I've been studying basic usage and I found wrong minus result on DecimalXX type.

**Example Query**
`
select toDecimal64(4,4) - toDecimal32(2,2)
`

**Expected output**
2.0000

**Actual output**
3.9800

Is there anything wrong or maybe my misunderstanding of toDecimal ?

![image](https://user-images.githubusercontent.com/8475371/107513855-cdd57a00-6be3-11eb-841d-d425a198d172.png)


**How to reproduce**
1. OS: macOS Catalina 10.15.5
2. server: clickhouse-server, built from source, version 21.3.1.1
3. client:  clickhouse-client,  built from source, version 21.3.1.1

DataTypeFromFieldType<typename Op::ResultType>>,
        
        
        Case<IsOperation<Operation>::plus, Switch<
            Case<IsIntegral<RightDataType>, LeftDataType>,
            Case<IsIntegral<LeftDataType>, RightDataType>>>,
        
        
        Case<IsOperation<Operation>::minus, Switch<
            Case<std::is_same_v<LeftDataType, RightDataType>, DataTypeInt32>,
            Case<IsDateOrDateTime<LeftDataType> && IsIntegral<RightDataType>, LeftDataType>>>,
        
        
        Case<std::is_same_v<LeftDataType, RightDataType> && (IsOperation<Operation>::least || IsOperation<Operation>::greatest),
            LeftDataType>,
        
        
        Case<IsOperation<Operation>::modulo, Switch<
            Case<IsDateOrDateTime<LeftDataType> && IsIntegral<RightDataType>, RightDataType>,
            Case<IsDateOrDateTime<LeftDataType> && IsFloatingPoint<RightDataType>, DataTypeFloat64>>>>;
};
}
namespace impl_
{
/** Arithmetic operations: +, -, *, /, %,
  * intDiv (integer division)
  * Bitwise operations: |, &, ^, ~.
  * Etc.
  */
enum class OpCase { Vector, LeftConstant, RightConstant };