func NewNull() *Constant {
	return &Constant{
		Value:   types.NewDatum(nil),
		RetType: types.NewFieldType(mysql.TypeTiny),
	}
}
NewNull stands for null constant.
package expression
import (
	"fmt"
	"github.com/pingcap/parser/mysql"
	"github.com/pingcap/parser/terror"
	"github.com/pingcap/tidb/sessionctx"
	"github.com/pingcap/tidb/sessionctx/stmtctx"
	"github.com/pingcap/tidb/types"
	"github.com/pingcap/tidb/types/json"
	"github.com/pingcap/tidb/util/chunk"
	"github.com/pingcap/tidb/util/codec"
)
func NewOne() *Constant {
	return &Constant{
		Value:   types.NewDatum(1),
		RetType: types.NewFieldType(mysql.TypeTiny),
	}
}
func NewZero() *Constant {
	return &Constant{
		Value:   types.NewDatum(0),
		RetType: types.NewFieldType(mysql.TypeTiny),
	}
}