From 8b5627f9ef5e780d51a86984a65db6b4344b77be Mon Sep 17 00:00:00 2001
From: abdelrahim <abdelrahimsaad544@gmail.com>
Date: Sun, 8 Feb 2026 01:28:58 +0200
Subject: [PATCH] MDEV-38670 Unary minus on empty string returns -0

Normalize -0.0 to +0.0 in Item_func_neg::real_op() to match
binary subtraction behavior. Add test case in type_newdecimal.

add result for tests

fix tests

Revert whitespace changes in type_newdecimal.result

Revert whitespace changes in type_newdecimal.test
---
 mysql-test/main/func_math.result | 9 +++++++++
 mysql-test/main/func_math.test   | 7 +++++++
 sql/item_func.cc                 | 9 ++++++++-
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/mysql-test/main/func_math.result b/mysql-test/main/func_math.result
index 475485cbcae59..7cbff4266544f 100644
--- a/mysql-test/main/func_math.result
+++ b/mysql-test/main/func_math.result
@@ -3739,3 +3739,12 @@ cast(1 as unsigned) - cast(2 as unsigned)
 -1
 set sql_mode=default;
 # End of 10.5 tests
+# MDEV-38670 Unary minus on empty string should return 0
+@x := -''
+0
+# Ensure binary subtraction matches
+@y := 0 - ''
+0
+# Additional edge cases
+-0.0	-'0'	-'   '	-@x	--@x
+0.0	0	0	0	0
\ No newline at end of file
diff --git a/mysql-test/main/func_math.test b/mysql-test/main/func_math.test
index 363d84eab3e07..7fbce63011957 100644
--- a/mysql-test/main/func_math.test
+++ b/mysql-test/main/func_math.test
@@ -1998,3 +1998,10 @@ select cast(1 as unsigned) - cast(2 as unsigned);
 set sql_mode=default;
 
 --echo # End of 10.5 tests
+
+--echo #
+--echo # MDEV-38670 Unary minus on empty string should return 0
+--echo #
+SELECT @x := -'';
+SELECT @y := 0 - '';
+SELECT -0.0, -'0', -'   ', -@x, --@x;
\ No newline at end of file
diff --git a/sql/item_func.cc b/sql/item_func.cc
index 0d2b43d100844..31d2d8881adff 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -1825,7 +1825,11 @@ double Item_func_neg::real_op()
 {
   double value= args[0]->val_real();
   null_value= args[0]->null_value;
-  return -value;
+  double res= -value;
+  // Normalize -0.0 to +0.0 to match binary subtraction behavior
+  if (res == 0.0)
+    res= 0.0;
+  return res;
 }
 
 
@@ -1858,6 +1862,9 @@ my_decimal *Item_func_neg::decimal_op(my_decimal *decimal_value)
   {
     my_decimal2decimal(value.ptr(), decimal_value);
     my_decimal_neg(decimal_value);
+    // Normalize -0 to +0 in decimal representation
+    if (my_decimal_is_zero(decimal_value))
+        decimal_value->sign(false);
     return decimal_value;
   }
   return 0;