Merge pull request #35 from dwallace0723/add-cardinality-test

Add cardinality equality schema test

README.md

@@ -72,7 +72,7 @@ model_name:
 ```
 
 #### at_least_one ([source](macros/schema_tests/at_least_one.sql))
-This schema test asserts if column has at least one value. 
+This schema test asserts if column has at least one value.
 
 Usage:
 ```
@@ -95,6 +95,17 @@ model_name:
 
 ```
 
+#### cardinality_equality ([source](macros/schema_tests/cardinality_equality.sql))
+This schema test asserts if values in a given column have exactly the same cardinality as values from a different column in a different model.
+
+Usage:
+```
+model_name:
+  constraints:
+    cardinality_equality:
+    - {from: column_name, to: ref('other_model_name'), field: other_column_name}
+```
+
 ---
 ### SQL helpers
 #### group_by ([source](macros/sql/groupby.sql))

macros/schema_tests/cardinality_equality.sql

@@ -0,0 +1,46 @@
+{% macro test_cardinality_equality(model, from, to, field) %}
+
+with table_a as (
+select
+  {{ from }},
+  count(*) as num_rows
+from {{ model }}
+group by 1
+),
+
+table_b as (
+select
+  {{ field }},
+  count(*) as num_rows
+from {{ to }}
+group by 1
+),
+
+except_a as (
+  select *
+  from table_a
+  except
+  select *
+  from table_b
+),
+
+except_b as (
+  select *
+  from table_b
+  except
+  select *
+  from table_a
+),
+
+unioned as (
+  select *
+  from except_a
+  union all
+  select *
+  from except_b
+)
+
+select count(*)
+from unioned
+
+{% endmacro %}