path: root/doc/faq/faq.yml

---
- "How do I do a database query?":
  - "I just want an array of the rows...": >-

      Use the Database#execute method. If you don't give it a block, it will
      return an array of all the rows:


      <pre>
        require 'sqlite3'

        db = SQLite3::Database.new( "test.db" )
        rows = db.execute( "select * from test" )
      </pre>

  - "I'd like to use a block to iterate through the rows...": >-

      Use the Database#execute method. If you give it a block, each row of the
      result will be yielded to the block:


      <pre>
        require 'sqlite3'

        db = SQLite3::Database.new( "test.db" )
        db.execute( "select * from test" ) do |row|
          ...
        end
      </pre>

  - "I need to get the column names as well as the rows...": >-

      Use the Database#execute2 method. This works just like Database#execute;
      if you don't give it a block, it returns an array of rows; otherwise, it
      will yield each row to the block. _However_, the first row returned is
      always an array of the column names from the query:


      <pre>
        require 'sqlite3'

        db = SQLite3::Database.new( "test.db" )
        columns, *rows = db.execute2( "select * from test" )

        # or use a block:

        columns = nil
        db.execute2( "select * from test" ) do |row|
          if columns.nil?
            columns = row
          else
            # process row
          end
        end
      </pre>

  - "I just want the first row of the result set...": >-

      Easy. Just call Database#get_first_row:


      <pre>
        row = db.get_first_row( "select * from table" )
      </pre>


      This also supports bind variables, just like Database#execute
      and friends.

  - "I just want the first value of the first row of the result set...": >-

      Also easy. Just call Database#get_first_value:


      <pre>
        count = db.get_first_value( "select count(*) from table" )
      </pre>


      This also supports bind variables, just like Database#execute
      and friends.

- "How do I prepare a statement for repeated execution?": >-
    If the same statement is going to be executed repeatedly, you can speed
    things up a bit by _preparing_ the statement. You do this via the
    Database#prepare method. It returns a Statement object, and you can
    then invoke #execute on that to get the ResultSet:

    
    <pre>
      stmt = db.prepare( "select * from person" )

      1000.times do
        stmt.execute do |result|
          ...
        end
      end

      stmt.close

      # or, use a block

      db.prepare( "select * from person" ) do |stmt|
        1000.times do
          stmt.execute do |result|
            ...
          end
        end
      end
    </pre>


    This is made more useful by the ability to bind variables to placeholders
    via the Statement#bind_param and Statement#bind_params methods. (See the
    next FAQ for details.)

- "How do I use placeholders in an SQL statement?": >-
    Placeholders in an SQL statement take any of the following formats:


    * @?@

    * @?_nnn_@

    * @:_word_@


    Where _n_ is an integer, and _word_ is an alpha-numeric identifier (or
    number). When the placeholder is associated with a number, that number
    identifies the index of the bind variable to replace it with. When it
    is an identifier, it identifies the name of the correponding bind
    variable. (In the instance of the first format--a single question
    mark--the placeholder is assigned a number one greater than the last
    index used, or 1 if it is the first.)


    For example, here is a query using these placeholder formats:


    <pre>
      select *
        from table
       where ( c = ?2 or c = ? )
         and d = :name
         and e = :1
    </pre>


    This defines 5 different placeholders: 1, 2, 3, and "name".


    You replace these placeholders by _binding_ them to values. This can be
    accomplished in a variety of ways.


    The Database#execute, and Database#execute2 methods all accept additional
    arguments following the SQL statement. These arguments are assumed to be
    bind parameters, and they are bound (positionally) to their corresponding
    placeholders:


    <pre>
      db.execute( "select * from table where a = ? and b = ?",
                  "hello",
                  "world" )
    </pre>


    The above would replace the first question mark with 'hello' and the
    second with 'world'. If the placeholders have an explicit index given, they
    will be replaced with the bind parameter at that index (1-based).


    If a Hash is given as a bind parameter, then its key/value pairs are bound
    to the placeholders. This is how you bind by name:


    <pre>
      db.execute( "select * from table where a = :name and b = :value",
                  "name" => "bob",
                  "value" => "priceless" )
    </pre>


    You can also bind explicitly using the Statement object itself. Just pass
    additional parameters to the Statement#execute statement:
    

    <pre>
      db.prepare( "select * from table where a = :name and b = ?" ) do |stmt|
        stmt.execute "value", "name" => "bob"
      end
    </pre>


    Or do a Database#prepare to get the Statement, and then use either
    Statement#bind_param or Statement#bind_params:


    <pre>
      stmt = db.prepare( "select * from table where a = :name and b = ?" )

      stmt.bind_param( "name", "bob" )
      stmt.bind_param( 1, "value" )

      # or

      stmt.bind_params( "value", "name" => "bob" )
    </pre>

- "How do I discover metadata about a query?": >-
  
    If you ever want to know the names or types of the columns in a result
    set, you can do it in several ways.


    The first way is to ask the row object itself. Each row will have a
    property "fields" that returns an array of the column names. The row
    will also have a property "types" that returns an array of the column
    types:


    <pre>
      rows = db.execute( "select * from table" )
      p rows[0].fields
      p rows[0].types
    </pre>


    Obviously, this approach requires you to execute a statement that actually
    returns data. If you don't know if the statement will return any rows, but
    you still need the metadata, you can use Database#query and ask the
    ResultSet object itself:


    <pre>
      db.query( "select * from table" ) do |result|
        p result.columns
        p result.types
        ...
      end
    </pre>


    Lastly, you can use Database#prepare and ask the Statement object what
    the metadata are:


    <pre>
      stmt = db.prepare( "select * from table" )
      p stmt.columns
      p stmt.types
    </pre>

- "I'd like the rows to be indexible by column name.": >-
    By default, each row from a query is returned as an Array of values. This
    means that you can only obtain values by their index. Sometimes, however,
    you would like to obtain values by their column name.


    The first way to do this is to set the Database property "results_as_hash"
    to true. If you do this, then all rows will be returned as Hash objects,
    with the column names as the keys. (In this case, the "fields" property
    is unavailable on the row, although the "types" property remains.)


    <pre>
      db.results_as_hash = true
      db.execute( "select * from table" ) do |row|
        p row['column1']
        p row['column2']
      end
    </pre>


    The other way is to use Ara Howard's
    "ArrayFields":http://rubyforge.org/projects/arrayfields
    module. Just require "arrayfields", and all of your rows will be indexable
    by column name, even though they are still arrays!


    <pre>
      require 'arrayfields'

      ...
      db.execute( "select * from table" ) do |row|
        p row[0] == row['column1']
        p row[1] == row['column2']
      end
    </pre>

- "I'd like the values from a query to be the correct types, instead of String.": >-
    You can turn on "type translation" by setting Database#type_translation to
    true:


    <pre>
      db.type_translation = true
      db.execute( "select * from table" ) do |row|
        p row
      end
    </pre>


    By doing this, each return value for each row will be translated to its
    correct type, based on its declared column type.


    You can even declare your own translation routines, if (for example) you are
    using an SQL type that is not handled by default:


    <pre>
      # assume "objects" table has the following schema:
      #   create table objects (
      #     name varchar2(20),
      #     thing object
      #   )

      db.type_translation = true
      db.translator.add_translator( "object" ) do |type, value|
        db.decode( value )
      end

      h = { :one=>:two, "three"=>"four", 5=>6 }
      dump = db.encode( h )

      db.execute( "insert into objects values ( ?, ? )", "bob", dump )

      obj = db.get_first_value( "select thing from objects where name='bob'" )
      p obj == h
    </pre>

- "How do insert binary data into the database?": >-
    Use blobs. Blobs are new features of SQLite3. You have to use bind
    variables to make it work:


    <pre>
      db.execute( "insert into foo ( ?, ? )",
        SQLite3::Blob.new( "\0\1\2\3\4\5" ),
        SQLite3::Blob.new( "a\0b\0c\0d ) )
    </pre>


    The blob values must be indicated explicitly by binding each parameter to
    a value of type SQLite3::Blob.

- "How do I do a DDL (insert, update, delete) statement?": >-
    You can actually do inserts, updates, and deletes in exactly the same way
    as selects, but in general the Database#execute method will be most
    convenient:

    
    <pre>
      db.execute( "insert into table values ( ?, ? )", *bind_vars )
    </pre>

- "How do I execute multiple statements in a single string?": >-
    The standard query methods (Database#execute, Database#execute2,
    Database#query, and Statement#execute) will only execute the first
    statement in the string that is given to them. Thus, if you have a
    string with multiple SQL statements, each separated by a string,
    you can't use those methods to execute them all at once.


    Instead, use Database#execute_batch:


    <pre>
      sql = <<SQL
        create table the_table (
          a varchar2(30),
          b varchar2(30)
        );

        insert into the_table values ( 'one', 'two' );
        insert into the_table values ( 'three', 'four' );
        insert into the_table values ( 'five', 'six' );
      SQL

      db.execute_batch( sql )
    </pre>


    Unlike the other query methods, Database#execute_batch accepts no
    block. It will also only ever return +nil+. Thus, it is really only
    suitable for batch processing of DDL statements.

- "How do I begin/end a transaction?":
    Use Database#transaction to start a transaction. If you give it a block,
    the block will be automatically committed at the end of the block,
    unless an exception was raised, in which case the transaction will be
    rolled back. (Never explicitly call Database#commit or Database#rollback
    inside of a transaction block--you'll get errors when the block
    terminates!)


    <pre>
      database.transaction do |db|
        db.execute( "insert into table values ( 'a', 'b', 'c' )" )
        ...
      end
    </pre>


    Alternatively, if you don't give a block to Database#transaction, the
    transaction remains open until you explicitly call Database#commit or
    Database#rollback.


    <pre>
      db.transaction
      db.execute( "insert into table values ( 'a', 'b', 'c' )" )
      db.commit
    </pre>


    Note that SQLite does not allow nested transactions, so you'll get errors
    if you try to open a new transaction while one is already active. Use
    Database#transaction_active? to determine whether a transaction is
    active or not.

#- "How do I discover metadata about a table/index?":
#
#- "How do I do tweak database settings?":