Add uniqueness documentation for database identifiers

docs/relational-databases/databases/database-identifiers.md

@@ -39,6 +39,43 @@ CREATE TABLE TableX (
 
 This table also has an unnamed constraint. The `PRIMARY KEY` constraint has no identifier.
 
+Some identifiers, such as constraint names and other schema-scoped objects, must be unique within a database schema. For example, two primary key constraints in the same schema can't share a name, so the second table creation statement isn't allowed:
+
+```sql
+USE AdventureWorks2022;
+GO
+
+CREATE TABLE [SalesOrderGeneral Table] (
+   [Order] INT NOT NULL,
+   [SalesOrderID] INT IDENTITY(1, 1) NOT NULL,
+   [SalesOrderDetailID] INT NOT NULL,
+   [ModifiedDate] DATETIME NOT NULL,
+   CONSTRAINT [PK_SalesOrder] PRIMARY KEY CLUSTERED (
+     [Order] ASC,
+     [SalesOrderID] ASC
+   )
+);
+GO
+
+-- Primary key identifier conflicts with existing primary key, and so will result in an error.
+CREATE TABLE [SalesOrderDetail Table] (
+   [Order] INT NOT NULL,
+   [SalesOrderDetailID] INT IDENTITY(1, 1) NOT NULL,
+   [OrderQty] SMALLINT NOT NULL,
+   [ProductID] INT NOT NULL,
+   [UnitPrice] MONEY NOT NULL,
+   [UnitPriceDiscount] MONEY NOT NULL,
+   [ModifiedDate] DATETIME NOT NULL,
+   CONSTRAINT [PK_SalesOrder] PRIMARY KEY CLUSTERED (
+       [Order] ASC,
+       [SalesOrderDetailID] ASC
+   )
+);
+GO
+```
+
+However, each table can contain its own column named `Order`, because column names only need to be unique within each table, not within the schema.
+
 The collation of an identifier depends on the level at which it's defined. Identifiers of instance-level objects, such as logins and database names, are assigned the default collation of the instance. Identifiers of objects in a database, such as tables, views, and column names, are assigned the default collation of the database. For example, two tables with names that differ only in case can be created in a database that has case-sensitive collation, but can't be created in a database that has case-insensitive collation.
 
 > [!NOTE]