fix issue:pingcap#11102

                                                            Unexcepted result in `SELECT ... CASE WHEN ... ELSE NULL...`

expression/constant_fold.go

@@ -81,12 +81,11 @@ func ifNullFoldHandler(expr *ScalarFunction) (Expression, bool) {
 
 func caseWhenHandler(expr *ScalarFunction) (Expression, bool) {
 	args, l := expr.GetArgs(), len(expr.GetArgs())
-	isDeferredConst, hasNonConstCondition := false, true
+	isDeferred, isDeferredConst, hasNonConstCondition := false, false, true
 	for i := 0; i < l-1; i += 2 {
-		foldedArg, isDeferred := foldConstant(args[i])
-		expr.GetArgs()[i] = foldedArg
+		expr.GetArgs()[i], isDeferred = foldConstant(args[i])
 		isDeferredConst = isDeferredConst || isDeferred
-		if _, isConst := foldedArg.(*Constant); isConst && hasNonConstCondition {
+		if _, isConst := expr.GetArgs()[i].(*Constant); isConst && hasNonConstCondition {
 			// If the condition is const and true, and the previous conditions
 			// has no expr, then the folded execution body is returned, otherwise
 			// the arguments of the casewhen are folded and replaced.
@@ -96,9 +95,8 @@ func caseWhenHandler(expr *ScalarFunction) (Expression, bool) {
 			}
 			if val != 0 && !isNull {
 				foldedExpr, isDeferred := foldConstant(args[i+1])
-				foldedExpr.GetType().Decimal = expr.GetType().Decimal
 				isDeferredConst = isDeferredConst || isDeferred
-				return foldedExpr, isDeferredConst
+				return BuildCastFunction(expr.GetCtx(), foldedExpr, expr.GetType()), isDeferredConst
 			}
 		} else {
 			hasNonConstCondition = false
@@ -112,12 +110,10 @@ func caseWhenHandler(expr *ScalarFunction) (Expression, bool) {
 		// is const and false, then the folded else execution body is returned. otherwise
 		// the execution body of the else are folded and replaced.
 		foldedExpr, isDeferred := foldConstant(args[l-1])
-		foldedExpr.GetType().Decimal = expr.GetType().Decimal
 		isDeferredConst = isDeferredConst || isDeferred
-		return foldedExpr, isDeferredConst
+		return BuildCastFunction(expr.GetCtx(), foldedExpr, expr.GetType()), isDeferredConst
 	} else if l%2 == 1 {
-		foldedArg, isDeferred := foldConstant(args[l-1])
-		expr.GetArgs()[l-1] = foldedArg
+		expr.GetArgs()[l-1], isDeferred = foldConstant(args[l-1])
 		isDeferredConst = isDeferredConst || isDeferred
 	}