Add safe add macro (#191)

* Add safe add macro

* Use do instead of set

README.md

@@ -498,6 +498,14 @@ Usage:
 {{ dbt_utils.surrogate_key('field_a', 'field_b'[,...]) }}
 ```
 
+#### safe_add ([source](macros/sql/safe_add.sql))
+Implements a cross-database way to sum nullable fiellds using the fields specified.
+
+Usage:
+```
+{{ dbt_utils.safe_add('field_a', 'field_b'[,...]) }}
+```
+
 #### pivot ([source](macros/sql/pivot.sql))
 This macro pivots values from rows to columns.
 

integration_tests/data/sql/data_safe_add.csv

@@ -0,0 +1,5 @@
+field_1,field_2,field_3,expected
+1,2,3,6
+1,,3,4
+,,2,2
+,,,0

integration_tests/models/sql/schema.yml

@@ -75,6 +75,12 @@ models:
         tests:
           - not_empty_string
 
+  - name: test_safe_add
+    tests:
+      - assert_equal:
+          actual: actual
+          expected: expected
+
   - name: test_pivot
     tests:
       - dbt_utils.equality:

integration_tests/models/sql/test_safe_add.sql

@@ -0,0 +1,12 @@
+
+with data as (
+
+    select * from {{ ref('data_safe_add') }}
+
+)
+
+select
+    {{ dbt_utils.safe_add('field_1', 'field_2', 'field_3') }} as actual,
+    expected
+
+from data

macros/sql/safe_add.sql

@@ -0,0 +1,13 @@
+{%- macro safe_add() -%}
+
+{% set fields = [] %}
+
+{%- for field in varargs -%}
+
+    {% do fields.append("coalesce(" ~ field ~ ", 0)") %}
+
+{%- endfor -%}
+
+{{ fields|join(' +\n  ') }}
+
+{%- endmacro -%}