Database.HDBC

HDBC-2.3.1.2: Haskell Database Connectivity

Portability | portable
Stability | provisional
Maintainer | John Goerzen <jgoerzen@complete.org>
Safe Haskell | None

Database.HDBC

Contents

- Introduction
  - Features
  - Available Drivers
- Typing of transfer data
- Database Connections
  - Wrapped Connections
  - Preparing Queries
  - Transaction Handling
  - Connection Inquiries
- Statements
  - Execution
  - Fetching Results
  - Statement Inquires
  - Miscellaneous
- Exceptions
- Column Types
- Threading

Description

Welcome to HDBC, the Haskell Database Connectivity library.

Written by John Goerzen, jgoerzen@complete.org

Synopsis

- data SqlValue
  - = SqlString String
  - | SqlByteString ByteString
  - | SqlWord32 Word32
  - | SqlWord64 Word64
  - | SqlInt32 Int32
  - | SqlInt64 Int64
  - | SqlInteger Integer
  - | SqlChar Char
  - | SqlBool Bool
  - | SqlDouble Double
  - | SqlRational Rational
  - | SqlLocalDate Day
  - | SqlLocalTimeOfDay TimeOfDay
  - | SqlZonedLocalTimeOfDay TimeOfDay TimeZone
  - | SqlLocalTime LocalTime
  - | SqlZonedTime ZonedTime
  - | SqlUTCTime UTCTime
  - | SqlDiffTime NominalDiffTime
  - | SqlPOSIXTime POSIXTime
  - | SqlEpochTime Integer
  - | SqlTimeDiff Integer
  - | SqlNull
- toSql :: Convertible a SqlValue => a -> SqlValue
- fromSql :: Convertible SqlValue a => SqlValue -> a
- safeFromSql :: Convertible SqlValue a => SqlValue -> ConvertResult a
- nToSql :: Integral a => a -> SqlValue
- iToSql :: Int -> SqlValue
- posixToSql :: POSIXTime -> SqlValue
- class IConnection conn where
  - disconnect :: conn -> IO ()
  - commit :: conn -> IO ()
  - rollback :: conn -> IO ()
  - runRaw :: conn -> String -> IO ()
  - run :: conn -> String -> [SqlValue] -> IO Integer
  - prepare :: conn -> String -> IO Statement
  - clone :: conn -> IO conn
  - hdbcDriverName :: conn -> String
  - hdbcClientVer :: conn -> String
  - proxiedClientName :: conn -> String
  - proxiedClientVer :: conn -> String
  - dbServerVer :: conn -> String
  - dbTransactionSupport :: conn -> Bool
  - getTables :: conn -> IO [String]
  - describeTable :: conn -> String -> IO [(String, SqlColDesc)]
- data ConnWrapper = forall conn . IConnection conn => ConnWrapper conn
- withWConn :: forall b. ConnWrapper -> (forall conn. IConnection conn => conn -> b) -> b
- sRun :: IConnection conn => conn -> String -> [Maybe String] -> IO Integer
- quickQuery' :: IConnection conn => conn -> String -> [SqlValue] -> IO [[SqlValue]]
- quickQuery :: IConnection conn => conn -> String -> [SqlValue] -> IO [[SqlValue]]
- withTransaction :: IConnection conn => conn -> (conn -> IO a) -> IO a
- data Statement
- execute :: Statement -> [SqlValue] -> IO Integer
- executeRaw :: Statement -> IO ()
- sExecute :: Statement -> [Maybe String] -> IO Integer
- executeMany :: Statement -> [[SqlValue]] -> IO ()
- sExecuteMany :: Statement -> [[Maybe String]] -> IO ()
- fetchRow :: Statement -> IO (Maybe [SqlValue])
- fetchRowAL :: Statement -> IO (Maybe [(String, SqlValue)])
- fetchRowMap :: Statement -> IO (Maybe (Map String SqlValue))
- sFetchRow :: Statement -> IO (Maybe [Maybe String])
- fetchAllRows :: Statement -> IO [[SqlValue]]
- fetchAllRows' :: Statement -> IO [[SqlValue]]
- fetchAllRowsAL :: Statement -> IO [[(String, SqlValue)]]
- fetchAllRowsAL' :: Statement -> IO [[(String, SqlValue)]]
- fetchAllRowsMap :: Statement -> IO [Map String SqlValue]
- fetchAllRowsMap' :: Statement -> IO [Map String SqlValue]
- sFetchAllRows :: Statement -> IO [[Maybe String]]
- sFetchAllRows' :: Statement -> IO [[Maybe String]]
- getColumnNames :: Statement -> IO [String]
- describeResult :: Statement -> IO [(String, SqlColDesc)]
- finish :: Statement -> IO ()
- originalQuery :: Statement -> String
- data SqlError = SqlError {
  - seState :: String
  - seNativeError :: Int
  - seErrorMsg :: String
  }
- throwSqlError :: SqlError -> IO a
- catchSql :: IO a -> (SqlError -> IO a) -> IO a
- handleSql :: (SqlError -> IO a) -> IO a -> IO a
- sqlExceptions :: SqlError -> Maybe SqlError
- handleSqlError :: IO a -> IO a
- module Database.HDBC.ColTypes

# Introduction

Welcome to HDBC, Haskell Database Connectivity.

HDBC provides an abstraction layer between Haskell programs and SQL relational databases. This lets you write database code once, in Haskell, and have it work with any number of backend SQL databases (MySQL, Oracle, PostgreSQL, ODBC-compliant databases, etc.)

HDBC is modeled loosely on Perl's DBI interface http://search.cpan.org/~timb/DBI/DBI.pm, though it has also been influenced by Python's DB-API v2, JDBC in Java, and HSQL in Haskell.

HDBC is a from-scratch effort. It is not a reimplementation of HSQL, though its purpose is the same.

## Features

Features of HDBC include:

- Ability to use replacable parameters to let one query be executed multiple times (eliminates the need for an escape function)
- Ability to access returned rows by column number
- Ability to read data from the SQL server on-demand rather than reading the entire result set up front
- HUnit testsuite for each backend driver
- Well-defined standard API and easy backend driver implementation
- Lazy reading of the entire result set (think hGetContents, but for the results of SELECT) (see `sFetchAllRows`)
- Support for translation between Haskell and SQL types
- Support for querying database server properties
- Add-on package (hdbc-missingh) to integrate with MissingH, providing a database backend for AnyDBM.
- Support for querying metadata such as column names.
- Support for querying additional metadata (column types, etc.)

## Available Drivers

Here is a list of known drivers as of January 26, 2009:

`Sqlite v3`
  Available from http://software.complete.org/hdbc-sqlite3. Or, to participate in development, use `git clone git://git.complete.org/hdbc-sqlite3`
`PostgreSQL`
  Available from http://software.complete.org/hdbc-postgresql. Or, to participate in development, use `git clone git://git.complete.org/hdbc-postgresql`
`ODBC`
  Available from http://software.complete.org/hdbc-odbc. Or, to partitipace in development, use `git clone git://git.complete.org/hdbc-odbc`
`MySQL`
  MySQL users have two choices: the first is the ODBC driver, which works and has been tested against MySQL on both Linux/Unix and Windows platforms. There is also an alpha-quality native MySQL driver available for download at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-mysql with a homepage at http://www.maubi.net/~waterson/hacks/hdbc-mysql.html.

In addition, there is one integration package: hdbc-anydbm. This integrates with the AnyDBM library http://software.complete.org/anydbm. It lets any HDBC database act as a backend for the AnyDBM interface. Available from http://software.complete.org/hdbc-anydbm. Or, to participate in development, use `darcs get --partial http://darcs.complete.org/hdbc-anydbm`

The latest version of HDBC itself is available from http://software.complete.org/hdbc. Or, to participate in development, use `git clone git://git.complete.org/hdbc`.

# Typing of transfer data

data SqlValue

`SqlValue` is he main type for expressing Haskell values to SQL databases.

INTRODUCTION TO SQLVALUE

This type is used to marshall Haskell data to and from database APIs. HDBC driver interfaces will do their best to use the most accurate and efficient way to send a particular value to the database server.

Values read back from the server are constructed with the most appropriate `SqlValue` constructor. `fromSql` or `safeFromSql` can then be used to convert them into whatever type is needed locally in Haskell.

Most people will use `toSql` and `fromSql` instead of manipulating `SqlValue`s directly.

EASY CONVERSIONS BETWEEN HASKELL TYPES

Conversions are powerful; for instance, you can call `fromSql` on a SqlInt32 and get a String or a Double out of it. This class attempts to Do The Right Thing whenever possible, and will raise an error when asked to do something incorrect. In particular, when converting to any type except a Maybe, `SqlNull` as the input will cause an error to be raised.

Conversions are implemented in terms of the Data.Convertible module, part of the convertible package. You can refer to its documentation, and import that module, if you wish to parse the Left result from `safeFromSql` yourself, or write your own conversion instances.

Here are some notes about conversion:

- Fractions of a second are not preserved on time values
- There is no `safeToSql` because `toSql` never fails.

See also `toSql`, `safeFromSql`, `fromSql`, `nToSql`, `iToSql`, `posixToSql`.

ERROR CONDITIONS

There may sometimes be an error during conversion. For instance, if you have a `SqlString` and are attempting to convert it to an Integer, but it doesn't parse as an Integer, you will get an error. This will be indicated as an exception if using `fromSql`, or a Left result if using `safeFromSql`.

SPECIAL NOTE ON POSIXTIME

Note that a `NominalDiffTime` or `POSIXTime` is converted to `SqlDiffTime` by `toSql`. HDBC cannot differentiate between `NominalDiffTime` and `POSIXTime` since they are the same underlying type. You must construct `SqlPOSIXTime` manually or via `posixToSql`, or use `SqlUTCTime`.

DETAILS ON SQL TYPES

HDBC database backends are expected to marshal date and time data back and forth using the appropriate representation for the underlying database engine. Databases such as PostgreSQL with builtin date and time types should see automatic conversion between these Haskell types to database types. Other databases will be presented with an integer or a string. Care should be taken to use the same type on the Haskell side as you use on the database side. For instance, if your database type lacks timezone information, you ought not to use ZonedTime, but instead LocalTime or UTCTime. Database type systems are not always as rich as Haskell. For instance, for data stored in a TIMESTAMP WITHOUT TIME ZONE column, HDBC may not be able to tell if it is intended as UTCTime or LocalTime data, and will happily convert it to both, upon your request. It is your responsibility to ensure that you treat timezone issues with due care.

This behavior also exists for other types. For instance, many databases do not have a Rational type, so they will just use the show function and store a Rational as a string.

The conversion between Haskell types and database types is complex, and generic code in HDBC or its backends cannot possibly accomodate every possible situation. In some cases, you may be best served by converting your Haskell type to a String, and passing that to the database.

UNICODE AND BYTESTRINGS

Beginning with HDBC v2.0, interactions with a database are presumed to occur in UTF-8.

To accomplish this, whenever a ByteString must be converted to or from a String, the ByteString is assumed to be in UTF-8 encoding, and will be decoded or encoded as appropriate. Database drivers will generally present text or string data they have received from the database as a SqlValue holding a ByteString, which `fromSql` will automatically convert to a String, and thus automatically decode UTF-8, when you need it. In the other direction, database drivers will generally convert a `SqlString` to a ByteString in UTF-8 encoding before passing it to the database engine.

If you are handling some sort of binary data that is not in UTF-8, you can of course work with the ByteString directly, which will bypass any conversion.

Due to lack of support by database engines, lazy ByteStrings are not passed to database drivers. When you use `toSql` on a lazy ByteString, it will be converted to a strict ByteString for storage. Similarly, `fromSql` will convert a strict ByteString to a lazy ByteString if you demand it.

EQUALITY OF SQLVALUE

Two SqlValues are considered to be equal if one of these hold. The first comparison that can be made is controlling; if none of these comparisons can be made, then they are not equal:

- Both are NULL
- Both represent the same type and the encapsulated values are considered equal by applying (==) to them
- The values of each, when converted to a string, are equal.

STRING VERSIONS OF TIMES

Default string representations are given as comments below where such are non-obvious. These are used for `fromSql` when a `String` is desired. They are also defaults for representing data to SQL backends, though individual backends may override them when a different format is demanded by the underlying database. Date and time formats use ISO8601 date format, with HH:MM:SS added for time, and -HHMM added for timezone offsets.

DEPRECATED CONSTRUCTORS

`SqlEpochTime` and `SqlTimeDiff` are no longer created automatically by any `toSql` or `fromSql` functions or database backends. They may still be manually constructed, but are expected to be removed in a future version. Although these two constructures will be removed, support for marshalling to and from the old System.Time data will be maintained as long as System.Time is, simply using the newer data types for conversion.

Constructors

SqlString String |
SqlByteString ByteString |
SqlWord32 Word32 |
SqlWord64 Word64 |
SqlInt32 Int32 |
SqlInt64 Int64 |
SqlInteger Integer |
SqlChar Char |
SqlBool Bool |
SqlDouble Double |
SqlRational Rational |
SqlLocalDate Day | Local YYYY-MM-DD (no timezone)
SqlLocalTimeOfDay TimeOfDay | Local HH:MM:SS (no timezone)
SqlZonedLocalTimeOfDay TimeOfDay TimeZone | Local HH:MM:SS -HHMM. Converts to and from (TimeOfDay, TimeZone).
SqlLocalTime LocalTime | Local YYYY-MM-DD HH:MM:SS (no timezone)
SqlZonedTime ZonedTime | Local YYYY-MM-DD HH:MM:SS -HHMM. Considered equal if both convert to the same UTC time.
SqlUTCTime UTCTime | UTC YYYY-MM-DD HH:MM:SS
SqlDiffTime NominalDiffTime | Calendar diff between seconds. Rendered as Integer when converted to String, but greater precision may be preserved for other types or to underlying database.
SqlPOSIXTime POSIXTime | Time as seconds since midnight Jan 1 1970 UTC. Integer rendering as for `SqlDiffTime`.
SqlEpochTime Integer | DEPRECATED Representation of ClockTime or CalendarTime. Use SqlPOSIXTime instead.
SqlTimeDiff Integer | DEPRECATED Representation of TimeDiff. Use SqlDiffTime instead.
SqlNull | NULL in SQL or Nothing in Haskell

Instances

Eq SqlValue |
Show SqlValue |
Typeable SqlValue |
Convertible Bool SqlValue |
Convertible Char SqlValue |
Convertible Double SqlValue |
Convertible Int SqlValue |
Convertible Int32 SqlValue |
Convertible Int64 SqlValue |
Convertible Integer SqlValue |
Convertible Rational SqlValue |
Convertible Word32 SqlValue |
Convertible Word64 SqlValue |
Convertible String SqlValue |
Convertible ByteString SqlValue |
Convertible ByteString SqlValue |
Convertible ClockTime SqlValue |
Convertible CalendarTime SqlValue |
Convertible TimeDiff SqlValue |
Convertible Text SqlValue |
Convertible Text SqlValue |
Convertible LocalTime SqlValue |
Convertible ZonedTime SqlValue |
Convertible TimeOfDay SqlValue |
Convertible UTCTime SqlValue |
Convertible NominalDiffTime SqlValue |
Convertible Day SqlValue |
Convertible DiffTime SqlValue |
Convertible SqlValue Bool |
Convertible SqlValue Char |
Convertible SqlValue Double |
Convertible SqlValue Int |
Convertible SqlValue Int32 |
Convertible SqlValue Int64 |
Convertible SqlValue Integer |
Convertible SqlValue Rational |
Convertible SqlValue Word32 |
Convertible SqlValue Word64 |
Convertible SqlValue String |
Convertible SqlValue ByteString |
Convertible SqlValue ByteString |
Convertible SqlValue ClockTime |
Convertible SqlValue CalendarTime |
Convertible SqlValue TimeDiff |
Convertible SqlValue Text |
Convertible SqlValue Text |
Convertible SqlValue LocalTime |
Convertible SqlValue ZonedTime |
Convertible SqlValue TimeOfDay |
Convertible SqlValue UTCTime |
Convertible SqlValue NominalDiffTime |
Convertible SqlValue Day |
Convertible SqlValue DiffTime |
Convertible SqlValue SqlValue |
Convertible SqlValue a => Convertible SqlValue (Maybe a) |
Convertible SqlValue (TimeOfDay, TimeZone) |
Convertible a SqlValue => Convertible (Maybe a) SqlValue |
Convertible (TimeOfDay, TimeZone) SqlValue |

toSql :: Convertible a SqlValue => a -> SqlValue

Convert a value to an `SqlValue`. This function is simply a restricted-type wrapper around `convert`. See extended notes on `SqlValue`.

fromSql :: Convertible SqlValue a => SqlValue -> a

Convert from an `SqlValue` to a Haskell value. Any problem is indicated by calling `error`. This function is simply a restricted-type wrapper around `convert`. See extended notes on `SqlValue`.

safeFromSql :: Convertible SqlValue a => SqlValue -> ConvertResult a

Conversions to and from `SqlValue`s and standard Haskell types.

This function converts from an `SqlValue` to a Haskell value. Many people will use the simpler `fromSql` instead. This function is simply a restricted-type wrapper around `safeConvert`.

nToSql :: Integral a => a -> SqlValue

Converts any Integral type to a `SqlValue` by using toInteger.

iToSql :: Int -> SqlValue

Convenience function for using numeric literals in your program.

posixToSql :: POSIXTime -> SqlValue

Convenience function for converting `POSIXTime` to a `SqlValue`, because `toSql` cannot do the correct thing in this instance.

# Database Connections

class IConnection conn where

Main database handle object.

An `IConnection` object is created by specific functions in the module for an individual database. That is, the connect function -- which creates this object -- is not standardized through the HDBC interface.

A connection is closed by a call to `disconnect`.

A call to `commit` is required to make sure that your changes get committed to the database. In other words, HDBC has no support for autocommit, which we consider an outdated notion.

Methods

disconnect :: conn -> IO ()

Disconnect from the remote database.

You do not need to explicitly close an IConnection object, but you may do so if you so desire. If you don't, the object will disconnect from the database in a sane way when it is garbage-collected. However, a disconnection may raise an error, so you are encouraged to explicitly call `disconnect`. Also, garbage collection may not run when the program terminates, and some databases really like an explicit disconnect.

So, bottom line is, you're best off calling `disconnect` directly, but the world won't end if you forget.

This function discards any data not committed already. Database driver implementators should explicitly call `rollback` if their databases don't do this automatically on disconnect.

Bad Things (TM) could happen if you call this while you have `Statement`s active. In more precise language, the results in such situations are undefined and vary by database. So don't do it.

commit :: conn -> IO ()

Commit any pending data to the database.

Required to make any changes take effect.

rollback :: conn -> IO ()

Roll back to the state the database was in prior to the last `commit` or `rollback`.

runRaw :: conn -> String -> IO ()

Execute an SQL string, which may contain multiple queries. This is intended for situations where you need to run DML or DDL queries and aren't interested in results.

run :: conn -> String -> [SqlValue] -> IO Integer

Execute a single SQL query. Returns the number of rows modified (see `execute` for details). The second parameter is a list of replacement values, if any.

prepare :: conn -> String -> IO Statement

Prepares a statement for execution.

Question marks in the statement will be replaced by positional parameters in a later call to `execute`.

Please note that, depending on the database and the driver, errors in your SQL may be raised either here or by `execute`. Make sure you handle exceptions both places if necessary.

clone :: conn -> IO conn

Create a new `Connection` object, pointed at the same server as this object is. This will generally establish a separate physical connection.

When you wish to establish multiple connections to a single server, the correct way to do so is to establish the first connection with the driver-specific connection function, and then clone it for each additional connection.

This can be important when a database doesn't provide much thread support itself, and the HDBC driver module must serialize access to a particular database.

This can also be a handy utility function whenever you need a separate connection to whatever database you are connected to already.

hdbcDriverName :: conn -> String

The name of the HDBC driver module for this connection. Ideally would be the same as the database name portion of the Cabal package name. For instance, "sqlite3" or "odbc". This is the layer that is bound most tightly to HDBC.

hdbcClientVer :: conn -> String

The version of the C (or whatever) client library that the HDBC driver module is bound to. The meaning of this is driver-specific. For an ODBC or similar proxying driver, this should be the version of the ODBC library, not the eventual DB client driver.

proxiedClientName :: conn -> String

In the case of a system such as ODBC, the name of the database client/server in use, if available. For others, identical to `hdbcDriverName`.

proxiedClientVer :: conn -> String

In the case of a system such as ODBC, the version of the database client in use, if available. For others, identical to `hdbcClientVer`. This is the next layer out past the HDBC driver.

dbServerVer :: conn -> String

The version of the database server, if available.

dbTransactionSupport :: conn -> Bool

Whether or not the current database supports transactions. If False, then `commit` and `rollback` should be expected to raise errors.

MySQL is the only commonly-used database that is known to not support transactions entirely. Please see the MySQL notes in the ODBC driver for more information.

getTables :: conn -> IO [String]

The names of all tables accessible by the current connection, excluding special meta-tables (system tables).

You should expect this to be returned in the same manner as a result from `fetchAllRows'`.

All results should be converted to lowercase for you before you see them.

describeTable :: conn -> String -> IO [(String, SqlColDesc)]

Obtain information about the columns in a specific table. The String in the result set is the column name.

You should expect this to be returned in the same manner as a result from `fetchAllRows'`.

All results should be converted to lowercase for you before you see them.

Instances

IConnection ConnWrapper |

## Wrapped Connections

data ConnWrapper

Sometimes, it is annoying to use typeclasses with Haskell's type system. In those situations, you can use a ConnWrapper. You can create one with:

    let wrapped = ConnWrapper iconn

You can then use this directly, since a ConnWrapper is also an `IConnection`. However, you will not be able to use private database functions on it.

Or, you can use `withWConn`.

Constructors

forall conn . IConnection conn => ConnWrapper conn |

Instances

IConnection ConnWrapper |

withWConn :: forall b. ConnWrapper -> (forall conn. IConnection conn => conn -> b) -> b

Unwrap a `ConnWrapper` and pass the embedded `IConnection` to a function. Example:

    withWConn wrapped run $ "SELECT * from foo where bar = 1" []

## Preparing Queries

sRun :: IConnection conn => conn -> String -> [Maybe String] -> IO Integer

Like `run`, but take a list of Maybe Strings instead of `SqlValue`s.

quickQuery' :: IConnection conn => conn -> String -> [SqlValue] -> IO [[SqlValue]]

Strict version of `quickQuery`.

quickQuery :: IConnection conn => conn -> String -> [SqlValue] -> IO [[SqlValue]]

A quick way to do a query. Similar to preparing, executing, and then calling `fetchAllRows` on a statement. See also `quickQuery'`

## Transaction Handling

This section concerns itself with writing (updating) a database.

In HDBC, as with many RDBMS implementations, every write to the database occurs within a transaction. No changes are visible (outside the current transaction) until a commit operation occurs, in which case all changes since the transaction started are atomically committed. Also, there is a rollback operation that can undo all changes since the transaction started.

HDBC does everything within a transaction. A transaction is implicitly entered when a connection to a database is established, and a transaction is implicitly entered after each call to `commit` or `rollback` as well.

The practical effect of this is that you must call `commit` after making changes to a database in order for those changes to become visible. You don't have to call `commit` after every change, just after a batch of them.

(Exceptions exist for databases that don't offer a high level of transaction isolation; but you should always play it safe and commit anyway.)

Database developers will also be experienced with the atomicity benefits of transactions, an explanation of which is outside the scope of this manual.

Errors occuring at the database level can leave a transaction in an indeterminate state, depending on the database. Some databases will refuse all queries until the next `commit` or `rollback`. The safe thing to do is to issue a `commit` or `rollback` after trapping any `SqlError`. Alternatively, you could use `withTransaction`, which will automatically handle this detail for you.

withTransaction :: IConnection conn => conn -> (conn -> IO a) -> IO a

Execute some code. If any uncaught exception occurs, run `rollback` and re-raise it. Otherwise, run `commit` and return.

This function, therefore, encapsulates the logical property that a transaction is all about: all or nothing.

The `IConnection` object passed in is passed directly to the specified function as a convenience.

This function traps all uncaught exceptions, not just SqlErrors. Therefore, you will get a rollback for any exception that you don't handle. That's probably what you want anyway.

Since all operations in HDBC are done in a transaction, this function doesn't issue an explicit "begin" to the server. You should ideally have called `commit` or `rollback` before calling this function. If you haven't, this function will commit or rollback more than just the changes made in the included action.

If there was an error while running `rollback`, this error will not be reported since the original exception will be propogated back. (You'd probably like to know about the root cause for all of this anyway.) Feedback on this behavior is solicited.

## Connection Inquiries

# Statements

data Statement

## Execution

execute :: Statement -> [SqlValue] -> IO Integer

Execute the prepared statement, passing in the given positional parameters (that should take the place of the question marks in the call to `prepare`).

For non-SELECT queries, the return value is the number of rows modified, if known. If no rows were modified, you get 0. If the value is unknown, you get -1. All current HDBC drivers support this function and should never return -1.

For SELECT queries, you will always get 0.

This function should automatically call finish() to finish the previous execution, if necessary.

executeRaw :: Statement -> IO ()

Execute the statement as-is, without supplying any positional parameters. This is intended for statements for which the results aren't interesting or present (e.g., DDL or DML commands). If your query contains placeholders, this will certainly fail; use `execute` instead.

sExecute :: Statement -> [Maybe String] -> IO Integer

Like `execute`, but take a list of Maybe Strings instead of `SqlValue`s.

executeMany :: Statement -> [[SqlValue]] -> IO ()

Execute the query with many rows. The return value is the return value from the final row as if you had called `execute` on it.

Due to optimizations that are possible due to different databases and driver designs, this can often be significantly faster than using `execute` multiple times since queries need to be compiled only once.

This is most useful for non-SELECT statements.

sExecuteMany :: Statement -> [[Maybe String]] -> IO ()

Like `executeMany`, but take a list of Maybe Strings instead of `SqlValue`s.

## Fetching Results

fetchRow :: Statement -> IO (Maybe [SqlValue])

Fetches one row from the DB. Returns `Nothing` if there are no more rows. Will automatically call `finish` when the last row is read.

fetchRowAL :: Statement -> IO (Maybe [(String, SqlValue)])

Like `fetchRow`, but instead of returning a list, return an association list from column name to value.

The keys of the column names are lowercase versions of the data returned by `getColumnNames`. Please heed the warnings there. Additionally, results are undefined if multiple columns are returned with identical names.

fetchRowMap :: Statement -> IO (Maybe (Map String SqlValue))

Similar to `fetchRowAL`, but return a Map instead of an association list.

sFetchRow :: Statement -> IO (Maybe [Maybe String])

Like `fetchRow`, but return a list of Maybe Strings instead of `SqlValue`s.

fetchAllRows :: Statement -> IO [[SqlValue]]

Lazily fetch all rows from an executed `Statement`.

You can think of this as hGetContents applied to a database result set.

The result of this is a lazy list, and each new row will be read, lazily, from the database as the list is processed.

When you have exhausted the list, the `Statement` will be `finish`ed.

Please note that the careless use of this function can lead to some unpleasant behavior. In particular, if you have not consumed the entire list, then attempt to `finish` or re-execute the statement, and then attempt to consume more elements from the list, the result will almost certainly not be what you want.

But then, similar caveats apply with hGetContents.

Bottom line: this is a very convenient abstraction; use it wisely.

Use `fetchAllRows'` if you need something that is strict, without all these caveats.

fetchAllRows' :: Statement -> IO [[SqlValue]]

Strict version of `fetchAllRows`. Does not have the side-effects of `fetchAllRows`, but forces the entire result set to be buffered in memory.

fetchAllRowsAL :: Statement -> IO [[(String, SqlValue)]]

Like `fetchAllRows`, but instead of returning a list for each row, return an association list for each row, from column name to value.

See `fetchRowAL` for more details.

fetchAllRowsAL' :: Statement -> IO [[(String, SqlValue)]]

Strict version of `fetchAllRowsAL`

fetchAllRowsMap :: Statement -> IO [Map String SqlValue]

Like `fetchAllRowsAL`, but return a list of Maps instead of a list of association lists.

fetchAllRowsMap' :: Statement -> IO [Map String SqlValue]

Strict version of `fetchAllRowsMap`

sFetchAllRows :: Statement -> IO [[Maybe String]]

Like `fetchAllRows`, but return Maybe Strings instead of `SqlValue`s.

sFetchAllRows' :: Statement -> IO [[Maybe String]]

Strict version of `sFetchAllRows`.

getColumnNames :: Statement -> IO [String]

Returns a list of the column names in the result. For maximum portability, you should not assume that information is available until after an `execute` function has been run.

Information is returned here directly as returned by the underlying database layer. Note that different databases have different rules about capitalization of return values and about representation of names of columns that are not simple columns. For this reason, it is suggested that you treat this information for display purposes only. Failing that, you should convert to lower (or upper) case, and use `AS` clauses for anything other than simple columns.

A simple getColumnNames implementation could simply apply `map fst` to the return value of `describeResult`.

## Statement Inquires

describeResult :: Statement -> IO [(String, SqlColDesc)]

Obtain information about the columns in the result set. Must be run only after `execute`. The String in the result set is the column name.

You should expect this to be returned in the same manner as a result from `fetchAllRows'`.

All results should be converted to lowercase for you before you see them.

Please see caveats under `getColumnNames` for information on the column name field here.

## Miscellaneous

finish :: Statement -> IO ()

Abort a query in progress -- usually not needed.

originalQuery :: Statement -> String

The original query that this `Statement` was prepared with.

# Exceptions

data SqlError

The main HDBC exception object. As much information as possible is passed from the database through to the application through this object.

Errors generated in the Haskell layer will have seNativeError set to -1.

Constructors

SqlError |
Fields seState :: String seNativeError :: Int seErrorMsg :: String

Instances

Eq SqlError |
Read SqlError |
Show SqlError |
Typeable SqlError |
Exception SqlError |

throwSqlError :: SqlError -> IO a

A utility function to throw a `SqlError`. The mechanics of throwing such a thing differ between GHC 6.8.x, Hugs, and GHC 6.10. This function takes care of the special cases to make it simpler.

With GHC 6.10, it is a type-restricted alias for throw. On all other systems, it is a type-restricted alias for throwDyn.

catchSql :: IO a -> (SqlError -> IO a) -> IO a

Execute the given IO action.

If it raises a `SqlError`, then execute the supplied handler and return its return value. Otherwise, proceed as normal.

handleSql :: (SqlError -> IO a) -> IO a -> IO a

Like `catchSql`, with the order of arguments reversed.

sqlExceptions :: SqlError -> Maybe SqlError

Given an Exception, return Just SqlError if it was an SqlError, or Nothing otherwise. Useful with functions like catchJust.

handleSqlError :: IO a -> IO a

Catches `SqlError`s, and re-raises them as IO errors with fail. Useful if you don't care to catch SQL errors, but want to see a sane error message if one happens. One would often use this as a high-level wrapper around SQL calls.

# Column Types

These are defined in Database.HDBC.ColTypes but are available to programs importing Database.HDBC by default as well. See Database.HDBC.ColTypes for documentation.

module Database.HDBC.ColTypes

# Threading

FIXME: this is draft information

Thread support in a generalized interface such as HDBC can be complicated because support for threading varies across database interfaces.

However, applications using HDBC should be able to rely upon at least a few basic guarantees:

- The HDBC modules may freely be imported and used across all threads.
- HDBC modules may also freely share database connections and statements; the database or HDBC driver will be responsible for locking if necessary.

I use "share" in the same sense as Python's DB-API: multiple threads may use the resource without wrapping it in any lock.

However, there are some caveats to the above:

- Not all databases support more than one active statement for a single connection. Therefore, for maximum portability, you should use a different connection to the database for each simultaneous query you wish to use. FIXME: describe when a statement is active.
- Not all databases may support the level of multithreading described above. For those that don't, safe access will be restriced in the HDBC driver by using locks. Therefore, you can write portable code, but you only get real multithreading when databases really support it. Details of thread support should be documented in the HDBC driver for each specific database.