- Document how SQL Server does MAX with VARCHAR, NVARCHAR

Fixes #3760

Change-Id: I0613eb66bfdc9d7118688c74e29c8da322c3b4db

1 files changed, 20 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/mssql/base.py b/lib/sqlalchemy/dialects/mssql/base.py
index bc1ad5cdf..d1c7452a1 100644
--- a/lib/sqlalchemy/dialects/mssql/base.py
+++ b/lib/sqlalchemy/dialects/mssql/base.py
@@ -166,6 +166,26 @@ how SQLAlchemy handles this:
 This
 is an auxilliary use case suitable for testing and bulk insert scenarios.
 
+MAX on VARCHAR / NVARCHAR
+-------------------------
+
+SQL Server supports the special string "MAX" within the
+:class:`.sqltypes.VARCHAR` and :class:`.sqltypes.NVARCHAR` datatypes,
+to indicate "maximum length possible".   The dialect currently handles this as
+a length of "None" in the base type, rather than supplying a
+dialect-specific version of these types, so that a base type
+specified such as ``VARCHAR(None)`` can assume "unlengthed" behavior on
+more than one backend without using dialect-specific types.
+
+To build a SQL Server VARCHAR or NVARCHAR with MAX length, use None::
+
+    my_table = Table(
+        'my_table', metadata,
+        Column('my_data', VARCHAR(None)),
+        Column('my_n_data', NVARCHAR(None))
+    )
+
+
 Collation Support
 -----------------