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
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.
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
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
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.