Fix/557 split part negative part number (#559)

* Add support for negative part number for Postgres, Redshift and BigQuery

* Add test for negative part_number

* Keep the positive behaviour for part_number = 0

* Update CHANGELOG

Co-authored-by: Doug Beatty <doug.beatty@dbtlabs.com>
Co-authored-by: Doug Beatty <44704949+dbeatty10@users.noreply.github.com>

Author: b-per

CHANGELOG.md

@@ -1,9 +1,11 @@
 # Unreleased
 ## New features
 - Add an optional `where` clause parameter to `get_column_values()` to filter values returned ([#511](https://github.com/dbt-labs/dbt-utils/issues/511), [#583](https://github.com/dbt-labs/dbt-utils/pull/583)) 
-
-## New features
 - Add `where` parameter to `union_relations` macro ([#554](https://github.com/dbt-labs/dbt-utils/pull/554))
+
+## Fixes
+- Enable a negative part_number for `split_part()` ([#557](https://github.com/dbt-labs/dbt-utils/issues/557), [#559](https://github.com/dbt-labs/dbt-utils/pull/559))
+
 ## Quality of life
 - Documentation about listagg macro ([#544](https://github.com/dbt-labs/dbt-utils/issues/544), [#560](https://github.com/dbt-labs/dbt-utils/pull/560))
 - Fix links to macro section in table of contents ([#555](https://github.com/dbt-labs/dbt-utils/pull/555))
@@ -18,6 +20,7 @@
 - [@judahrand](https://github.com/judahrand) (#552)
 - [@clausherther](https://github.com/clausherther) (#555)
 - [@epapineau](https://github.com/epapineau) (#583)
+- [@b-per](https://github.com/b-per) (#559)
 
 # dbt-utils v0.8.4
 ## Fixes

integration_tests/models/cross_db_utils/test_split_part.sql

@@ -26,3 +26,11 @@ select
     result_3 as expected
 
 from data
+
+union all
+
+select
+    {{ dbt_utils.split_part('parts', 'split_on', -1) }} as actual,
+    result_3 as expected
+
+from data

macros/cross_db_utils/split_part.sql

@@ -14,11 +14,59 @@
 {% endmacro %}
 
 
+{% macro _split_part_negative(string_text, delimiter_text, part_number) %}
+
+    split_part(
+        {{ string_text }},
+        {{ delimiter_text }},
+          length({{ string_text }}) 
+          - length(
+              replace({{ string_text }},  {{ delimiter_text }}, '')
+          ) + 2 {{ part_number }}
+        )
+
+{% endmacro %}
+
+
+{% macro postgres__split_part(string_text, delimiter_text, part_number) %}
+
+  {% if part_number >= 0 %}
+    {{ dbt_utils.default__split_part(string_text, delimiter_text, part_number) }}
+  {% else %}
+    {{ dbt_utils._split_part_negative(string_text, delimiter_text, part_number) }}
+  {% endif %}
+
+{% endmacro %}
+
+
+{% macro redshift__split_part(string_text, delimiter_text, part_number) %}
+
+  {% if part_number >= 0 %}
+    {{ dbt_utils.default__split_part(string_text, delimiter_text, part_number) }}
+  {% else %}
+    {{ dbt_utils._split_part_negative(string_text, delimiter_text, part_number) }}
+  {% endif %}
+
+{% endmacro %}
+
+
 {% macro bigquery__split_part(string_text, delimiter_text, part_number) %}
 
+  {% if part_number >= 0 %}
     split(
         {{ string_text }},
         {{ delimiter_text }}
         )[safe_offset({{ part_number - 1 }})]
+  {% else %}
+    split(
+        {{ string_text }},
+        {{ delimiter_text }}
+        )[safe_offset(
+          length({{ string_text }}) 
+          - length(
+              replace({{ string_text }},  {{ delimiter_text }}, '')
+          ) + 1
+        )]
+  {% endif %}
 
 {% endmacro %}