Use JSON_VALUE() for scalar JSON values if available or workaround weird JSON_EXTRACT() behavior in regards to null otherwise

src/EFCore.MySql/Query/ExpressionVisitors/Internal/MySqlQuerySqlGenerator.cs

@@ -106,13 +106,33 @@ private Expression VisitInlinedParameterExpression(MySqlInlinedParameterExpressi
 
         protected virtual Expression VisitJsonPathTraversal(MySqlJsonTraversalExpression expression)
         {
-            // If the path contains parameters, then the -> and ->> aliases are not supported by MySQL, because
-            // we need to concatenate the path and the parameters.
-            // We will use JSON_EXTRACT (and JSON_UNQUOTE if needed) only in this case, because the aliases
-            // are much more readable.
-            var isSimplePath = expression.Path.All(
-                l => l is SqlConstantExpression ||
-                     l is MySqlJsonArrayIndexExpression e && e.Expression is SqlConstantExpression);
+            var isScalar = expression.Type.IsPrimitive();
+            var useJsonValue = _options.ServerVersion.Supports.JsonValue &&
+                               isScalar;
+
+            if (useJsonValue)
+            {
+                VisitJsonPathTraversalCore(expression, "JSON_VALUE");
+                return expression;
+            }
+
+            // JSON_EXTRACT() returns the SQL string `null` for a JSON value of `null`, instead of a SQL value of `NULL`.
+            // This is an unfortunate oversight by the MySQL team, that is unlikely to be corrected (see
+            // https://bugs.mysql.com/bug.php?id=85755).
+            // While JSON_VALUE() handles this scenario correctly for scalar values, for MySQL versions without JSON_VALUE()
+            // support, we need to explicitly ensure that the returned value is not `null`, or we could return a string
+            // containing `null` independent of what is expected.
+
+            Sql.Append("CASE ");
+
+            VisitJsonPathTraversalCore(
+                expression,
+                "JSON_CONTAINS",
+                new SqlConstantExpression(
+                    Expression.Constant("null"),
+                    _typeMappingSource.GetMapping(typeof(string))));
+
+            Sql.Append(" WHEN TRUE THEN NULL ELSE ");
 
             if (expression.ReturnsText)
             {
@@ -121,67 +141,100 @@ protected virtual Expression VisitJsonPathTraversal(MySqlJsonTraversalExpression
 
             if (expression.Path.Count > 0)
             {
-                Sql.Append("JSON_EXTRACT(");
+                VisitJsonPathTraversalCore(expression, "JSON_EXTRACT");
             }
 
+            if (expression.ReturnsText)
+            {
+                Sql.Append(")");
+            }
+
+            Sql.Append(" END");
+
+            return expression;
+        }
+
+        protected virtual Expression VisitJsonPathTraversalCore(
+            MySqlJsonTraversalExpression expression,
+            string functionName,
+            SqlExpression secondArgumentExpression = null)
+        {
+            Sql.Append(functionName)
+                .Append("(");
+
             Visit(expression.Expression);
 
+            if (secondArgumentExpression is not null)
+            {
+                Sql.Append(", ");
+
+                Visit(secondArgumentExpression);
+            }
+
             if (expression.Path.Count > 0)
             {
                 Sql.Append(", ");
 
-                if (!isSimplePath)
-                {
-                    Sql.Append("CONCAT(");
-                }
+                GenerateJsonPathExpression(expression.Path);
+            }
 
-                Sql.Append("'$");
+            Sql.Append(")");
 
-                foreach (var location in expression.Path)
-                {
-                    if (location is MySqlJsonArrayIndexExpression arrayIndexExpression)
-                    {
-                        var isConstantExpression = arrayIndexExpression.Expression is SqlConstantExpression;
+            return expression;
+        }
 
-                        Sql.Append("[");
+        private void GenerateJsonPathExpression(IReadOnlyList<SqlExpression> path)
+        {
+            // If the path contains parameters, then the -> and ->> aliases are not supported by MySQL, because
+            // we need to concatenate the path and the parameters.
+            // We will use JSON_EXTRACT (and JSON_UNQUOTE if needed) only in this case, because the aliases
+            // are much more readable.
+            var isSimplePath = path.All(
+                l => l is SqlConstantExpression ||
+                     l is MySqlJsonArrayIndexExpression e && e.Expression is SqlConstantExpression);
 
-                        if (!isConstantExpression)
-                        {
-                            Sql.Append("', ");
-                        }
+            if (!isSimplePath)
+            {
+                Sql.Append("CONCAT(");
+            }
 
-                        Visit(arrayIndexExpression.Expression);
+            Sql.Append("'$");
 
-                        if (!isConstantExpression)
-                        {
-                            Sql.Append(", '");
-                        }
+            foreach (var location in path)
+            {
+                if (location is MySqlJsonArrayIndexExpression arrayIndexExpression)
+                {
+                    var isConstantExpression = arrayIndexExpression.Expression is SqlConstantExpression;
 
-                        Sql.Append("]");
-                    }
-                    else
+                    Sql.Append("[");
+
+                    if (!isConstantExpression)
                     {
-                        Sql.Append(".");
-                        Visit(location);
+                        Sql.Append("', ");
                     }
-                }
 
-                Sql.Append("'");
+                    Visit(arrayIndexExpression.Expression);
+
+                    if (!isConstantExpression)
+                    {
+                        Sql.Append(", '");
+                    }
 
-                if (!isSimplePath)
+                    Sql.Append("]");
+                }
+                else
                 {
-                    Sql.Append(")");
+                    Sql.Append(".");
+                    Visit(location);
                 }
-
-                Sql.Append(")");
             }
 
-            if (expression.ReturnsText)
+            Sql.Append("'");
+
+            if (!isSimplePath)
             {
                 Sql.Append(")");
             }
-
-            return expression;
         }
 
         protected override Expression VisitColumn(ColumnExpression columnExpression)

src/EFCore.MySql/Query/Expressions/Internal/MySqlJsonTraversalExpression.cs

@@ -39,7 +39,7 @@ public MySqlJsonTraversalExpression(
             bool returnsText,
             [NotNull] Type type,
             [CanBeNull] RelationalTypeMapping typeMapping)
-            : this(expression, new SqlExpression[0], returnsText, type, typeMapping)
+            : this(expression, new SqlExpression[0], returnsText, type.UnwrapNullableType(), typeMapping)
         {
         }
 

test/EFCore.MySql.FunctionalTests/Query/NorthwindFunctionsQueryMySqlTest.MySql.cs

@@ -813,7 +813,7 @@ await AssertQueryScalar(
 
             AssertSql(
 $"""
-SELECT LOG(7.0, CAST(`o`.`Discount` AS double))
+SELECT LOG(7.0, {MySqlTestHelpers.CastAsDouble("`o`.`Discount`")})
 FROM `Order Details` AS `o`
 WHERE ((`o`.`OrderID` = 11077) AND (`o`.`Discount` > 0)) AND (LOG(7.0, {MySqlTestHelpers.CastAsDouble("`o`.`Discount`")}) < -1.0)
 """);