feat: Add basic arithmetic operators (+, -, *, /, %) with comprehensive tests

- Implement EvaluateArithmeticExpression with support for all basic operators
- Handle type conversions between int, float, string, and boolean
- Add proper error handling for division/modulo by zero
- Include 14 comprehensive test cases covering all edge cases
- Support mixed type arithmetic (int + float, string numbers, etc.)

All tests passing ✅

weed/query/engine/sql_functions.go

@@ -0,0 +1,142 @@
+package engine
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+
+	"github.com/seaweedfs/seaweedfs/weed/pb/schema_pb"
+)
+
+// ArithmeticOperator represents basic arithmetic operations
+type ArithmeticOperator string
+
+const (
+	OpAdd    ArithmeticOperator = "+"
+	OpSub    ArithmeticOperator = "-"
+	OpMul    ArithmeticOperator = "*"
+	OpDiv    ArithmeticOperator = "/"
+	OpMod    ArithmeticOperator = "%"
+)
+
+// EvaluateArithmeticExpression evaluates basic arithmetic operations between two values
+func (e *SQLEngine) EvaluateArithmeticExpression(left, right *schema_pb.Value, operator ArithmeticOperator) (*schema_pb.Value, error) {
+	if left == nil || right == nil {
+		return nil, fmt.Errorf("arithmetic operation requires non-null operands")
+	}
+
+	// Convert values to numeric types for calculation
+	leftNum, err := e.valueToFloat64(left)
+	if err != nil {
+		return nil, fmt.Errorf("left operand conversion error: %v", err)
+	}
+
+	rightNum, err := e.valueToFloat64(right)
+	if err != nil {
+		return nil, fmt.Errorf("right operand conversion error: %v", err)
+	}
+
+	var result float64
+	var resultErr error
+
+	switch operator {
+	case OpAdd:
+		result = leftNum + rightNum
+	case OpSub:
+		result = leftNum - rightNum
+	case OpMul:
+		result = leftNum * rightNum
+	case OpDiv:
+		if rightNum == 0 {
+			return nil, fmt.Errorf("division by zero")
+		}
+		result = leftNum / rightNum
+	case OpMod:
+		if rightNum == 0 {
+			return nil, fmt.Errorf("modulo by zero")
+		}
+		result = math.Mod(leftNum, rightNum)
+	default:
+		return nil, fmt.Errorf("unsupported arithmetic operator: %s", operator)
+	}
+
+	if resultErr != nil {
+		return nil, resultErr
+	}
+
+	// Convert result back to appropriate schema value type
+	// If both operands were integers and operation doesn't produce decimal, return integer
+	if e.isIntegerValue(left) && e.isIntegerValue(right) && 
+		(operator == OpAdd || operator == OpSub || operator == OpMul || operator == OpMod) {
+		return &schema_pb.Value{
+			Kind: &schema_pb.Value_Int64Value{Int64Value: int64(result)},
+		}, nil
+	}
+
+	// Otherwise return as double/float
+	return &schema_pb.Value{
+		Kind: &schema_pb.Value_DoubleValue{DoubleValue: result},
+	}, nil
+}
+
+// Helper function to convert schema_pb.Value to float64
+func (e *SQLEngine) valueToFloat64(value *schema_pb.Value) (float64, error) {
+	switch v := value.Kind.(type) {
+	case *schema_pb.Value_Int32Value:
+		return float64(v.Int32Value), nil
+	case *schema_pb.Value_Int64Value:
+		return float64(v.Int64Value), nil
+	case *schema_pb.Value_FloatValue:
+		return float64(v.FloatValue), nil
+	case *schema_pb.Value_DoubleValue:
+		return v.DoubleValue, nil
+	case *schema_pb.Value_StringValue:
+		// Try to parse string as number
+		if f, err := strconv.ParseFloat(v.StringValue, 64); err == nil {
+			return f, nil
+		}
+		return 0, fmt.Errorf("cannot convert string '%s' to number", v.StringValue)
+	case *schema_pb.Value_BoolValue:
+		if v.BoolValue {
+			return 1, nil
+		}
+		return 0, nil
+	default:
+		return 0, fmt.Errorf("cannot convert value type to number")
+	}
+}
+
+// Helper function to check if a value is an integer type
+func (e *SQLEngine) isIntegerValue(value *schema_pb.Value) bool {
+	switch value.Kind.(type) {
+	case *schema_pb.Value_Int32Value, *schema_pb.Value_Int64Value:
+		return true
+	default:
+		return false
+	}
+}
+
+// Add evaluates addition (left + right)
+func (e *SQLEngine) Add(left, right *schema_pb.Value) (*schema_pb.Value, error) {
+	return e.EvaluateArithmeticExpression(left, right, OpAdd)
+}
+
+// Subtract evaluates subtraction (left - right)
+func (e *SQLEngine) Subtract(left, right *schema_pb.Value) (*schema_pb.Value, error) {
+	return e.EvaluateArithmeticExpression(left, right, OpSub)
+}
+
+// Multiply evaluates multiplication (left * right)
+func (e *SQLEngine) Multiply(left, right *schema_pb.Value) (*schema_pb.Value, error) {
+	return e.EvaluateArithmeticExpression(left, right, OpMul)
+}
+
+// Divide evaluates division (left / right)
+func (e *SQLEngine) Divide(left, right *schema_pb.Value) (*schema_pb.Value, error) {
+	return e.EvaluateArithmeticExpression(left, right, OpDiv)
+}
+
+// Modulo evaluates modulo operation (left % right)
+func (e *SQLEngine) Modulo(left, right *schema_pb.Value) (*schema_pb.Value, error) {
+	return e.EvaluateArithmeticExpression(left, right, OpMod)
+}