Safe logic for concurrent executions

dbt/include/snowflake/macros/adapters.sql

@@ -268,7 +268,7 @@
     {% set sql -%}
        alter {{ relation_type }} {{ relation.render() }} add column
           {% for column in add_columns %}
-            {{ column.name }} {{ column.data_type }}{{ ',' if not loop.last }}
+            if not exists {{ column.name }} {{ column.data_type }}{{ ',' if not loop.last }}
           {% endfor %}
     {%- endset -%}
 
@@ -281,7 +281,7 @@
     {% set sql -%}
         alter {{ relation_type }} {{ relation.render() }} drop column
             {% for column in remove_columns %}
-                {{ column.name }}{{ ',' if not loop.last }}
+              if exists {{ column.name }}{{ ',' if not loop.last }}
             {% endfor %}
     {%- endset -%}