Fix query when only aggregate fields are requested (#140)

`pypinfo numpy percent3` creates a query with an empty `GROUP BY` clause which results in a BigQuery exception:
`google.api_core.exceptions.BadRequest: 400 Syntax error: Unexpected keyword ORDER at [10:1]`

This commit fixes the logic used to conditionally build the `GROUP BY` clause.

pypinfo/core.py

@@ -149,20 +149,12 @@ def build_query(
     if where:
         query += f'  AND {where}\n'
 
-    if len(fields) > 1:
-        gb = 'GROUP BY\n'
-        initial_length = len(gb)
-
-        non_aggregate_fields = []
-        for field in fields[:-1]:
-            if field not in AGGREGATES:
-                non_aggregate_fields.append(field.name)
-        gb += '  '
-        gb += ', '.join(non_aggregate_fields)
-        gb += '\n'
-
-        if len(gb) > initial_length:
-            query += gb
+    non_aggregate_fields = [field.name for field in fields if field not in AGGREGATES]
+    if non_aggregate_fields:
+        query += 'GROUP BY\n'
+        query += '  '
+        query += ', '.join(non_aggregate_fields)
+        query += '\n'
 
     query += f'ORDER BY\n  {order or Downloads.name} DESC\n'
     query += f'LIMIT {limit}'

pypinfo/fields.py

@@ -32,4 +32,4 @@
 Libc = Field('libc_name', 'details.distro.libc.lib')
 LibcVersion = Field('libc_version', 'details.distro.libc.version')
 
-AGGREGATES = {Percent3, Percent2}
+AGGREGATES = {Downloads, Percent3, Percent2}

tests/test_core.py

@@ -3,7 +3,7 @@
 import pytest
 
 from pypinfo import core
-from pypinfo.fields import File, PythonVersion
+from pypinfo.fields import File, Percent3, PythonVersion
 
 ROWS = [
     ['python_version', 'percent', 'download_count'],
@@ -259,6 +259,37 @@ def test_build_query_no_project():
     assert output == expected
 
 
+def test_build_query_only_aggregate():
+    # pypinfo -sd -2 -ed -1 -l 20 numpy percent3
+    project = "numpy"
+    all_fields = [Percent3]
+    start_date = "-2"
+    end_date = "-1"
+    days = None
+    limit = 20
+    where = None
+    order = None
+    pip = True
+    expected = r"""
+SELECT
+  ROUND(100 * SUM(CASE WHEN REGEXP_EXTRACT(details.python, r"^([^\.]+)") = "3" THEN 1 ELSE 0 END) / COUNT(*), 1) as percent_3,
+  COUNT(*) as download_count,
+FROM `bigquery-public-data.pypi.file_downloads`
+WHERE timestamp BETWEEN TIMESTAMP_ADD(CURRENT_TIMESTAMP(), INTERVAL -2 DAY) AND TIMESTAMP_ADD(CURRENT_TIMESTAMP(), INTERVAL -1 DAY)
+  AND file.project = "numpy"
+  AND details.installer.name = "pip"
+ORDER BY
+  download_count DESC
+LIMIT 20
+        """.strip()  # noqa: E501
+
+    # Act
+    output = core.build_query(project, all_fields, start_date, end_date, days, limit, where, order, pip)
+
+    # Assert
+    assert output == expected
+
+
 def test_build_query_bad_end_date():
     # Arrange
     project = "pycodestyle"