调用sqlContext.udf.register()

此时注册的方法 只能在sql()中可见，对DataFrame API不可见

用法：sqlContext.udf.register("makeDt", makeDT(_:String,_:String,_:String))

示例：

def makeDT(date: String, time: String, tz: String) = s"$date $time $tz"
sqlContext.udf.register("makeDt", makeDT(_:String,_:String,_:String))
 
// Now we can use our function directly in SparkSQL.
sqlContext.sql("SELECT amount, makeDt(date, time, tz) from df").take(2)
// but not outside
df.select($"customer_id", makeDt($"date", $"time", $"tz"), $"amount").take(2) // fails

2）调用spark.sql.function.udf()方法

此时注册的方法，对外部可见

用法：valmakeDt = udf(makeDT(_:String,_:String,_:String))

示例：

import org.apache.spark.sql.functions.udf
val makeDt = udf(makeDT(_:String,_:String,_:String))
// now this works
df.select($"customer_id", makeDt($"date", $"time", $"tz"), $"amount").take(2)