Retrieve every data from database without one field using rule

A queryColumn can only have one column name. And hence you will have to add multiple query columns. 
Check the code below..

cast('type!{urn:com:appian:types:HSM}HSM_Elements?list',
  a!queryEntity(
    entity: cons!HSM_Entity,
    query: a!query(
      selection:a!querySelection(
        columns: {
          a!queryColumn(
            field:"S_No",
          ),
          a!queryColumn(
            field:"Employee_Name",
          ),
          a!queryColumn(
            field:"Employee_Id",
          )
        }
      ),
      pagingInfo: a!pagingInfo(
        startIndex: 1,
        batchSize: 5000,
        sort: a!sortInfo(
          field:"S_No",
          ascending: true,
        )
      )
    )
  ).data
)