odbc

Erlang ODBC application

This application provides an Erlang interface to communicate with relational SQL-databases. It is built on top of Microsofts ODBC interface and therefore requires that you have an ODBC driver to the database that you want to connect to.

Note!

The functions first/[1,2], last/[1,2], next/[1,2], prev[1,2] and select/[3,4] assumes there is a result set associated with the connection to work on. Calling the function select_count/[2,3] associates such a result set with the connection. Calling select_count again will remove the current result set association and create a new one. Calling a function which dose not operate on an associated result sets, such as sql_query/[2,3], will remove the current result set association.

Alas some drivers only support sequential traversal of the result set, e.i. they do not support what in the ODBC world is known as scrollable cursors. This will have the effect that functions such as first/[1,2], last/[1,2], prev[1,2], etc will return {error, driver_does_not_support_function}

COMMON DATA TYPES

Here follows type definitions that are used by more than one function in the ODBC API.

Note!

The type TimeOut has the default value infinity, so for instance:
commit(Ref, CommitMode) is the same as commit(Ref, CommitMode, infinity). If the timeout expires the client will exit with the reason timeout. 

 connection_reference() - as returned by connect/2    
 time_out() = milliseconds() | infinity    
 milliseconds() = integer() >= 0     
 common_reason() = connection_closed | extended_error() | term() - some kind of
 explanation of what went wrong    
 extended_error() = {string(), integer(), Reason} - extended error type with ODBC
 and native database error codes, as well as the base reason that would have been
 returned had extended_errors not been enabled. 
 string() = list of ASCII characters    
 col_name() = string() - Name of column in the result set    
 col_names() - [col_name()] - e.g. a list of the names of the
           selected columns in the result set.     
 row() = {value()} - Tuple of column values e.g. one row of the
           result set.     
 value() = null | term() - A column value.     
 rows() = [row()] - A list of rows from the result set.    
 result_tuple() =
      {updated, n_rows()} | {selected, col_names(), rows()}    
 n_rows() = integer() - The number of affected rows for UPDATE,
           INSERT, or DELETE queries. For other query types the value
           is driver defined, and hence should be ignored.    
 odbc_data_type() = sql_integer | sql_smallint | sql_tinyint |
      {sql_decimal, precision(), scale()} |
      {sql_numeric, precision(), scale()} |
      {sql_char, size()} |
      {sql_wchar, size()} |
      {sql_varchar, size()} |
      {sql_wvarchar, size()}|
      {sql_float, precision()} |
      {sql_wlongvarchar, size()} |
      {sql_float, precision()} |
      sql_real | sql_double | sql_bit | atom()
    
 precision() = integer()    
 scale() = integer()    
 size() = integer()

ERROR HANDLING

The error handling strategy and possible errors sources are described in the Erlang ODBC User's Guide.

Functions

commit(Ref, CommitMode) ->

commit(Ref, CommitMode, TimeOut) -> ok | {error, Reason}

  • Ref = connection_reference()
  • CommitMode = commit | rollback
  • TimeOut = time_out()
  • Reason = not_an_explicit_commit_connection | process_not_owner_of_odbc_connection | common_reason()

Commits or rollbacks a transaction. Needed on connections where automatic commit is turned off.

connect(ConnectStr, Options) -> {ok, Ref} | {error, Reason}

  • ConnectStr = string()
    • An example of a connection string: "DSN=sql-server;UID=aladdin;PWD=sesame" where DSN is your ODBC Data Source Name, UID is a database user id and PWD is the password for that user. These are usually the attributes required in the connection string, but some drivers have other driver specific attributes, for example "DSN=Oracle8;DBQ=gandalf;UID=aladdin;PWD=sesame" where DBQ is your TNSNAMES.ORA entry name e.g. some Oracle specific configuration attribute.
  • Options = [] | [option()]
    • All options has default values.
  • option() = {auto_commit, on | off} | {timeout, milliseconds()} | {binary_strings, on | off} | {tuple_row, on | off} | {scrollable_cursors, on | off} | {trace_driver, on | off} | {extended_errors, on | off}
  • Ref = connection_reference() - should be used to access the connection.
  • Reason = port_program_executable_not_found | common_reason()

Opens a connection to the database. The connection is associated with the process that created it and can only be accessed through it. This function may spawn new processes to handle the connection. These processes will terminate if the process that created the connection dies or if you call disconnect/1.

If automatic commit mode is turned on, each query will be considered as an individual transaction and will be automatically committed after it has been executed. If you want more than one query to be part of the same transaction the automatic commit mode should be turned off. Then you will have to call commit/3 explicitly to end a transaction.

The default timeout is infinity

>If the option binary_strings is turned on all strings will be returned as binaries and strings inputed to param_query will be expected to be binaries. The user needs to ensure that the binary is in an encoding that the database expects. By default this option is turned off.

As default result sets are returned as a lists of tuples. The TupleMode option still exists to keep some degree of backwards compatibility. If the option is set to off, result sets will be returned as a lists of lists instead of a lists of tuples.

Scrollable cursors are nice but causes some overhead. For some connections speed might be more important than flexible data access and then you can disable scrollable cursor for a connection, limiting the API but gaining speed.

Note!

Turning the scrollable_cursors option off is noted to make old odbc-drivers able to connect that will otherwhise fail.

If trace mode is turned on this tells the ODBC driver to write a trace log to the file SQL.LOG that is placed in the current directory of the erlang emulator. This information may be useful if you suspect there might be a bug in the erlang ODBC application, and it might be relevant for you to send this file to our support. Otherwise you will probably not have much use of this.

Note!

For more information about the ConnectStr see description of the function SQLDriverConnect in [1].

The extended_errors option enables extended ODBC error information when an operation fails. Rather than returning {error, Reason}, the failing function will reutrn {error, {ODBCErrorCode, NativeErrorCode, Reason}}. Note that this information is probably of little use when writing database-independent code, but can be of assistance in providing more sophisticated error handling when dealing with a known underlying database.

ODBCErrorCode is the ODBC error string returned by the ODBC driver. NativeErrorCode is the numberic error code returned by the underlying database. The possible values and their meanings are dependent on the database being used. Reason is as per the Reason field when extended errors are not enabled.

disconnect(Ref) -> ok | {error, Reason}

  • Ref = connection_reference()
  • Reason = process_not_owner_of_odbc_connection | extended_error()

Closes a connection to a database. This will also terminate all processes that may have been spawned when the connection was opened. This call will always succeed. If the connection can not be disconnected gracefully it will be brutally killed. However you may receive an error message as result if you try to disconnect a connection started by another process.

describe_table(Ref, Table) ->

describe_table(Ref, Table, Timeout) -> {ok, Description} | {error, Reason}

  • Ref = connection_reference()
  • Table = string() - Name of databas table.
  • TimeOut = time_out()
  • Description = [{col_name(), odbc_data_type()}]
  • Reason = common_reason()

Queries the database to find out the ODBC data types of the columns of the table Table.

first(Ref) ->

first(Ref, Timeout) -> {selected, ColNames, Rows} | {error, Reason}

  • Ref = connection_reference()
  • TimeOut = time_out()
  • ColNames = col_names()
  • Rows = rows()
  • Reason = result_set_does_not_exist | driver_does_not_support_function | scrollable_cursors_disabled | process_not_owner_of_odbc_connection | common_reason()

Returns the first row of the result set and positions a cursor at this row.

last(Ref) ->

last(Ref, TimeOut) -> {selected, ColNames, Rows} | {error, Reason}

  • Ref = connection_reference()
  • TimeOut = time_out()
  • ColNames = col_names()
  • Rows = rows()
  • Reason = result_set_does_not_exist | driver_does_not_support_function | scrollable_cursors_disabled | process_not_owner_of_odbc_connection | common_reason()

Returns the last row of the result set and positions a cursor at this row.

next(Ref) ->

next(Ref, TimeOut) -> {selected, ColNames, Rows} | {error, Reason}

  • Ref = connection_reference()
  • TimeOut = time_out()
  • ColNames = col_names()
  • Rows = rows()
  • Reason = result_set_does_not_exist | process_not_owner_of_odbc_connection | common_reason()

Returns the next row of the result set relative the current cursor position and positions the cursor at this row. If the cursor is positioned at the last row of the result set when this function is called the returned value will be {selected, ColNames,[]} e.i. the list of row values is empty indicating that there is no more data to fetch.

param_query(Ref, SQLQuery, Params) ->

param_query(Ref, SQLQuery, Params, TimeOut) -> ResultTuple | {error, Reason}

  • Ref = connection_reference()
  • SQLQuery = string() - a SQL query with parameter markers/place holders in form of question marks.
  • Params = [{odbc_data_type(), [value()]}] |[{odbc_data_type(), in_or_out(), [value()]}]
  • in_or_out = in | out | inout
    • Defines IN, OUT, and IN OUT Parameter Modes for stored procedures.
  • TimeOut = time_out()
  • Values = term() - Must be consistent with the Erlang data type that corresponds to the ODBC data type ODBCDataType

Executes a parameterized SQL query. For an example see the "Using the Erlang API" in the Erlang ODBC User's Guide.

Note!

Use the function describe_table/[2,3] to find out which ODBC data type that is expected for each column of that table. If a column has a data type that is described with capital letters, alas it is not currently supported by the param_query function. Too know which Erlang data type corresponds to an ODBC data type see the Erlang to ODBC data type mapping in the User's Guide.

prev(Ref) ->

prev(ConnectionReference, TimeOut) -> {selected, ColNames, Rows} | {error, Reason}

  • Ref = connection_reference()
  • TimeOut = time_out()
  • ColNames = col_names()
  • Rows = rows()
  • Reason = result_set_does_not_exist | driver_does_not_support_function | scrollable_cursors_disabled | process_not_owner_of_odbc_connection | common_reason()

Returns the previous row of the result set relative the current cursor position and positions the cursor at this row.

start() ->

start(Type) -> ok | {error, Reason}

  • Type = permanent | transient | temporary

Starts the odbc application. Default type is temporary. See application(3)

stop() -> ok

Stops the odbc application. See application(3)

sql_query(Ref, SQLQuery) ->

sql_query(Ref, SQLQuery, TimeOut) -> ResultTuple | [ResultTuple] |{error, Reason}

  • Ref = connection_reference()
  • SQLQuery = string() - The string may be composed by several SQL-queries separated by a ";", this is called a batch.
  • TimeOut = time_out()
  • ResultTuple = result_tuple()
  • Reason = process_not_owner_of_odbc_connection | common_reason()

Executes a SQL query or a batch of SQL queries. If it is a SELECT query the result set is returned, on the format {selected, ColNames, Rows}. For other query types the tuple {updated, NRows} is returned, and for batched queries, if the driver supports them, this function can also return a list of result tuples.

Note!

Some drivers may not have the information of the number of affected rows available and then the return value may be {updated, undefined} .

The list of column names is ordered in the same way as the list of values of a row, e.g. the first ColName is associated with the first Value in a Row.


select_count(Ref, SelectQuery) ->

select_count(Ref, SelectQuery, TimeOut) -> {ok, NrRows} | {error, Reason}

  • Ref = connection_reference()
  • SelectQuery = string()
    • SQL SELECT query.
  • TimeOut = time_out()
  • NrRows = n_rows()
  • Reason = process_not_owner_of_odbc_connection | common_reason()

Executes a SQL SELECT query and associates the result set with the connection. A cursor is positioned before the first row in the result set and the tuple {ok, NrRows} is returned.

Note!

Some drivers may not have the information of the number of rows in the result set, then NrRows will have the value undefined.

select(Ref, Position, N) ->

select(Ref, Position, N, TimeOut) -> {selected, ColNames, Rows} | {error, Reason}

  • Ref = connection_reference()
  • Position = next | {relative, Pos} | {absolute, Pos}
    • Selection strategy, determines at which row in the result set to start the selection.
  • Pos = integer()
    • Should indicate a row number in the result set. When used together with the option relativeit will be used as an offset from the current cursor position, when used together with the option absoluteit will be interpreted as a row number.
  • N = integer()
  • TimeOut = time_out()
  • Reason = result_set_does_not_exist | driver_does_not_support_function | scrollable_cursors_disabled | process_not_owner_of_odbc_connection | common_reason()

Selects N consecutive rows of the result set. If Position is next it is semantically equivalent of calling next/[1,2] N times. If Position is {relative, Pos}, Pos will be used as an offset from the current cursor position to determine the first selected row. If Position is {absolute, Pos}, Pos will be the number of the first row selected. After this function has returned the cursor is positioned at the last selected row. If there is less then N rows left of the result set the length of Rows will be less than N. If the first row to select happens to be beyond the last row of the result set, the returned value will be {selected, ColNames,[]} e.i. the list of row values is empty indicating that there is no more data to fetch.

REFERENCES

[1]: Microsoft ODBC 3.0, Programmer's Reference and SDK Guide
See also http://msdn.microsoft.com/