The Haskell types that stand for SQL Server's, and which SQL types each decodes from and is sent as. The value types of hmssql's own (Money, DateTimeOffset, RawValue, RawParam; Hmssql.SqlValue for a value of any type) are defined or re-exported here; the rest are the usual base, text, bytestring, time, Decimal and uuid-types types.

Decoding: FromSqlField

A column's SQL type is checked against the Haskell type when the result set's columns arrive, before any row is decoded. Maybe a decodes NULL as Nothing for any a below; a NULL in a column decoded as a bare type is a DecodeError.

rowversion is binary(8) on the wire, json is nvarchar(max) and vector is varchar: each decodes as what it is sent as. A CLR user-defined type (hierarchyid, geometry, geography) decodes only as RawValue; CAST it or call .ToString() in the query instead.

Encoding: ToSqlField

A parameter's wire type is fixed by its Haskell type. The server converts it to the column's type where it can, but sending nvarchar against a varchar index, or datetime2 against a datetime column, can turn a seek into a scan; RawParam chooses the wire type yourself. Nothing is a NULL of the type the bare type would be sent as.

Date and time values are rounded to 100 ns as the server rounds them; a value the server would reject (a year outside 1 through 9999, an offset beyond ±14:00) is an EncodeError before anything is sent. A NULL of Text, SqlParam or RawParam is nvarchar(max), which the server converts to most types but not to binary or sql_variant: send a typed NULL (Nothing :: Maybe Int32) there.

When a string's declared type matters

A string or binary parameter's declared type depends on the value's length, as the table shows, and on whether it is NULL. The declared type doesn't limit what can be sent, and it doesn't change the result of storing the value in a column or comparing it with one. It does matter in three cases:

• An expression typed by the parameter. SELECT @p1 returns the declared type, and concatenating bounded types stops at their limit without an error. With two values of 3000 characters, @p1 + @p2 returns 4000 characters and CAST(@p1 AS nvarchar(max)) + @p2 returns 6000. Binary values stop at 8000 bytes in the same way. Cast a parameter whose result can outgrow the limit.
• A sql_variant target. The server won't convert a (max) type to sql_variant, so a longer value or an untyped NULL fails with error 206.
• Plan caching. The server caches a plan per declaration, so one statement can have one plan for short values and another for long values and NULLs. Values within one declaration share a plan whatever their lengths.