getColumnForTable = function(table) {
    return table.getEffectiveName() ==
        joinStep.predicate.rightColumn.getTable().getEffectiveName() ?
            joinStep.predicate.rightColumn : joinStep.predicate.leftColumn;
  }
Finds which of the two joined columns corresponds to the given table.
@param {!lf.schema.Table} table
@return {!lf.schema.Column}