Output a warning when star finds no columns, not '*' (#732)

* Change star() behaviour when no columns returned

* Code review: return a * in compile mode

integration_tests/models/sql/schema.yml

@@ -178,6 +178,12 @@ models:
       - dbt_utils.equality:
           compare_model: ref('data_star_expected')
 
+  - name: test_star_no_columns
+    columns: 
+      - name: canary_column #If the no-columns state isn't hit, this table won't be queryable because there will be a missing comma
+        tests: 
+          - not_null
+
   - name: test_generate_surrogate_key
     tests:
       - assert_equal:

integration_tests/models/sql/test_star_no_columns.sql

@@ -0,0 +1,11 @@
+with data as (
+
+    select
+        {{ dbt_utils.star(from=ref('data_star'), except=['field_1', 'field_2', 'field_3']) }}
+        -- if star() returns `*` or a list of columns, this query will fail because there's no comma between the columns
+        1 as canary_column
+    from {{ ref('data_star') }}
+
+)
+
+select * from data

macros/sql/star.sql

@@ -8,13 +8,23 @@
 
     {#-- Prevent querying of db in parsing mode. This works because this macro does not create any new refs. #}
     {%- if not execute -%}
-        {{ return('*') }}
+        {% do return('*') %}
     {%- endif -%}
 
     {% set cols = dbt_utils.get_filtered_columns_in_relation(from, except) %}
 
     {%- if cols|length <= 0 -%}
-      {{- return('*') -}}
+        {% if flags.WHICH == 'compile' %}
+            {% set response %}
+*
+/* No columns were returned. Maybe the relation doesn't exist yet 
+or all columns were excluded. This star is only output during  
+dbt compile, and exists to keep SQLFluff happy. */
+            {% endset %}
+            {% do return(response) %}
+        {% else %}
+            {% do return("/* no columns returned from star() macro */") %}
+        {% endif %}
     {%- else -%}
         {%- for col in cols %}
             {%- if relation_alias %}{{ relation_alias }}.{% else %}{%- endif -%}