docs: document populating NOT NULL columns from bulk procedures (#833)

* docs: document populating NOT NULL columns from bulk procedures

Add guidance under 'Adding a column to a table' on keeping a NOT NULL
column backwards compatible during rolling deployments when it is
populated through an OPENJSON/TVP bulk procedure, using ISNULL at each
insert/update site.

* Remove TVP callout

Author: Hinton

docs/contributing/code-style/sql.md

@@ -766,6 +766,54 @@ variants), as this can lead to unnecessary storage overhead and performance issu
 
 :::
 
+##### Populating a `NOT NULL` column from a bulk JSON procedure
+
+When a `NOT NULL` column is populated through a bulk procedure that reads from `OPENJSON` (e.g.
+`OrganizationUser_CreateMany`, `OrganizationUser_UpdateMany`), guard the value with `ISNULL` at
+every insert and update site so that a rolling deployment stays backwards compatible.
+
+A scalar procedure parameter stays backwards compatible through its own default value
+(`@Column BIT = 0`), so an old server that calls the procedure without the new argument still works.
+A JSON field has no equivalent per-field default: during a rolling deployment an old server sends a
+payload that omits the field and `OPENJSON` yields `NULL`. The value you fall back to differs
+between inserts and updates.
+
+For **inserts**, fall back to the column's default, since the row is new.
+
+This breaks during a rolling deployment:
+
+```sql
+INSERT INTO [dbo].[Table] ([Column])
+SELECT
+    OUI.[Column] -- NULL when the payload predates the column
+FROM
+    OPENJSON(@jsonData) WITH ([Column] BIT '$.Column') OUI
+```
+
+This is safe:
+
+```sql
+INSERT INTO [dbo].[Table] ([Column])
+SELECT
+    ISNULL(OUI.[Column], 0)
+FROM
+    OPENJSON(@jsonData) WITH ([Column] BIT '$.Column') OUI
+```
+
+For **updates**, fall back to the existing value, not the default. Falling back to the default lets
+an old server that omits the field overwrite a value that a new server has already written:
+
+```sql
+UPDATE
+    OU
+SET
+    [Column] = ISNULL(OUI.[Column], OU.[Column])
+FROM
+    [dbo].[Table] OU
+    INNER JOIN OPENJSON(@jsonData) WITH ([Id] UNIQUEIDENTIFIER '$.Id', [Column] BIT '$.Column') OUI
+        ON OU.[Id] = OUI.[Id]
+```
+
 #### Changing a column data type
 
 You must wrap the `ALTER TABLE` statement in a conditional block, so that subsequent runs of the