Add unique_combination_of_columns test (#177)

README.md

@@ -346,6 +346,30 @@ the lower bound of the next record (common for date ranges).
 | 0           | 1           |
 | 2           | 3           |
 | 4           | 5           |
+
+#### unique_combination_of_columns ([source](macros/schema_tests/unique_combination_of_columns.sql))
+This test confirms that the combination of columns is unique. For example, the
+combination of month and product is unique, however neither column is unique
+in isolation.
+
+We generally recommend testing this uniqueness condition by either:
+* generating a [surrogate_key](#surrogate_key-source) for your model and testing
+the uniqueness of said key, OR
+* passing the `unique` test a coalesce of the columns (as discussed [here](https://docs.getdbt.com/docs/testing#section-testing-expressions)).
+
+However, these approaches can become non-perfomant on large data sets, in which
+case we recommend using this test instead.
+
+**Usage:**
+```yaml
+- name: revenue_by_product_by_month
+  tests:
+    - dbt_utils.unique_combination_of_columns:
+        combination_of_columns:
+          - month
+          - product
+```
+
 ---
 ### SQL helpers
 #### get_query_results_as_dict ([source](macros/sql/get_query_results_as_dict.sql))

integration_tests/data/schema_tests/data_unique_combination_of_columns.csv

@@ -0,0 +1,7 @@
+month,product,revenue
+2019-01-01,jaffle,500
+2019-01-01,lamington,100
+2019-01-01,pavlova,600
+2019-02-01,jaffle,300
+2019-02-01,lamington,300
+2019-02-01,pavlova,400

integration_tests/models/schema_tests/schema.yml

@@ -80,3 +80,10 @@ models:
           upper_bound_column: coalesce(valid_to, '2099-01-01')
           partition_by: subscription_id
           gaps: required
+
+  - name: data_unique_combination_of_columns
+    tests:
+      - dbt_utils.unique_combination_of_columns:
+          combination_of_columns:
+            - month
+            - product

macros/schema_tests/unique_combination_of_columns.sql

@@ -0,0 +1,22 @@
+{% macro test_unique_combination_of_columns(model) %}
+
+{%- set columns = kwargs.get('combination_of_columns', kwargs.get('arg')) %}
+
+{%- set columns_csv=columns | join(', ') %}
+
+with validation_errors as (
+
+    select
+        {{ columns_csv }}
+    from {{ model }}
+
+    group by {{ columns_csv }}
+    having count(*) > 1
+
+)
+
+select count(*)
+from validation_errors
+
+
+{% endmacro %}