MID doesn't allow a null value for the length parameter. Add a IIF wrapper around it if that argument is marked as nullable.

Added here instead of stringmethodtranslator as we need the nullability optimizations on the sqlfunctionexpression. When wrapped in a caseexpression we get some different optimizations. Produces the correct result on the test case but is different to SQL Server. This way we keep it closer to SQL Server

src/EFCore.Jet/Query/Sql/Internal/JetQuerySqlGenerator.cs

@@ -473,6 +473,32 @@ namespace EntityFrameworkCore.Jet.Query.Sql.Internal
                 return sqlFunctionExpression;
             }
 
+            if (sqlFunctionExpression.Name.Equals("MID", StringComparison.OrdinalIgnoreCase) &&
+                sqlFunctionExpression.Arguments != null && sqlFunctionExpression.Arguments.Count > 2)
+            {
+                if (sqlFunctionExpression.Arguments[2] is ColumnExpression { IsNullable: true })
+                {
+                    Sql.Append("IIF(");
+                    Visit(sqlFunctionExpression.Arguments[2]);
+                    Sql.Append(" IS NULL, NULL, ");
+                    base.VisitSqlFunction(sqlFunctionExpression);
+                    Sql.Append(")");
+                    return sqlFunctionExpression;
+                }
+                if (sqlFunctionExpression.Arguments[2] is SqlUnaryExpression { OperatorType: ExpressionType.Convert } unaryExpression)
+                {
+                    if (unaryExpression.Operand is ColumnExpression { IsNullable: true } || unaryExpression.Operand is SqlFunctionExpression { IsNullable: true })
+                    {
+                        Sql.Append("IIF(");
+                        Visit(unaryExpression.Operand);
+                        Sql.Append(" IS NULL, NULL, ");
+                        base.VisitSqlFunction(sqlFunctionExpression);
+                        Sql.Append(")");
+                        return sqlFunctionExpression;
+                    }
+                }
+            }
+
             return base.VisitSqlFunction(sqlFunctionExpression);
         }
 

test/EFCore.Jet.FunctionalTests/Query/GearsOfWarQueryJetTest.cs

@@ -1101,8 +1101,8 @@ ORDER BY `t`.`Nickname`");
             await base.Select_null_propagation_negative6(isAsync);
 
             AssertSql(
-"""
-SELECT IIF(`g`.`LeaderNickname` IS NOT NULL, FALSE, NULL)
+                """
+SELECT IIF(`g`.`LeaderNickname` IS NOT NULL, IIF(CLNG(LEN(`g`.`LeaderNickname`)) <> CLNG(LEN(`g`.`LeaderNickname`)), TRUE, FALSE), NULL)
 FROM `Gears` AS `g`
 """);
         }
@@ -6223,7 +6223,12 @@ WHERE `g`.`FullName` <> 'Dom' AND EXISTS (
             await base.Null_semantics_is_correctly_applied_for_function_comparisons_that_take_arguments_from_optional_navigation(isAsync);
 
             AssertSql(
-                $@"");
+                """
+    SELECT `t`.`Id`, `t`.`GearNickName`, `t`.`GearSquadId`, `t`.`IssueDate`, `t`.`Note`
+    FROM `Tags` AS `t`
+    LEFT JOIN `Gears` AS `g` ON `t`.`GearNickName` = `g`.`Nickname` AND `t`.`GearSquadId` = `g`.`SquadId`
+    WHERE IIF(`g`.`SquadId` IS NULL, NULL, MID(`t`.`Note`, 0 + 1, `g`.`SquadId`)) = `t`.`GearNickName` OR (((`t`.`Note` IS NULL) OR (`g`.`SquadId` IS NULL)) AND (`t`.`GearNickName` IS NULL))
+    """);
         }
 
         public override async Task
@@ -6233,15 +6238,13 @@ WHERE `g`.`FullName` <> 'Dom' AND EXISTS (
                 isAsync);
 
             AssertSql(
-                $@"SELECT `t`.`Id`, `t`.`GearNickName`, `t`.`GearSquadId`, `t`.`Note`
-FROM `Tags` AS `t`
-LEFT JOIN (
-    SELECT `g`.`Nickname`, `g`.`SquadId`, `g`.`AssignedCityName`, `g`.`CityOfBirthName`, `g`.`Discriminator`, `g`.`FullName`, `g`.`HasSoulPatch`, `g`.`LeaderNickname`, `g`.`LeaderSquadId`, `g`.`Rank`
-    FROM `Gears` AS `g`
-    WHERE `g`.`Discriminator` IN ('Gear', 'Officer')
-) AS `t0` ON (`t`.`GearNickName` = `t0`.`Nickname`) AND (`t`.`GearSquadId` = `t0`.`SquadId`)
-LEFT JOIN `Squads` AS `s` ON `t0`.`SquadId` = `s`.`Id`
-WHERE (SUBSTRING(`t`.`Note`, 0 + 1, CAST(LEN(`s`.`Name`) AS int)) = `t`.`GearNickName`) OR (SUBSTRING(`t`.`Note`, 0 + 1, CAST(LEN(`s`.`Name`) AS int)) IS NULL AND `t`.`GearNickName` IS NULL)");
+                """
+    SELECT `t`.`Id`, `t`.`GearNickName`, `t`.`GearSquadId`, `t`.`IssueDate`, `t`.`Note`
+    FROM (`Tags` AS `t`
+    LEFT JOIN `Gears` AS `g` ON `t`.`GearNickName` = `g`.`Nickname` AND `t`.`GearSquadId` = `g`.`SquadId`)
+    LEFT JOIN `Squads` AS `s` ON `g`.`SquadId` = `s`.`Id`
+    WHERE IIF(LEN(`s`.`Name`) IS NULL, NULL, MID(`t`.`Note`, 0 + 1, CLNG(LEN(`s`.`Name`)))) = `t`.`GearNickName` OR (((`t`.`Note` IS NULL) OR (`s`.`Name` IS NULL)) AND (`t`.`GearNickName` IS NULL))
+    """);
         }
 
         public override async Task Filter_with_new_Guid(bool isAsync)