Updating incremental merge where condition to handle nullable fields …

…with an appropriate null comparison.

.changes/unreleased/Fixes-20230511-142935.yaml

@@ -0,0 +1,7 @@
+kind: Fixes
+body: Fix incremental merge to use an `is null` check to allow for handling of nullable
+  fields in the unique_key
+time: 2023-05-11T14:29:35.400734+01:00
+custom:
+  Author: amardatar
+  Issue: "7597"

core/dbt/include/global_project/macros/materializations/models/incremental/merge.sql

@@ -62,12 +62,12 @@
 
     {% if unique_key %}
         {% if unique_key is sequence and unique_key is not string %}
-            delete from {{target }}
+            delete from {{ target }}
             using {{ source }}
             where (
                 {% for key in unique_key %}
-                    {{ source }}.{{ key }} = {{ target }}.{{ key }}
-                    {{ "and " if not loop.last}}
+                    ({{ source }}.{{ key }} = {{ target }}.{{ key }} or ({{ source }}.{{ key }} is null and {{ target }}.{{ key }} is null))
+                    {{ "and " if not loop.last }}
                 {% endfor %}
                 {% if incremental_predicates %}
                     {% for predicate in incremental_predicates %}