}.
- {employee, 115018, "Mattsson Hakan", 3, male, 99251, {203,348}}.
{employee, 104465, "Johnson Torbjorn", 1, male, 99184, {242,038}}. {employee, 107912, "Carlsson Tuula", 2, female,94556, {242,056}}. {employee, 114872, "Dacker Bjarne", 3, male, 99415, {221,035}}. {employee, 104531, "Nilsson Hans", 3, male, 99495, {222,026}}. {employee, 104659, "Tornkvist Torbjorn", 2, male, 99514, {222,022}}. {employee, 104732, "Wikstrom Claes", 2, male, 99586, {221,015}}. {employee, 117716, "Fedoriw Anna", 1, female,99143, {221,031}}. {employee, 115018, "Mattsson Hakan", 3, male, 99251, {203,348}}.
dept
- {dept, 'B/SF', "Open Telecom Platform"}.
- {dept, 'B/SFP', "OTP - Product Development"}.
- {dept, 'B/SFR', "Computer Science Laboratory"}.
{dept, 'B/SF', "Open Telecom Platform"}. {dept, 'B/SFP', "OTP - Product Development"}. {dept, 'B/SFR', "Computer Science Laboratory"}.
project
- {project, erlang, 1}.
- {project, otp, 2}.
- {project, beam, 3}.
- {project, mnesia, 5}.
- {project, wolf, 6}.
- {project, documentation, 7}.
- {project, www, 8}.
{project, erlang, 1}. {project, otp, 2}. {project, beam, 3}. {project, mnesia, 5}. {project, wolf, 6}. {project, documentation, 7}. {project, www, 8}.
manager
- {manager, 104465, 'B/SF'}.
- {manager, 104465, 'B/SFP'}.
- {manager, 114872, 'B/SFR'}.
{manager, 104465, 'B/SF'}. {manager, 104465, 'B/SFP'}. {manager, 114872, 'B/SFR'}.
at_dep
- {at_dep, 104465, 'B/SF'}.
- {at_dep, 107912, 'B/SF'}.
- {at_dep, 114872, 'B/SFR'}.
- {at_dep, 104531, 'B/SFR'}.
- {at_dep, 104659, 'B/SFR'}.
- {at_dep, 104732, 'B/SFR'}.
- {at_dep, 117716, 'B/SFP'}.
- {at_dep, 115018, 'B/SFP'}.
{at_dep, 104465, 'B/SF'}. {at_dep, 107912, 'B/SF'}. {at_dep, 114872, 'B/SFR'}. {at_dep, 104531, 'B/SFR'}. {at_dep, 104659, 'B/SFR'}. {at_dep, 104732, 'B/SFR'}. {at_dep, 117716, 'B/SFP'}. {at_dep, 115018, 'B/SFP'}.
in_proj
- {in_proj, 104465, otp}.
- {in_proj, 107912, otp}.
- {in_proj, 114872, otp}.
- {in_proj, 104531, otp}.
- {in_proj, 104531, mnesia}.
- {in_proj, 104545, wolf}.
- {in_proj, 104659, otp}.
- {in_proj, 104659, wolf}.
- {in_proj, 104732, otp}.
- {in_proj, 104732, mnesia}.
- {in_proj, 104732, erlang}.
- {in_proj, 117716, otp}.
- {in_proj, 117716, documentation}.
- {in_proj, 115018, otp}.
- {in_proj, 115018, mnesia}.
{in_proj, 104465, otp}. {in_proj, 107912, otp}. {in_proj, 114872, otp}. {in_proj, 104531, otp}. {in_proj, 104531, mnesia}. {in_proj, 104545, wolf}. {in_proj, 104659, otp}. {in_proj, 104659, wolf}. {in_proj, 104732, otp}. {in_proj, 104732, mnesia}. {in_proj, 104732, erlang}. {in_proj, 117716, otp}. {in_proj, 117716, documentation}. {in_proj, 115018, otp}. {in_proj, 115018, mnesia}.
写查询语句
从DBMS里获取数据的方法为mnesia:read/3或mnesia:read/1:
- raise(Eno, Raise) ->
- F = fun() ->
- [E] = mnesia:read(employee, Eno, write),
- Salary = E#employee.salary + Raise,
- New = E#employee{salary = Salary},
- mnesia:write(New)
- end,
- mnesia:transaction(F).
raise(Eno, Raise) -> F = fun() -> [E] = mnesia:read(employee, Eno, write), Salary = E#employee.salary + Raise, New = E#employee{salary = Salary}, mnesia:write(New) end, mnesia:transaction(F).
由于我们希望在增加salary之后使用mnesia:write/1来更新record,所以我们在从table读数据时获得一个写lock(read方法的第三个参数)
有时候我们需要搜索多个表才能获取数据,这种查询比直接的mnesia:read开销要大很多
有两种方式来写数据库查询:
1) Mnesia方法
2) QLC
Mnesia方法
从数据库获取女性employee的名字:
- mnesia:select(employee, [{#employee{sex = female, name = '$1', _ = '_'}, [], ['$1']}]).
mnesia:select(employee, [{#employee{sex = female, name = '$1', _ = '_'}, [], ['$1']}]).
select必须运行在transaction等activity里面,所有我们需要构造一个方法:
- all_females() ->
- F = fun() ->
- Female = #employee{sex = female, name = '$1', _ = '_'},
- mnesia:select(employee, [{Female, [], ['$1']}])
- end,
- mnesia:transaction(F).
all_females() -> F = fun() -> Female = #employee{sex = female, name = '$1', _ = '_'}, mnesia:select(employee, [{Female, [], ['$1']}]) end, mnesia:transaction(F).
select表达式匹配employee表里所有的记录中sex为female的记录
该方法可以从shell里直接调用:
- 1> company:all_females().
- {atomic, ["Carlsson Tuula", "Fedoriw Anna"]}
1> company:all_females(). {atomic, ["Carlsson Tuula", "Fedoriw Anna"]}
使用QLC
使用QLC可能比使用Mnesia方法开销更大,但是它提供了一个很好的语法
- Q = qlc:q([E#employee.name || E <- mnesia:table(employee), E#employee.sex == female]),
- qlc:e(Q).
Q = qlc:q([E#employee.name || E <- mnesia:table(employee), E#employee.sex == female]), qlc:e(Q).
使用QLC list comprehension来访问Mnesia表时必须运行在一个transaction里:
- females() ->
- F = fun() ->
- Q = qlc:q([E#employee.name || E <- mnesia:table(employee), E#employee.sex == female]),
- qlc:e(Q)
- end,
- mnesia:transaction(F).
females() -> F = fun() -> Q = qlc:q([E#employee.name || E <- mnesia:table(employee), E#employee.sex == female]), qlc:e(Q) end, mnesia:transaction(F).
调用QLC写的方法:
- company:females().
company:females().
list comprehension表达式:
1)[括号表示“构建list”
2)||表示“例如”,<-表示“从哪里取”
上面的list comprehension表示:构建list E#employee.name,E来自employee表,并且sex属性等于female
raise female salary的方法:
- raise_females(Amount) ->
- F = fun() ->
- Q = qlc:q([E || E <- mnesia:table(employee),
- E#employee.sex == female]),
- Fs = qlc:e(Q),
- over_write(Fs, Amount)
- end,
- mnesia:transaction(F).
- over_write([E|Tail], Amount) ->
- Salary = E#employee.salary + Amount,
- New = E#employee{salary = Salary},
- mnesia:write(New),
- 1 + over_write(Tail, Amount);
- over_write([], _) ->
- 0.
raise_females(Amount) -> F = fun() -> Q = qlc:q([E || E <- mnesia:table(employee), E#employee.sex == female]), Fs = qlc:e(Q), over_write(Fs, Amount) end, mnesia:transaction(F). over_write([E|Tail], Amount) -> Salary = E#employee.salary + Amount, New = E#employee{salary = Salary}, mnesia:write(New), 1 + over_write(Tail, Amount); over_write([], _) -> 0.