=== $ spark sql 的特别的方法

 /**
   * Equality test.
   * {{{
   *   // Scala:
   *   df.filter( df("colA") === df("colB") )
   *
   *   // Java
   *   import static org.apache.spark.sql.functions.*;
   *   df.filter( col("colA").equalTo(col("colB")) );
   * }}}
   *
   * @group expr_ops
   * @since 1.3.0
   */
  def === (other: Any): Column = withExpr {
    val right = lit(other).expr
    if (this.expr == right) {
      logWarning(
        s"Constructing trivially true equals predicate, '${this.expr} = $right'. " +
          "Perhaps you need to use aliases.")
    }
    EqualTo(expr, right)
  }

使用场景,join时判断等值时scala中使用===判断

  /**
   * Converts $"col name" into a [[Column]].
   *
   * @since 2.0.0
   */
  implicit class StringToColumn(val sc: StringContext) {
    def $(args: Any*): ColumnName = {
      new ColumnName(sc.s(args: _*))
    }
  }

使用场景,根据聚合之后的别名排序

.agg(count(userActionLog("logId")).alias("actionCount"))
// 第五步：进行排序
.sort($"actionCount".desc)