Include select alias for null expressions

Author: mythz

src/ServiceStack.OrmLite/Expressions/SqlExpression.cs

@@ -2192,9 +2192,11 @@ private object SetAnonTypePropertyNamesForSelectExpression(object expr, Expressi
                 return new PartialSqlString(OrmLiteUtils.UnquotedColumnName(strExpr) != member.Name 
                     ? strExpr + " AS " + member.Name 
                     : strExpr);
-            } 
+            }
 
-            return UseSelectPropertiesAsAliases
+            var usePropertyAlias = UseSelectPropertiesAsAliases ||
+                (expr is PartialSqlString p && Equals(p, PartialSqlString.Null)); // new { Alias = (DateTime?)null }
+            return usePropertyAlias
                 ? new SelectItemExpression(DialectProvider, expr.ToString(), member.Name)
                 : expr;
         }
@@ -2256,7 +2258,7 @@ protected virtual object VisitParameter(ParameterExpression p)
         protected virtual object VisitConstant(ConstantExpression c)
         {
             if (c.Value == null)
-                return new PartialSqlString("null");
+                return PartialSqlString.Null;
 
             return c.Value;
         }
@@ -3049,12 +3051,24 @@ public interface IHasDialectProvider
 
     public class PartialSqlString
     {
+        public static PartialSqlString Null = new("null");
+        
         public PartialSqlString(string text)
         {
             Text = text;
         }
         public string Text { get; internal set; }
         public override string ToString() => Text;
+
+        protected bool Equals(PartialSqlString other) => Text == other.Text;
+        public override bool Equals(object obj)
+        {
+            if (ReferenceEquals(null, obj)) return false;
+            if (ReferenceEquals(this, obj)) return true;
+            if (obj.GetType() != this.GetType()) return false;
+            return Equals((PartialSqlString) obj);
+        }
+        public override int GetHashCode() => (Text != null ? Text.GetHashCode() : 0);
     }
 
     public class EnumMemberAccess : PartialSqlString

tests/ServiceStack.OrmLite.Tests/ExpressionVisitorTests.cs

@@ -493,7 +493,7 @@ public void Can_Where_using_filter_with_Concat()
         public void Can_Where_using_constant_filter()
         {
             object left = null;
-            var right = new PartialSqlString("null");
+            var right = PartialSqlString.Null;
 
             System.Linq.Expressions.Expression<Func<TestType, bool>> filter = x => left == right;
             var q = Db.From<TestType>().Where(filter);//todo: here Where: null is NULL. May be need to change to 1=1 ?

tests/ServiceStack.OrmLite.Tests/Issues/CustomInsertIntoSelectJoinIssue.cs

@@ -73,6 +73,7 @@ public void Can_custom_join_into_select()
                     UserAuthCustomId = w.UserAuthCustomId,
                     DomainKey = w.DomainKey,
                     CreateDate = DateTime.UtcNow,
+                    DeferUntil = (DateTime?) null,
                     TargetDomainKey = t.DomainKey,
                     Tries = 0,
                     TargetDomainRecordId = w.TargetDomainRecordId,