Poison

Auxiliary Class Constructors

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辅助类构造器的使用 Column.scala at v2.4.4:

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/**
 * A column that will be computed based on the data in a `DataFrame`.
 *
 * A new column can be constructed based on the input columns present in a DataFrame:
 *
 * {{{
 *   df("columnName")            // On a specific `df` DataFrame.
 *   col("columnName")           // A generic column not yet associated with a DataFrame.
 *   col("columnName.field")     // Extracting a struct field
 *   col("`a.column.with.dots`") // Escape `.` in column names.
 *   $"columnName"               // Scala short hand for a named column.
 * }}}
 *
 * [[Column]] objects can be composed to form complex expressions:
 *
 * {{{
 *   $"a" + 1
 *   $"a" === $"b"
 * }}}
 *
 * @note The internal Catalyst expression can be accessed via [[expr]], but this method is for
 * debugging purposes only and can change in any future Spark releases.
 *
 * @groupname java_expr_ops Java-specific expression operators
 * @groupname expr_ops Expression operators
 * @groupname df_ops DataFrame functions
 * @groupname Ungrouped Support functions for DataFrames
 *
 * @since 1.3.0
 */
@InterfaceStability.Stable
class Column(val expr: Expression) extends Logging {

  def this(name: String) = this(name match {
    case "*" => UnresolvedStar(None)
    case _ if name.endsWith(".*") =>
      val parts = UnresolvedAttribute.parseAttributeName(name.substring(0, name.length - 2))
      UnresolvedStar(Some(parts))
    case _ => UnresolvedAttribute.quotedString(name)
  })

  // ...
}
References

Auxiliary Class Constructors | Scala Book | Scala Documentation