transform: coerce empty strings to NULL when converting TEXT columns to numeric types (#805)

* transform: coerce empty strings to NULL when converting TEXT columns to numeric types

When a TEXT column is transformed to INTEGER, FLOAT, or REAL and a row
contains an empty string, the empty string is now converted to NULL during
the INSERT...SELECT copy, matching the expected behavior described in #488.

Fixes #488

docs/changelog.rst

@@ -25,6 +25,7 @@ Unreleased
 - ``sqlite-utils convert --dry-run`` now works for table and column names containing closing square brackets. (:issue:`829`)
 - ``table.indexes`` and ``table.xindexes`` now work for table, index and column names containing double quotes. This also fixes ``table.transform()`` for tables with those identifiers. Thanks, `nyxst4ck <https://github.com/nyxst4ck>`__. (:issue:`824`, `#825 <https://github.com/simonw/sqlite-utils/pull/825>`__)
 - Improved type annotations throughout the package and added Pyright regression checks to CI. (:issue:`833`)
+- Changing a ``TEXT`` column to ``INTEGER``, ``FLOAT`` or ``REAL`` using ``table.transform()`` or ``sqlite-utils transform`` now converts exact empty strings to ``NULL``. Previously they remained empty strings in the numeric column. Thanks, `ikatyal2110 <https://github.com/ikatyal2110>`__. (:issue:`488`, `#805 <https://github.com/simonw/sqlite-utils/pull/805>`__)
 
 .. _v3_39_1:
 

docs/cli.rst

@@ -2236,7 +2236,7 @@ The ``transform`` command allows you to apply complex transformations to a table
 Every option for this table (with the exception of ``--pk-none``) can be specified multiple times. The options are as follows:
 
 ``--type column-name new-type``
-    Change the type of the specified column. Valid types are ``integer``, ``text``, ``float``, ``real``, ``blob`` and ``any``.
+    Change the type of the specified column. Valid types are ``integer``, ``text``, ``float``, ``real``, ``blob`` and ``any``. Changing a ``TEXT`` column to ``INTEGER``, ``FLOAT`` or ``REAL`` converts exact empty-string values to ``NULL``.
 
 ``--drop column-name``
     Drop the specified column.

docs/python-api.rst

@@ -1826,6 +1826,8 @@ To alter the type of a column, use the ``types=`` argument:
     # Convert the 'age' column to an integer, and 'weight' to a float
     table.transform(types={"age": int, "weight": float})
 
+When a ``TEXT`` column is changed to ``INTEGER``, ``FLOAT`` or ``REAL``, exact empty-string values are stored as ``NULL``. Other values, including whitespace-only strings, are copied normally.
+
 See :ref:`python_api_add_column` for a list of available types.
 
 .. _python_api_transform_strict: