diff options
Diffstat (limited to 'lib/mnesia/doc/src/Mnesia_chap2.xmlsrc')
| -rw-r--r-- | lib/mnesia/doc/src/Mnesia_chap2.xmlsrc | 559 |
1 files changed, 253 insertions, 306 deletions
diff --git a/lib/mnesia/doc/src/Mnesia_chap2.xmlsrc b/lib/mnesia/doc/src/Mnesia_chap2.xmlsrc index f464135a89..14999228a8 100644 --- a/lib/mnesia/doc/src/Mnesia_chap2.xmlsrc +++ b/lib/mnesia/doc/src/Mnesia_chap2.xmlsrc @@ -21,7 +21,7 @@ </legalnotice> - <title>Getting Started with Mnesia</title> + <title>Getting Started</title> <prepared>Claes Wikström, Hans Nilsson and Håkan Mattsson</prepared> <responsible></responsible> <docno></docno> @@ -31,27 +31,31 @@ <rev>C</rev> <file>Mnesia_chap2.xml</file> </header> - <p>This chapter introduces Mnesia. Following a brief discussion - about the first initial setup, a Mnesia database example is - demonstrated. This database example will be referenced in the - following chapters, where this example is modified in order to - illustrate various program constructs. In this chapter, the - following mandatory procedures are illustrated by examples: - </p> + <marker id="getting_started"></marker> + <p>This section introduces <c>Mnesia</c> with an example database. + This example is referenced in the + following sections, where the example is modified to + illustrate various program constructs. This section illustrates + the following mandatory procedures through examples:</p> <list type="bulleted"> - <item>Starting an Erlang session and specifying a directory for the - Mnesia database. + <item>Starting the Erlang session. </item> - <item>Initializing a database schema. + <item>Specifying the <c>Mnesia</c> directory where the database + is to be stored. + </item> + <item>Initializing a new database schema with an attribute that + specifies on which node, or nodes, that database is to operate. + </item> + <item>Starting <c>Mnesia</c>. + </item> + <item>Creating and populating the database tables. </item> - <item>Starting Mnesia and creating the required tables.</item> </list> <section> - <title>Starting Mnesia for the first time</title> - <p>Following is a simplified demonstration of a Mnesia system startup. This is the dialogue from the Erlang - shell: - </p> + <title>Starting Mnesia for the First Time</title> + <p>This section provides a simplified demonstration of a <c>Mnesia</c> + system startup. The dialogue from the Erlang shell is as follows:</p> <pre><![CDATA[ unix> erl -mnesia dir '"/tmp/funky"' Erlang (BEAM) emulator version 4.9 @@ -89,79 +93,62 @@ 0 transactions waits for other nodes: [] ok ]]></pre> - <p>In the example above the following actions were performed: - </p> + <p>In this example, the following actions are performed:</p> <list type="bulleted"> - <item>The Erlang system was started from the UNIX prompt - with a flag <c>-mnesia dir '"/tmp/funky"'</c>. This flag indicates - to Mnesia which directory will store the data. + <item><em>Step 1:</em> The Erlang system is started from the UNIX + prompt with a flag <c>-mnesia dir '"/tmp/funky"'</c>, which indicates + in which directory to store the data. </item> - <item>A new empty schema was initialized on the local node by evaluating - <c>mnesia:create_schema([node()]).</c> The schema contains - information about the database in general. This will be - thoroughly explained later on. + <item><em>Step 2:</em> A new empty schema is initialized on the local + node by evaluating + <seealso marker="mnesia#create_schema/1">mnesia:create_schema([node()])</seealso>. + The schema contains information about the database in general. + This is explained in detail later. </item> - <item>The DBMS was started by evaluating <c>mnesia:start()</c>. + <item><em>Step 3:</em> The DBMS is started by evaluating + <seealso marker="mnesia#start/0">mnesia:start()</seealso>. </item> - <item>A first table was created, called <c>funky</c> by evaluating - the expression <c>mnesia:create_table(funky, [])</c>. The table - was given default properties. + <item><em>Step 4:</em> A first table is created, called <c>funky</c>, + by evaluating the expression <c>mnesia:create_table(funky, [])</c>. + The table is given default properties. </item> - <item><c>mnesia:info()</c> was evaluated and subsequently displayed - information regarding the status of the database on the terminal. + <item><em>Step 5:</em> <seealso marker="mnesia#info/0">mnesia:info()</seealso> + is evaluated to + display information on the terminal about the status of the database. </item> </list> </section> <section> - <title>An Introductory Example</title> - <p>A Mnesia database is organized as a set of tables. + <title>Example</title> + <p>A <c>Mnesia</c> database is organized as a set of tables. Each table is populated with instances (Erlang records). - A table also has a number of properties, such as location and - persistence. - </p> - <p>In this example we shall: - </p> - <list type="bulleted"> - <item>Start an Erlang system, and specify the directory where - the database will be located. - </item> - <item>Initiate a new schema with an attribute that specifies - on which node, or nodes, the database will operate. - </item> - <item>Start Mnesia itself. - </item> - <item>Create and populate the database tables. - </item> - </list> + A table has also a number of properties, such as location and + persistence.</p> <section> - <title>The Example Database</title> + <title>Database</title> </section> - <p>In this database example, we will create the database and - relationships depicted in the following diagram. We will call this - database the <em>Company</em> database. - </p> + <p>This example shows how to create a database called <c>Company</c> + and the relationships shown in the following diagram:</p> <image file="company.gif"> <icaption>Company Entity-Relation Diagram</icaption> </image> - <p>The database model looks as follows: - </p> + <p>The database model is as follows:</p> <list type="bulleted"> - <item>There are three entities: employee, project, and - department. + <item>There are three entities: department, employee, and project. </item> <item> <p>There are three relationships between these entities:</p> <list type="ordered"> - <item>A department is managed by an employee, hence the - <em>manager</em> relationship. + <item>A department is managed by an employee, + hence the <c>manager</c> relationship. </item> <item>An employee works at a department, hence the - <em>at_dep</em> relationship. + <c>at_dep</c> relationship. </item> - <item>Each employee works on a number of projects, hence - the <em>in_proj</em> relationship. + <item>Each employee works on a number of projects, + hence the <c>in_proj</c> relationship. </item> </list> </item> @@ -169,35 +156,32 @@ <section> <title>Defining Structure and Content</title> - <p>We first enter our record definitions into a text file + <p>First the record definitions are entered into a text file named <c>company.hrl</c>. This file defines the following - structure for our sample database: - </p> + structure for the example database: </p> <codeinclude file="company.hrl" tag="%0" type="erl"></codeinclude> - <p>The structure defines six tables in our database. In Mnesia, - the function <c>mnesia:create_table(Name, ArgList)</c> is - used to create tables. <c>Name</c> is the table - name <em>Note:</em> The current version of Mnesia does - not require that the name of the table is the same as the record - name, See Chapter 4: - <seealso marker="Mnesia_chap4#recordnames_tablenames">Record Names Versus Table Names.</seealso></p> - <p>For example, the table - for employees will be created with the function - <c>mnesia:create_table(employee, [{attributes, record_info(fields, employee)}]).</c> The table + <p>The structure defines six tables in the database. In <c>Mnesia</c>, + the function + <seealso marker="mnesia#create_table/2">mnesia:create_table(Name, ArgList)</seealso> + creates tables. <c>Name</c> is the table name.</p> + <note><p>The current version of <c>Mnesia</c> does not require that + the name of the table is the same as the record name, see + <seealso marker="Mnesia_chap4#recordnames_tablenames">Record Names versus Table Names.</seealso>.</p></note> + <p>For example, the table for employees is created with the + function <c>mnesia:create_table(employee, + [{attributes, record_info(fields, employee)}])</c>. The table name <c>employee</c> matches the name for records specified - in <c>ArgList</c>. The expression <c>record_info(fields, RecordName)</c> is processed by the Erlang preprocessor and - evaluates to a list containing the names of the different - fields for a record. - </p> + in <c>ArgList</c>. The expression + <c>record_info(fields, RecordName)</c> is processed by the Erlang + preprocessor and evaluates to a list containing the names of the + different fields for a record.</p> </section> <section> - <title>The Program</title> - <p>The following shell interaction starts Mnesia and - initializes the schema for our <c>company</c> database: - </p> + <title>Program</title> + <p>The following shell interaction starts <c>Mnesia</c> and + initializes the schema for the <c>Company</c> database:</p> <pre> - % <input>erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"'</input> Erlang (BEAM) emulator version 4.9 @@ -205,37 +189,39 @@ 1> mnesia:create_schema([node()]). ok 2> mnesia:start(). - ok - </pre> - <p>The following program module creates and populates previously defined tables: - </p> + ok</pre> + <p>The following program module creates and populates previously + defined tables:</p> <codeinclude file="company.erl" tag="%0" type="erl"></codeinclude> </section> <section> - <title>The Program Explained</title> - <p>The following commands and functions were used to initiate the - Company database: - </p> + <title>Program Explained</title> + <p>The following commands and functions are used to initiate the + <c>Company</c> database:</p> <list type="bulleted"> - <item><c>% erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"'.</c> This is a UNIX - command line entry which starts the Erlang system. The flag - <c>-mnesia dir Dir</c> specifies the location of the + <item><c>% erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"'</c>. + This is a UNIX + command-line entry that starts the Erlang system. The flag + <c>-mnesia dir Dir</c> specifies the location of the database directory. The system responds and waits for - further input with the prompt <em>1></em>. + further input with the prompt <c>1></c>. </item> - <item><c>mnesia:create_schema([node()]).</c> This function - has the format <c>mnesia:create_schema(DiscNodeList)</c> and - initiates a new schema. In this example, we have created a - non-distributed system using only one node. Schemas are fully - explained in Chapter 3:<seealso marker="Mnesia_chap3#def_schema">Defining a Schema</seealso>. + <item> + <seealso marker="mnesia#create_schema/1">mnesia:create_schema([node()])</seealso>. + This function + has the format <c>mnesia:create_schema(DiscNodeList)</c> and + initiates a new schema. In this example, a non-distributed system + using only one node is created. Schemas are fully explained in + <seealso marker="Mnesia_chap3#def_schema">Define a Schema</seealso>. </item> - <item><c>mnesia:start().</c> This function starts - Mnesia. This function is fully explained in Chapter 3: - <seealso marker="Mnesia_chap3#start_mnesia">Starting Mnesia</seealso>. + <item><seealso marker="mnesia#start/0">mnesia:start()</seealso>. + This function starts <c>Mnesia</c> and is fully explained in + <seealso marker="Mnesia_chap3#start_mnesia">Start Mnesia</seealso>. </item> </list> - <p>Continuing the dialogue with the Erlang shell will produce the following:</p> + <p>Continuing the dialogue with the Erlang shell produces the + following:</p> <pre><![CDATA[ 3> company:init(). {atomic,ok} @@ -271,63 +257,58 @@ 0 transactions waits for other nodes: [] ok ]]></pre> - <p>A set of tables is created: - </p> - <list type="bulleted"> - <item><c>mnesia:create_table(Name,ArgList)</c>. This - function is used to create the required database tables. The - options available with <c>ArgList</c> are explained in - Chapter 3: <seealso marker="Mnesia_chap3#create_tables">Creating New Tables</seealso>. </item> - </list> - <p>The <c>company:init/0</c> function creates our tables. Two tables are - of type <c>bag</c>. This is the <c>manager</c> relation as well - the <c>in_proj</c> relation. This shall be interpreted as: An + <p>A set of tables is created. The function + <seealso marker="mnesia#create_table/2">mnesia:create_table(Name, ArgList)</seealso> + creates the required database tables. The + options available with <c>ArgList</c> are explained in + <seealso marker="Mnesia_chap3#create_tables">Create New Tables</seealso>.</p> + <p>The function <c>company:init/0</c> creates the tables. Two tables + are of type <c>bag</c>. This is the <c>manager</c> relation as well + the <c>in_proj</c> relation. This is interpreted as: an employee can be manager over several departments, and an employee can participate in several projects. However, the <c>at_dep</c> - relation is <c>set</c> because an employee can only work in one department. - In this data model we have examples of relations that are one-to-one (<c>set</c>), - as well as one-to-many (<c>bag</c>). - </p> - <p><c>mnesia:info()</c> now indicates that a database - which has seven local tables, of which, six are our - user defined tables and one is the schema. - Six transactions have been committed, as six successful transactions were run when - creating the tables. - </p> - <p>To write a function which inserts an employee record into the database, there must be an - <c>at_dep</c> record and a set of <c>in_proj</c> records inserted. Examine the following - code used to complete this action: - </p> - <codeinclude file="company.erl" tag="%1" type="erl"></codeinclude> + relation is <c>set</c>, as an employee can only work in one department. + In this data model, there are examples of relations that are 1-to-1 + (<c>set</c>) and 1-to-many (<c>bag</c>).</p> + <p><seealso marker="mnesia#info/0">mnesia:info()</seealso> + now indicates that a database has seven + local tables, where six are the user-defined tables and one is + the schema. Six transactions have been committed, as six successful + transactions were run when creating the tables.</p> + <p>To write a function that inserts an employee record into the + database, there must be an <c>at_dep</c> record and a set of + <c>in_proj</c> records inserted. Examine the following + code used to complete this action:</p> + <codeinclude file="company.erl" tag="%1" type="erl"></codeinclude> <list type="bulleted"> <item> - <p><c>insert_emp(Emp, DeptId, ProjNames) -></c>. The - <c>insert_emp/3</c> arguments are:</p> + <p>The <c>insert_emp/3</c> arguments are as follows:</p> <list type="ordered"> <item><c>Emp</c> is an employee record. </item> - <item><c>DeptId</c> is the identity of the department where the employee is working. + <item><c>DeptId</c> is the identity of the department where + the employee works. </item> - <item><c>ProjNames</c> is a list of the names of the projects where the employee are working.</item> + <item><c>ProjNames</c> is a list of the names of the projects + where the employee works.</item> </list> </item> </list> - <p>The <c>insert_emp(Emp, DeptId, ProjNames) -></c> function - creates a <em>functional object</em>. Functional objects - are identified by the term <c>Fun</c>. The Fun is passed + <p>The function <c>insert_emp/3</c> creates a Functional Object (Fun). + <c>Fun</c> is passed as a single argument to the function - <c>mnesia:transaction(Fun)</c>. This means that Fun is - run as a transaction with the following properties: - </p> + <seealso marker="mnesia#transaction/2">mnesia:transaction(Fun)</seealso>. + This means that <c>Fun</c> is + run as a transaction with the following properties:</p> <list type="bulleted"> - <item>Fun either succeeds or fails completely. + <item>A <c>Fun</c> either succeeds or fails. </item> - <item>Code which manipulates the same data records can be + <item>Code that manipulates the same data records can be run concurrently without the different processes interfering with each other. </item> </list> - <p>The function can be used as:</p> + <p>The function can be used as follows:</p> <code type="none"> Emp = #employee{emp_no= 104732, name = klacke, @@ -335,20 +316,17 @@ sex = male, phone = 98108, room_no = {221, 015}}, - insert_emp(Me, 'B/SFR', [Erlang, mnesia, otp]). - </code> - <note> - <p>Functional Objects (Funs) are described in the - Erlang Reference Manual, "Fun Expressions". - </p> + insert_emp(Me, 'B/SFR', [Erlang, mnesia, otp]).</code> + <note><p>For information about Funs, see "Fun Expressions" in + section <c>Erlang Reference Manual</c> in System + Documentation..</p> </note> </section> <section> <title>Initial Database Content</title> - <p>After the insertion of the employee named <c>klacke</c> - we have the following records in the database: - </p> + <p>After the insertion of the employee named <c>klacke</c>, + the databse has the following records:</p> <marker id="table2_1"></marker> <table> <row> @@ -364,14 +342,14 @@ <cell align="left" valign="middle">klacke</cell> <cell align="left" valign="middle">7</cell> <cell align="left" valign="middle">male</cell> - <cell align="left" valign="middle">99586</cell> + <cell align="left" valign="middle">98108</cell> <cell align="left" valign="middle">{221, 015}</cell> </row> - <tcaption> -Employee</tcaption> + <tcaption>employee Database Record</tcaption> </table> - <p>An employee record has the following Erlang record/tuple - representation: <c>{employee, 104732, klacke, 7, male, 98108, {221, 015}}</c></p> + <p>This <c>employee</c> record has the Erlang record/tuple + representation + <c>{employee, 104732, klacke, 7, male, 98108, {221, 015}}</c>.</p> <marker id="table2_2"></marker> <table> <row> @@ -382,12 +360,10 @@ Employee</tcaption> <cell align="left" valign="middle">klacke</cell> <cell align="left" valign="middle">B/SFR</cell> </row> - <tcaption> -At_dep</tcaption> + <tcaption>at_dep Database Record</tcaption> </table> - <p>At_dep has the following Erlang tuple representation: - <c>{at_dep, klacke, 'B/SFR'}</c>. - </p> + <p>This <c>at_dep</c> record has the Erlang tuple representation + <c>{at_dep, klacke, 'B/SFR'}</c>.</p> <marker id="table3_3"></marker> <table> <row> @@ -406,39 +382,36 @@ At_dep</tcaption> <cell align="left" valign="middle">klacke</cell> <cell align="left" valign="middle">mnesia</cell> </row> - <tcaption> -In_proj</tcaption> + <tcaption>in_proj Database Record</tcaption> </table> - <p>In_proj has the following Erlang tuple representation: - <c>{in_proj, klacke, 'Erlang', klacke, 'otp', klacke, 'mnesia'}</c></p> - <p>There is no difference between rows in a table and Mnesia - records. Both concepts are the same and will be used - interchangeably throughout this book. - </p> - <p>A Mnesia table is populated by Mnesia records. For example, - the tuple <c>{boss, klacke, bjarne}</c> is a record. The - second element in this tuple is the key. In order to uniquely - identify a table row both the key and the table name is - needed. The term <em>object identifier</em>, - (oid) is sometimes used for the arity two tuple {Tab, Key}. The oid for - the <c>{boss, klacke, bjarne}</c> record is the arity two + <p>This <c>in_proj</c> record has the Erlang tuple representation + <c>{in_proj, klacke, 'Erlang', klacke, 'otp', klacke, + 'mnesia'}</c>.</p> + <p>There is no difference between rows in a table and <c>Mnesia</c> + records. Both concepts are the same and are used + interchangeably throughout this User's Guide.</p> + <p>A <c>Mnesia</c> table is populated by <c>Mnesia</c> records. For + example, the tuple <c>{boss, klacke, bjarne}</c> is a record. The + second element in this tuple is the key. To identify a table + uniquely, both the key and the table name is needed. + The term Object Identifier (OID) is + sometimes used for the arity two tuple {Tab, Key}. The OID for + the record <c>{boss, klacke, bjarne}</c> is the arity two tuple <c>{boss, klacke}</c>. The first element of the tuple is the type of the record and the second element is the key. An - oid can lead to zero, one, or more records depending on - whether the table type is <c>set</c> or <c>bag</c>. - </p> - <p>We were also able to insert the <c>{boss, klacke, bjarne}</c> record which contains an implicit reference to - another employee which does not yet exist in the - database. Mnesia does not enforce this. - </p> + OID can lead to zero, one, or more records depending on + whether the table type is <c>set</c> or <c>bag</c>.</p> + <p>The record <c>{boss, klacke, bjarne}</c> can also be inserted. + This record contains an implicit reference to + another employee that does not yet exist in the + database. <c>Mnesia</c> does not enforce this.</p> </section> <section> - <title>Adding Records and Relationships to the Database</title> - <p>After adding additional record to the Company database, we - may end up with the following records: - </p> - <p><em>Employees</em></p> + <title>Adding Records and Relationships to Database</title> + <p>After adding more records to the <c>Company</c> database, the + result can be the following records:</p> + <p><c>employees</c>:</p> <code type="none"> {employee, 104465, "Johnson Torbjorn", 1, male, 99184, {242,038}}. {employee, 107912, "Carlsson Tuula", 2, female,94556, {242,056}}. @@ -447,16 +420,13 @@ In_proj</tcaption> {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}}. - </code> - <p><em>Dept</em></p> + {employee, 115018, "Mattsson Hakan", 3, male, 99251, {203,348}}.</code> + <p><c>dept</c>:</p> <code type="none"> - {dept, 'B/SF', "Open Telecom Platform"}. {dept, 'B/SFP', "OTP - Product Development"}. - {dept, 'B/SFR', "Computer Science Laboratory"}. - </code> - <p><em>Projects</em></p> + {dept, 'B/SFR', "Computer Science Laboratory"}.</code> + <p><c>projects</c>:</p> <code type="none"> %% projects {project, erlang, 1}. @@ -465,23 +435,19 @@ In_proj</tcaption> {project, mnesia, 5}. {project, wolf, 6}. {project, documentation, 7}. - {project, www, 8}. - </code> - <p>The above three tables, titled <c>employees</c>, - <c>dept</c>, and <c>projects</c>, are the tables which are + {project, www, 8}.</code> + <p>These three tables, <c>employees</c>, <c>dept</c>, and + <c>projects</c>, are made up of real records. The following database content is - stored in the tables which is built on - relationships. These tables are titled <c>manager</c>, - <c>at_dep</c>, and <c>in_proj</c>. - </p> - <p><em>Manager</em></p> + stored in the tables and is built on + relationships. These tables are <c>manager</c>, + <c>at_dep</c>, and <c>in_proj</c>.</p> + <p><c>manager</c>:</p> <code type="none"> - {manager, 104465, 'B/SF'}. {manager, 104465, 'B/SFP'}. - {manager, 114872, 'B/SFR'}. - </code> - <p><em>At_dep</em></p> + {manager, 114872, 'B/SFR'}.</code> + <p><c>at_dep</c>:</p> <code type="none"> {at_dep, 104465, 'B/SF'}. {at_dep, 107912, 'B/SF'}. @@ -490,9 +456,8 @@ In_proj</tcaption> {at_dep, 104659, 'B/SFR'}. {at_dep, 104732, 'B/SFR'}. {at_dep, 117716, 'B/SFP'}. - {at_dep, 115018, 'B/SFP'}. - </code> - <p><em>In_proj</em></p> + {at_dep, 115018, 'B/SFP'}.</code> + <p><c>in_proj</c>:</p> <code type="none"> {in_proj, 104465, otp}. {in_proj, 107912, otp}. @@ -508,136 +473,118 @@ In_proj</tcaption> {in_proj, 117716, otp}. {in_proj, 117716, documentation}. {in_proj, 115018, otp}. - {in_proj, 115018, mnesia}. - </code> + {in_proj, 115018, mnesia}.</code> <p>The room number is an attribute of the employee - record. This is a structured attribute which consists of a + record. This is a structured attribute that consists of a tuple. The first element of the tuple identifies a corridor, - and the second element identifies the actual room in the - corridor. We could have chosen to represent this as a record + and the second element identifies the room in that + corridor. An alternative is to represent this as a record <c>-record(room, {corr, no}).</c> instead of an anonymous - tuple representation. - </p> - <p>The Company database is now initialized and contains - data. </p> + tuple representation.</p> + <p>The <c>Company</c> database is now initialized and contains + data.</p> </section> <section> <title>Writing Queries</title> - <p>Retrieving data from DBMS should usually be done with <c>mnesia:read/3</c> or - <c>mnesia:read/1</c> functions. The following function raises the salary:</p> + <p>Retrieving data from DBMS is usually to be done with the + functions + <seealso marker="mnesia#read/3">mnesia:read/3</seealso> or + <seealso marker="mnesia#read/2">mnesia:read/1</seealso>. + The following function raises the salary:</p> <codeinclude file="company.erl" tag="%5" type="erl"></codeinclude> - <p>Since we want to update the record using <c>mnesia:write/1</c> after we have - increased the salary we acquire a write lock (third argument to read) when we read the - record from the table. - </p> - <p>It is not always the case that we can directly read the values from the table, - we might need to search the table or several tables to get the data we want, this - is done by writing database queries. Queries are always more expensive operations - than direct lookups done with <c>mnesia:read</c> and should be avoided in performance - critical code.</p> - <p>There are two methods for writing database queries: - </p> + <p>Since it is desired to update the record using the function + <seealso marker="mnesia#write/1">mnesia:write/1</seealso> + after the salary has been increased, a write + lock (third argument to <c>read</c>) is acquired when the record from + the table is read.</p> + <p>To read the values from the table directly is not always possible. + It can be needed to search one or more tables to get the + wanted data, and this is done by writing database queries. Queries + are always more expensive operations than direct lookups done with + <c>mnesia:read</c>. Therefore, avoid queries in + performance-critical code.</p> + <p>Two methods are available for writing database queries:</p> <list type="bulleted"> - <item>Mnesia functions - </item> + <item><c>Mnesia</c> functions</item> <item>QLC</item> </list> <section> - <title>Mnesia functions </title> - <p></p> + <title>Using Mnesia Functions</title> <p>The following function extracts the names of the female employees - stored in the database: - </p> + stored in the database:</p> <pre> -mnesia:select(employee, [{#employee{sex = female, name = '$1', _ = '_'},[], ['$1']}]). - </pre> - <p>Select must always run within an activity such as a - transaction. To be able to call from the shell we might - construct a function as: - </p> +mnesia:select(employee, [{#employee{sex = female, name = '$1', _ = '_'},[], ['$1']}]).</pre> + <p><c>select</c> must always run within an activity, such as a + transaction. The following function can be constructed to call + from the shell:</p> <codeinclude file="company.erl" tag="%20" type="erl"></codeinclude> - <p>The select expression matches all entries in table employee with - the field sex set to female. - </p> - <p>This function can be called from the shell as follows: - </p> + <p>The <c>select</c> expression matches all entries in table + employee with the field <c>sex</c> set to <c>female</c>.</p> + <p>This function can be called from the shell as follows:</p> <pre> (klacke@gin)1> <input>company:all_females().</input> - {atomic, ["Carlsson Tuula", "Fedoriw Anna"]} - </pre> - <p>See also the <seealso marker="Mnesia_chap4#matching">Pattern Matching </seealso> - chapter for a description of select and its syntax. - </p> + {atomic, ["Carlsson Tuula", "Fedoriw Anna"]}</pre> + <p>For a description of <c>select</c> and its syntax, see + <seealso marker="Mnesia_chap4#matching">Pattern Matching</seealso>. + </p> </section> <section> <title>Using QLC </title> - <p>This section contains simple introductory examples - only. Refer to <em>QLC reference manual</em> for a - full description of the QLC query language. Using QLC - might be more expensive than using Mnesia functions directly but - offers a nice syntax. - </p> + <p>This section contains simple introductory examples only. For + a full description of the QLC query language, see the + <seealso marker="stdlib:qlc">qlc</seealso> manual page in + <c>STDLIB</c>.</p> + <p>Using QLC can be more expensive than using <c>Mnesia</c> + functions directly but offers a nice syntax.</p> <p>The following function extracts a list of female employees - from the database: - </p> + from the database:</p> <pre> Q = qlc:q([E#employee.name || E <![CDATA[<-]]> mnesia:table(employee), E#employee.sex == female]), - qlc:e(Q), - </pre> - <p>Accessing mnesia tables from a QLC list comprehension must + qlc:e(Q),</pre> + <p>Accessing <c>Mnesia</c> tables from a QLC list comprehension must always be done within a transaction. Consider the following - function: - </p> + function:</p> <codeinclude file="company.erl" tag="%2" type="erl"></codeinclude> - <p>This function can be called from the shell as follows: - </p> + <p>This function can be called from the shell as follows:</p> <pre> (klacke@gin)1> <input>company:females().</input> - {atomic, ["Carlsson Tuula", "Fedoriw Anna"]} - </pre> - <p>In traditional relational database terminology, the above - operation would be called a selection, followed by a projection. - </p> - <p>The list comprehension expression shown above contains a - number of syntactical elements. - </p> + {atomic, ["Carlsson Tuula", "Fedoriw Anna"]}</pre> + <p>In traditional relational database terminology, this + operation is called a selection, followed by a projection.</p> + <p>The previous list comprehension expression contains a + number of syntactical elements:</p> <list type="bulleted"> - <item>the first <c>[</c> bracket should be read as "build the - list" + <item>The first <c>[</c> bracket is read as "build the + list". </item> - <item>the <c>||</c> "such that" and the arrow <c><![CDATA[<-]]></c> should - be read as "taken from" + <item>The <c>||</c> "such that" and the arrow <c><![CDATA[<-]]></c> + is read as "taken from". </item> </list> - <p>Hence, the above list comprehension demonstrates the - formation of the list <c>E#employee.name</c> such that <c>E</c> is - taken from the table of employees and the <c>sex</c> attribute - of each records is equal with the atom <c>female</c>. - </p> - <p>The whole list comprehension must be given to the - <c>qlc:q/1</c> function. - </p> - <p>It is possible to combine list comprehensions with low - level Mnesia functions in the same transaction. If we want to - raise the salary of all female employees we execute: - </p> + <p>Hence, the previous list comprehension demonstrates the + formation of the list <c>E#employee.name</c> such that <c>E</c> is + taken from the table of employees, and attribute <c>sex</c> + of each record is equal to the atom <c>female</c>.</p> + <p>The whole list comprehension must be given to the function + <c>qlc:q/1</c>.</p> + <p>List comprehensions with low-level <c>Mnesia</c> functions + can be combined in the same transaction. To raise the + salary of all female employees, execute the following:</p> <codeinclude file="company.erl" tag="%4" type="erl"></codeinclude> <p>The function <c>raise_females/1</c> returns the tuple <c>{atomic, Number}</c>, where <c>Number</c> is the number of - female employees who received a salary increase. Should an error - occur, the value <c>{aborted, Reason}</c> is returned. In the - case of an error, Mnesia guarantees that the salary is not - raised for any employees at all. - </p> + female employees who received a salary increase. If an error + occurs, the value <c>{aborted, Reason}</c> is returned, and + <c>Mnesia</c> guarantees that the salary is not + raised for any employee.</p> + <p><em>Example:</em></p> <pre> - 33><input>company:raise_females(33).</input> - {atomic,2} - </pre> + {atomic,2}</pre> </section> </section> </section> |