sparkSQL实战之二：编程指定模式

当样本类不能提前确定（例如，记录的结构是经过编码的字符串，或者一个文本集合将会被解析，不同的字段投影给不同的用户），一个schemaRDD可以通过三步来创建。

1. 从原来的RDD创建一个行的RDD
2. 创建由一个structType表示的schema匹配第一步创建的RDD的行结构
3. 通过SQLContext提供的applySchema方法应用这个schema到行的RDD
   例如：

// Create an RDD val people = sc.textFile("examples/src/main/resources/people.txt") // The schema is encoded in a string val schemaString = "name age" // Import Spark SQL data types and Row. import org.apache.spark.sql._ // Generate the schema based on the string of schema val schema = StructType( schemaString.split(" ").map(fieldName => StructField(fieldName, StringType, true))) // Convert records of the RDD (people) to Rows. val rowRDD = people.map(_.split(",")).map(p => Row(p(0), p(1).trim)) // Apply the schema to the RDD. val peopleSchemaRDD = sqlContext.applySchema(rowRDD, schema) // Register the SchemaRDD as a table. peopleSchemaRDD.registerTempTable("people") // SQL statements can be run by using the sql methods provided by sqlContext. val results = sqlContext.sql("SELECT name FROM people") // The results of SQL queries are SchemaRDDs and support all the normal RDD operations. // The columns of a row in the result can be accessed by ordinal. results.map(t => "Name: " + t(0)).collect().foreach(println)