Document SQL Server sparse columns

Closes #3017

entity-framework/core/providers/sql-server/columns.md

@@ -0,0 +1,22 @@
+---
+title: Microsoft SQL Server Database Provider - Columns - EF Core
+description: Column features specific to the Entity Framework Core SQL Server provider
+author: roji
+ms.date: 10/1/2021
+uid: core/providers/sql-server/columns
+---
+# Column features specific to the Entity Framework Core SQL Server provider
+
+This page details column configuration options that are specific to the SQL Server provider.
+
+## Sparse columns
+
+Sparse columns are ordinary columns that have an optimized storage for null values, reducing the space requirements for null values at the cost of more overhead to retrieve non-NULL values.
+
+As an example, consider a type hierarchy mapped via [the table-per-hierarchy (TPH) strategy](xref:core/modeling/inheritance#table-per-hierarchy-and-discriminator-configuration). In TPH, a single database table is used to hold all types in a hierarchy; this means that the table must contain columns for each and every property across the entire hierarchy, and for columns belonging to rare types, most rows will contain a NULL value for that column. In these cases, it may make sense to configure the column as *sparse*, in order to reduce the space requirements. The decision whether to make a column sparse must be made by the user, and depends on expectations for actual data in the table.
+
+A column can be made sparse via the Fluent API:
+
+[!code-csharp[SparseColumn](../../../../samples/core/SqlServer/Columns/SparseColumnContext.cs?name=SparseColumn&highlight=5)]
+
+For more information on sparse columns, [see the SQL Server docs](/sql/relational-databases/tables/use-sparse-columns).

entity-framework/toc.yml

@@ -321,6 +321,8 @@
           href: core/providers/sql-server/value-generation.md
         - name: Function mappings
           href: core/providers/sql-server/functions.md
+        - name: Columns
+          href: core/providers/sql-server/columns.md
         - name: Indexes
           href: core/providers/sql-server/indexes.md
         - name: Memory-optimized tables

samples/core/SqlServer/Columns/Blog.cs

@@ -0,0 +1,11 @@
+using System;
+
+namespace SqlServer.Columns
+{
+    public class Blog
+    {
+        public int BlogId { get; set; }
+        public string Url { get; set; }
+        public DateTime PublishedOn { get; set; }
+    }
+}

samples/core/SqlServer/Columns/RareBlog.cs

@@ -0,0 +1,7 @@
+namespace SqlServer.Columns
+{
+    public class RareBlog : Blog
+    {
+        public string RareProperty { get; set; }
+    }
+}

samples/core/SqlServer/Columns/SparseColumnContext.cs

@@ -0,0 +1,19 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace SqlServer.Columns
+{
+    public class SparseColumnContext : DbContext
+    {
+        public DbSet<Blog> Blogs { get; set; }
+        public DbSet<RareBlog> RareBlogs { get; set; }
+
+        #region SparseColumn
+        protected override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+            modelBuilder.Entity<RareBlog>()
+                .Property(b => b.RareProperty)
+                .IsSparse();
+        }
+        #endregion
+    }
+}

samples/core/SqlServer/SqlServer.csproj

@@ -2,11 +2,11 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net5.0</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="5.0.2" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="6.0.0-rc.1.21452.10" />
   </ItemGroup>
 
 </Project>