diff --git a/src/mizugaki/parser/sql_driver.cpp b/src/mizugaki/parser/sql_driver.cpp
index e66e4f0..f4ee9ee 100644
--- a/src/mizugaki/parser/sql_driver.cpp
+++ b/src/mizugaki/parser/sql_driver.cpp
@@ -9,6 +9,9 @@
 
 #include <mizugaki/ast/name/qualified.h>
 #include <mizugaki/ast/type/user_defined.h>
+#include <mizugaki/ast/literal/numeric.h>
+#include <mizugaki/ast/scalar/literal_expression.h>
+#include <mizugaki/ast/scalar/unary_expression.h>
 #include <mizugaki/ast/scalar/variable_reference.h>
 #include <mizugaki/ast/scalar/builtin_function_invocation.h>
 
@@ -163,5 +166,42 @@ sql_driver::to_delimited_identifier(ast::common::chars const& str, location_type
             location);
 }
 
+sql_driver::node_ptr<ast::scalar::expression>
+sql_driver::try_fold_literal(node_ptr<ast::scalar::expression> expression) {
+    if (expression->node_kind() != ast::scalar::unary_expression::tag) {
+        return expression;
+    }
+
+    auto&& unary = unsafe_downcast<ast::scalar::unary_expression>(*expression);
+    if (unary.operator_kind() != ast::scalar::unary_operator::plus &&
+            unary.operator_kind() != ast::scalar::unary_operator::minus) {
+        return expression;
+    }
+    if (unary.operand()->node_kind() != ast::scalar::literal_expression::tag) {
+        return expression;
+    }
+
+    auto&& literal = unsafe_downcast<ast::scalar::literal_expression>(*unary.operand()).value();
+    if (literal->node_kind() != ast::literal::kind::exact_numeric &&
+            literal->node_kind() != ast::literal::kind::approximate_numeric) {
+        return expression;
+    }
+
+    // only set if the original literal does not have a sign
+    auto&& value = unsafe_downcast<ast::literal::numeric>(*literal);
+    if (value.sign().has_value()) {
+        return expression;
+    }
+
+    if (unary.operator_kind() == ast::scalar::unary_operator::plus) {
+        value.sign() = { ast::literal::sign::plus, unary.operator_kind().region() };
+    } else {
+        value.sign() = { ast::literal::sign::minus, unary.operator_kind().region() };
+    }
+    unary.operand()->region() = expression->region();
+
+    return std::move(unary.operand());
+}
+
 } // namespace mizugaki::parser
 
diff --git a/src/mizugaki/parser/sql_driver.h b/src/mizugaki/parser/sql_driver.h
index 7267a65..33cb66d 100644
--- a/src/mizugaki/parser/sql_driver.h
+++ b/src/mizugaki/parser/sql_driver.h
@@ -2,7 +2,6 @@
 
 #include <cstddef>
 
-
 #include <mizugaki/ast/common/vector.h>
 #include <mizugaki/ast/name/name.h>
 #include <mizugaki/ast/name/simple.h>
@@ -90,6 +89,8 @@ class sql_driver {
 
     [[nodiscard]] node_ptr<ast::type::type> try_build_type(node_ptr<ast::scalar::expression>& expr);
 
+    [[nodiscard]] node_ptr<ast::scalar::expression> try_fold_literal(node_ptr<ast::scalar::expression> expression);
+
 private:
     ::takatori::util::maybe_shared_ptr<document_type const> document_;
     std::vector<location_type> comments_;
diff --git a/src/mizugaki/parser/sql_parser.yy b/src/mizugaki/parser/sql_parser.yy
index f457e5d..da5c47d 100644
--- a/src/mizugaki/parser/sql_parser.yy
+++ b/src/mizugaki/parser/sql_parser.yy
@@ -2839,17 +2839,19 @@ scalar_value_expression
         }
     | "+"[o] scalar_value_expression[e] %prec UNARY_PLUS
         {
-            $$ = driver.node<ast::scalar::unary_expression>(
-                    regioned { ast::scalar::unary_operator::plus, @o },
-                    $e,
-                    @$);
+            $$ = driver.try_fold_literal(
+                    driver.node<ast::scalar::unary_expression>(
+                            regioned { ast::scalar::unary_operator::plus, @o },
+                            $e,
+                            @$));
         }
     | "-"[o] scalar_value_expression[e] %prec UNARY_MINUS
         {
-            $$ = driver.node<ast::scalar::unary_expression>(
-                    regioned { ast::scalar::unary_operator::minus, @o },
-                    $e,
-                    @$);
+            $$ = driver.try_fold_literal(
+                    driver.node<ast::scalar::unary_expression>(
+                            regioned { ast::scalar::unary_operator::minus, @o },
+                            $e,
+                            @$));
         }
     // 6.28 <datetime value expression> - <time zone>
     | value_expression_primary[e] AT[o1] LOCAL[o2]
@@ -2886,17 +2888,19 @@ scalar_value_expression
 signed_value_expression_primary
     : "+"[s] value_expression_primary[e]
         {
-            $$ = driver.node<ast::scalar::unary_expression>(
-                    regioned { ast::scalar::unary_operator::plus, @s },
-                    $e,
-                    @$);
+            $$ = driver.try_fold_literal(
+                    driver.node<ast::scalar::unary_expression>(
+                            regioned { ast::scalar::unary_operator::plus, @s },
+                            $e,
+                            @$));
         }
     | "-"[s] value_expression_primary[e]
         {
-            $$ = driver.node<ast::scalar::unary_expression>(
-                    regioned { ast::scalar::unary_operator::minus, @s },
-                    $e,
-                    @$);
+            $$ = driver.try_fold_literal(
+                    driver.node<ast::scalar::unary_expression>(
+                            regioned { ast::scalar::unary_operator::minus, @s },
+                            $e,
+                            @$));
         }
     | value_expression_primary[e]
         {
diff --git a/test/mizugaki/parser/sql_parser_scalar_test.cpp b/test/mizugaki/parser/sql_parser_scalar_test.cpp
index 011058a..4ff45d0 100644
--- a/test/mizugaki/parser/sql_parser_scalar_test.cpp
+++ b/test/mizugaki/parser/sql_parser_scalar_test.cpp
@@ -4,7 +4,6 @@
 
 #include <mizugaki/ast/statement/select_statement.h>
 
-#include <mizugaki/ast/query/query.h>
 #include <mizugaki/ast/query/table_value_constructor.h>
 #include <mizugaki/ast/query/table_reference.h>
 
@@ -478,6 +477,32 @@ TEST_F(sql_parser_scalar_test, unary_expression_minus) {
     }));
 }
 
+TEST_F(sql_parser_scalar_test, unary_expression_plus_literal) {
+    auto result = parse("+1");
+    ASSERT_TRUE(result) << diagnostics(result);
+
+    EXPECT_EQ(extract(result), (scalar::literal_expression {
+            ast::literal::numeric {
+                    ast::literal::kind::exact_numeric,
+                    ast::literal::sign::plus,
+                    { "1" },
+            },
+    }));
+}
+
+TEST_F(sql_parser_scalar_test, unary_expression_minus_literal) {
+    auto result = parse("-1");
+    ASSERT_TRUE(result) << diagnostics(result);
+
+    EXPECT_EQ(extract(result), (scalar::literal_expression {
+            ast::literal::numeric {
+                    ast::literal::kind::exact_numeric,
+                    ast::literal::sign::minus,
+                    { "1" },
+            },
+    }));
+}
+
 TEST_F(sql_parser_scalar_test, unary_expression_at_local) {
     auto result = parse("a at local");
     ASSERT_TRUE(result) << diagnostics(result);