Apache Spark SQL从SQL查询获取数据帧中的值

Question

I'm trying to get a string value from a SQL query with Apache Spark 2.2.0 as follows:

val result = spark.sql("SELECT AnswerText FROM datatable WHERE participantUUID='010A0550' AND assessmentNumber=0 AND Q_id_string = '1_Age'")

assertResult("23") {
  result.collect.head.getString(0)
}

I get the following exception:

next on empty iterator
java.util.NoSuchElementException: next on empty iterator

I've tried collectAsList to return a row but not getting any joy from that, either. I simply want to return the actual value from the query in the DataFrame, not the column, row or field. In this case, the result is a string but it could also be an int - the age of the person = 23.

Answer 1

This happens probably because query doesn't return any items. It would be better to use headOption

assertResult(Some("23")) {
  result.take(1).headOption.map(_.getAs[String]("AnswerText"))
}

or push it to SQL:

assertResult(1) {
  spark
    .sql("""SELECT AnswerText 
            FROM datatable 
            WHERE participantUUID='010A0550' AND 
                  assessmentNumber=0 AND
                  Q_id_string = '1_Age'""")
   .where($"AnswerText" === "23").count 
}