Can i access database table value using a constant?

I would rather use Appian expression and a!queryEntity function.

 .

a!queryEntity(
  entity: cons!TTC_USERDETAILS,
  query: a!query(
    logicalExpression: a!queryLogicalExpression(
      operator: "AND",
      filters: {
        a!queryFilter(
          field: "userName",
          operator: "=",
          value: ri!userName
        )
      },
      ignoreFiltersWithEmptyValues: true
    ),
    pagingInfo: a!pagingInfo(
      startIndex: 1,
      batchSize: 50,
      sort: a!sortInfo(
        field: "userId",
        ascending: true
      )
    )
  ),
  fetchTotalCount: false
)

By using a!queryentity we will fetch the data from the back end tables and directly we can use this expressions in the interface. for better understanding

please go through the above code .

I know this isn't your question but why are you storing usernames and passwords in a database? I also wouldn't suggest this, but if you're going to use a constant to control what's being retrieved from the database why not just directly store the password in the constant?

We probably need a little more information to answer fully.