问我们可以将数据库中的数据帧转换为字符串吗?为什么我们会得到错误查询，而流源必须使用writeStream.start()执行

EN

val DF = AppointmentDF
            .select("*")
            .filter($"xyz" === "abc")

DF.createOrReplaceTempView("MyTable")
val column1DF = spark.sql("SELECT column1 FROM MyTable")


// This is not getting resolved
val sql="select c.abc from c where c.column = \"" + String.valueOf(column1DF) + "\""
println(sql)

Error:
org.apache.spark.sql.AnalysisException: cannot resolve '`column1DF`' given input columns: []; line 1 pos 12;


DF.collect().foreach { row =>
  println(row.mkString(","))
 } 

 Error: 
 org.apache.spark.sql.AnalysisException: Queries with streaming sources must 
 be executed with writeStream.start();;