diff --git a/src/NHibernate.Test/Async/DialectTest/SchemaTests/NullInUnique.cs b/src/NHibernate.Test/Async/DialectTest/SchemaTests/NullInUnique.cs
new file mode 100644
index 00000000000..2c06fe1e453
--- /dev/null
+++ b/src/NHibernate.Test/Async/DialectTest/SchemaTests/NullInUnique.cs
@@ -0,0 +1,67 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by AsyncGenerator.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+
+using NHibernate.Cfg.MappingSchema;
+using NHibernate.Dialect;
+using NHibernate.Mapping.ByCode;
+using NUnit.Framework;
+
+namespace NHibernate.Test.DialectTest.SchemaTests
+{
+	using System.Threading.Tasks;
+	[TestFixture]
+	public class NullInUniqueFixtureAsync: TestCaseMappingByCode
+	{
+		protected override HbmMapping GetMappings()
+		{
+			var mapper = new ModelMapper();
+			mapper.Class<Entity>(
+				rc =>
+				{
+					rc.Id(x => x.Id, m => m.Generator(Generators.GuidComb));
+					rc.Property(x => x.Name, m => m.Unique(true));
+					rc.Property(
+						x => x.Name1,
+						m =>
+						{
+							m.NotNullable(true);
+							m.UniqueKey("Test");
+						});
+					rc.Property(x => x.Name2, m => m.UniqueKey("Test"));
+				});
+
+			return mapper.CompileMappingForAllExplicitlyAddedEntities();
+		}
+
+		protected override void OnTearDown()
+		{
+			using (var session = Sfi.OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				session.CreateQuery("delete from Entity").ExecuteUpdate();
+				transaction.Commit();
+			}
+		}
+
+		[Test]
+		public async Task InsertNullInUniqueAsync()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				await (session.SaveAsync(new Entity { Name1 = "1" }));
+				await (session.SaveAsync(new Entity { Name = "N", Name1 = "1", Name2 = "2"}));
+				await (session.SaveAsync(new Entity { Name = "Na", Name1 = "2", Name2 = "1"}));
+				await (session.SaveAsync(new Entity { Name = "Nam", Name1 = "2"}));
+				await (transaction.CommitAsync());
+			}
+		}
+	}
+}
diff --git a/src/NHibernate.Test/DialectTest/SchemaTests/DialectNotSupportingNullInUnique.cs b/src/NHibernate.Test/DialectTest/SchemaTests/DialectNotSupportingNullInUnique.cs
new file mode 100644
index 00000000000..fb94109c222
--- /dev/null
+++ b/src/NHibernate.Test/DialectTest/SchemaTests/DialectNotSupportingNullInUnique.cs
@@ -0,0 +1,48 @@
+using NHibernate.Cfg.MappingSchema;
+using NHibernate.Dialect;
+using NHibernate.Mapping.ByCode;
+using NUnit.Framework;
+
+namespace NHibernate.Test.DialectTest.SchemaTests
+{
+	public class DialectNotSupportingNullInUnique : GenericDialect
+	{
+		public override bool SupportsNullInUnique => false;
+	}
+
+	[TestFixture]
+	public class DialectNotSupportingNullInUniqueFixture
+	{
+		protected HbmMapping GetMappings()
+		{
+			var mapper = new ModelMapper();
+			mapper.Class<Entity>(
+				rc =>
+				{
+					rc.Id(x => x.Id, m => m.Generator(Generators.GuidComb));
+					rc.Property(x => x.Name, m => m.Unique(true));
+					rc.Property(
+						x => x.Name1,
+						m =>
+						{
+							m.NotNullable(true);
+							m.UniqueKey("Test");
+						});
+					rc.Property(x => x.Name2, m => m.UniqueKey("Test"));
+				});
+
+			return mapper.CompileMappingForAllExplicitlyAddedEntities();
+		}
+
+		[Test]
+		public void ScriptGenerationForDialectNotSupportingNullInUnique()
+		{
+			var configuration = TestConfigurationHelper.GetDefaultConfiguration();
+			configuration.AddMapping(GetMappings());
+
+			var script = configuration.GenerateSchemaCreationScript(new DialectNotSupportingNullInUnique());
+
+			Assert.That(script, Has.None.Contains("unique"));
+		}
+	}
+}
diff --git a/src/NHibernate.Test/DialectTest/SchemaTests/Entity.cs b/src/NHibernate.Test/DialectTest/SchemaTests/Entity.cs
new file mode 100644
index 00000000000..633dd5264e9
--- /dev/null
+++ b/src/NHibernate.Test/DialectTest/SchemaTests/Entity.cs
@@ -0,0 +1,12 @@
+using System;
+
+namespace NHibernate.Test.DialectTest.SchemaTests
+{
+	public class Entity
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+		public virtual string Name1 { get; set; }
+		public virtual string Name2 { get; set; }
+	}
+}
diff --git a/src/NHibernate.Test/DialectTest/SchemaTests/NullInUnique.cs b/src/NHibernate.Test/DialectTest/SchemaTests/NullInUnique.cs
new file mode 100644
index 00000000000..19effa5c1d6
--- /dev/null
+++ b/src/NHibernate.Test/DialectTest/SchemaTests/NullInUnique.cs
@@ -0,0 +1,56 @@
+using NHibernate.Cfg.MappingSchema;
+using NHibernate.Dialect;
+using NHibernate.Mapping.ByCode;
+using NUnit.Framework;
+
+namespace NHibernate.Test.DialectTest.SchemaTests
+{
+	[TestFixture]
+	public class NullInUniqueFixture: TestCaseMappingByCode
+	{
+		protected override HbmMapping GetMappings()
+		{
+			var mapper = new ModelMapper();
+			mapper.Class<Entity>(
+				rc =>
+				{
+					rc.Id(x => x.Id, m => m.Generator(Generators.GuidComb));
+					rc.Property(x => x.Name, m => m.Unique(true));
+					rc.Property(
+						x => x.Name1,
+						m =>
+						{
+							m.NotNullable(true);
+							m.UniqueKey("Test");
+						});
+					rc.Property(x => x.Name2, m => m.UniqueKey("Test"));
+				});
+
+			return mapper.CompileMappingForAllExplicitlyAddedEntities();
+		}
+
+		protected override void OnTearDown()
+		{
+			using (var session = Sfi.OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				session.CreateQuery("delete from Entity").ExecuteUpdate();
+				transaction.Commit();
+			}
+		}
+
+		[Test]
+		public void InsertNullInUnique()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				session.Save(new Entity { Name1 = "1" });
+				session.Save(new Entity { Name = "N", Name1 = "1", Name2 = "2"});
+				session.Save(new Entity { Name = "Na", Name1 = "2", Name2 = "1"});
+				session.Save(new Entity { Name = "Nam", Name1 = "2"});
+				transaction.Commit();
+			}
+		}
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/NH3749/TestDialect.cs b/src/NHibernate.Test/NHSpecificTest/NH3749/TestDialect.cs
index a5beb5d72b8..e8ec6ebe300 100644
--- a/src/NHibernate.Test/NHSpecificTest/NH3749/TestDialect.cs
+++ b/src/NHibernate.Test/NHSpecificTest/NH3749/TestDialect.cs
@@ -3,9 +3,9 @@ namespace NHibernate.Test.NHSpecificTest.NH3749
 {
 	public class TestDialect : Dialect.Dialect
 	{
-		public override bool SupportsNotNullUnique
+		public override bool SupportsNullInUnique
 		{
 			get { return false; }
 		}
 	}
-}
\ No newline at end of file
+}
diff --git a/src/NHibernate/Dialect/Dialect.cs b/src/NHibernate/Dialect/Dialect.cs
index 1dcdac383f0..b0689932306 100644
--- a/src/NHibernate/Dialect/Dialect.cs
+++ b/src/NHibernate/Dialect/Dialect.cs
@@ -468,11 +468,25 @@ public virtual bool SupportsCascadeDelete
 			get { return true; }
 		}
 
+		// Since v5.2
+		[Obsolete("Use or override SupportsNullInUnique instead")]
 		public virtual bool SupportsNotNullUnique
 		{
 			get { return true; }
 		}
 
+		/// <summary>
+		/// Does this dialect supports <c>null</c> values in columns belonging to an unique constraint/index?
+		/// </summary>
+		/// <remarks>Some databases do not accept <c>null</c> in unique constraints at all. In such case,
+		/// this property should be overriden for yielding <c>false</c>. This property is not meant for distinguishing
+		/// databases ignoring <c>null</c> when checking uniqueness (ANSI behavior) from those considering <c>null</c>
+		/// as a value and checking for its uniqueness.</remarks>
+		public virtual bool SupportsNullInUnique
+#pragma warning disable 618
+			=> SupportsNotNullUnique;
+#pragma warning restore 618
+
 		public virtual IDataBaseSchema GetDataBaseSchema(DbConnection connection)
 		{
 			throw new NotSupportedException();
diff --git a/src/NHibernate/Mapping/Table.cs b/src/NHibernate/Mapping/Table.cs
index 8fdd0f0528d..41643237fe2 100644
--- a/src/NHibernate/Mapping/Table.cs
+++ b/src/NHibernate/Mapping/Table.cs
@@ -406,7 +406,7 @@ public string SqlCreateString(Dialect.Dialect dialect, IMapping p, string defaul
 
 				if (col.IsUnique)
 				{
-					if (dialect.SupportsUnique)
+					if (dialect.SupportsUnique && (!col.IsNullable || dialect.SupportsNullInUnique))
 					{
 						buf.Append(" unique");
 					}
@@ -669,7 +669,7 @@ public string[] SqlAlterStrings(Dialect.Dialect dialect, IMapping p, ITableMetad
 				}
 
 				bool useUniqueConstraint = column.Unique && dialect.SupportsUnique
-										   && (!column.IsNullable || dialect.SupportsNotNullUnique);
+										   && (!column.IsNullable || dialect.SupportsNullInUnique);
 				if (useUniqueConstraint)
 				{
 					alter.Append(" unique");
diff --git a/src/NHibernate/Mapping/UniqueKey.cs b/src/NHibernate/Mapping/UniqueKey.cs
index 51148cbd3ad..12f5f1db0f8 100644
--- a/src/NHibernate/Mapping/UniqueKey.cs
+++ b/src/NHibernate/Mapping/UniqueKey.cs
@@ -32,7 +32,7 @@ public string SqlConstraintString(Dialect.Dialect dialect)
 				buf.Append(column.GetQuotedName(dialect));
 			}
 			//do not add unique constraint on DB not supporting unique and nullable columns
-			return !nullable || dialect.SupportsNotNullUnique ? buf.Append(StringHelper.ClosedParen).ToString() : null;
+			return !nullable || dialect.SupportsNullInUnique ? buf.Append(StringHelper.ClosedParen).ToString() : null;
 		}
 
 		/// <summary>
@@ -63,7 +63,7 @@ public override string SqlConstraintString(Dialect.Dialect dialect, string const
 			}
 
 			return
-				!nullable || dialect.SupportsNotNullUnique
+				!nullable || dialect.SupportsNullInUnique
 					? StringHelper.Replace(buf.Append(StringHelper.ClosedParen).ToString(), "primary key", "unique")
 					: null;
 		}
@@ -103,7 +103,7 @@ public override string SqlDropString(Dialect.Dialect dialect, string defaultCata
 
 		public override bool IsGenerated(Dialect.Dialect dialect)
 		{
-			if (dialect.SupportsNotNullUnique)
+			if (dialect.SupportsNullInUnique)
 				return true;
 			foreach (Column column in ColumnIterator)
 			{