Fix transform to convert empty strings to NULL when changing to integer or float type

sqlite_utils/db.py

@@ -2820,10 +2820,24 @@ def fk_with_renamed_columns(fk: ForeignKey) -> ForeignKey:
         if "rowid" not in new_cols:
             new_cols.insert(0, "rowid")
             old_cols.insert(0, "rowid")
+        # Columns explicitly converted to a numeric type need NULLIF(col, '') so
+        # that empty strings stored in a previously TEXT column become NULL rather
+        # than being coerced to 0 or raising a type error.
+        _numeric_kws = ("INT", "REAL", "FLOA", "DOUB", "NUMERIC", "DECIMAL")
+
+        def _col_expr(from_, to_):
+            if from_ in types:
+                raw = COLUMN_TYPE_MAPPING.get(types[from_])
+                if raw is None and isinstance(types[from_], str):
+                    raw = types[from_]
+                if raw and any(kw in raw.upper() for kw in _numeric_kws):
+                    return "NULLIF({}, '')".format(quote_identifier(from_))
+            return quote_identifier(from_)
+
         copy_sql = "INSERT INTO {} ({new_cols})\n   SELECT {old_cols} FROM {};".format(
             quote_identifier(new_table_name),
             quote_identifier(self.name),
-            old_cols=", ".join(quote_identifier(col) for col in old_cols),
+            old_cols=", ".join(_col_expr(f, t) for f, t in zip(old_cols, new_cols)),
             new_cols=", ".join(quote_identifier(col) for col in new_cols),
         )
         sqls.append(copy_sql)

tests/test_transform.py

@@ -21,7 +21,7 @@
             {"types": {"age": int}},
             [
                 'CREATE TABLE "dogs_new_suffix" (\n   "id" INTEGER PRIMARY KEY,\n   "name" TEXT,\n   "age" INTEGER\n);',
-                'INSERT INTO "dogs_new_suffix" ("rowid", "id", "name", "age")\n   SELECT "rowid", "id", "name", "age" FROM "dogs";',
+                'INSERT INTO "dogs_new_suffix" ("rowid", "id", "name", "age")\n   SELECT "rowid", "id", "name", NULLIF("age", \'\') FROM "dogs";',
                 'DROP TABLE "dogs";',
                 'ALTER TABLE "dogs_new_suffix" RENAME TO "dogs";',
             ],
@@ -51,7 +51,7 @@
             {"types": {"age": int}, "rename": {"age": "dog_age"}},
             [
                 'CREATE TABLE "dogs_new_suffix" (\n   "id" INTEGER PRIMARY KEY,\n   "name" TEXT,\n   "dog_age" INTEGER\n);',
-                'INSERT INTO "dogs_new_suffix" ("rowid", "id", "name", "dog_age")\n   SELECT "rowid", "id", "name", "age" FROM "dogs";',
+                'INSERT INTO "dogs_new_suffix" ("rowid", "id", "name", "dog_age")\n   SELECT "rowid", "id", "name", NULLIF("age", \'\') FROM "dogs";',
                 'DROP TABLE "dogs";',
                 'ALTER TABLE "dogs_new_suffix" RENAME TO "dogs";',
             ],
@@ -144,7 +144,7 @@ def tracer(sql, params):
             {"types": {"age": int}},
             [
                 'CREATE TABLE "dogs_new_suffix" (\n   "id" INTEGER,\n   "name" TEXT,\n   "age" INTEGER\n);',
-                'INSERT INTO "dogs_new_suffix" ("rowid", "id", "name", "age")\n   SELECT "rowid", "id", "name", "age" FROM "dogs";',
+                'INSERT INTO "dogs_new_suffix" ("rowid", "id", "name", "age")\n   SELECT "rowid", "id", "name", NULLIF("age", \'\') FROM "dogs";',
                 'DROP TABLE "dogs";',
                 'ALTER TABLE "dogs_new_suffix" RENAME TO "dogs";',
             ],
@@ -669,6 +669,20 @@ def test_transform_with_indexes_errors(fresh_db, transform_params):
     )
 
 
+def test_transform_converts_empty_strings_to_null_for_numeric_types(fresh_db):
+    # Regression test for: transform should convert '' to NULL when changing
+    # a TEXT column to an INTEGER or FLOAT type (issue #488).
+    fresh_db["test"].insert_all([
+        {"id": "1", "age": "3", "weight": "2.5", "name": "Alice"},
+        {"id": "2", "age": "", "weight": "", "name": ""},
+    ])
+    fresh_db["test"].transform(types={"age": int, "weight": float})
+    rows = list(fresh_db["test"].rows)
+    assert rows[0] == {"id": "1", "age": 3, "weight": 2.5, "name": "Alice"}
+    # Empty strings in numeric columns become NULL; text columns are unchanged
+    assert rows[1] == {"id": "2", "age": None, "weight": None, "name": ""}
+
+
 def test_transform_with_unique_constraint_implicit_index(fresh_db):
     dogs = fresh_db["dogs"]
     # Create a table with a UNIQUE constraint on 'name', which creates an implicit index