[SPARK-24849][SPARK-24911][SQL] Converting a value of StructType to a DDL string

sql/catalyst/src/main/scala/org/apache/spark/sql/types/StructType.scala

@@ -436,6 +436,14 @@ object StructType extends AbstractDataType {
    */
   def fromDDL(ddl: String): StructType = CatalystSqlParser.parseTableSchema(ddl)
 
+  /**
+   * Converts a value of StructType to a string in DDL format. For example:
+   * `StructType(Seq(StructField("a", IntegerType)))` should be converted to `a int`
+   */
+  def toDDL(struct: StructType): String = {
+    struct.map(field => s"${quoteIdentifier(field.name)} ${field.dataType.sql}").mkString(",")
+  }
+
   def apply(fields: Seq[StructField]): StructType = StructType(fields.toArray)
 
   def apply(fields: java.util.List[StructField]): StructType = {

sql/catalyst/src/test/scala/org/apache/spark/sql/types/StructTypeSuite.scala

@@ -18,6 +18,7 @@
 package org.apache.spark.sql.types
 
 import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.types.StructType.{fromDDL, toDDL}
 
 class StructTypeSuite extends SparkFunSuite {
 
@@ -37,4 +38,29 @@ class StructTypeSuite extends SparkFunSuite {
     val e = intercept[IllegalArgumentException](s.fieldIndex("c")).getMessage
     assert(e.contains("Available fields: a, b"))
   }
+
+  test("SPARK-24849: toDDL - simple struct") {
+    val struct = StructType(Seq(StructField("a", IntegerType)))
+
+    assert(toDDL(struct) == "`a` INT")
+  }
+
+  test("SPARK-24849: round trip toDDL - fromDDL") {
+    val struct = new StructType().add("a", IntegerType).add("b", StringType)
+
+    assert(fromDDL(toDDL(struct)) === struct)
+  }
+
+  test("SPARK-24849: round trip fromDDL - toDDL") {
+    val struct = "`a` MAP<INT, STRING>,`b` INT"
+
+    assert(toDDL(fromDDL(struct)) === struct)
+  }
+
+  test("SPARK-24849: toDDL must take into account case") {
+    val struct = new StructType()
+      .add("metaData", new StructType().add("eventId", StringType))
+
+    assert(toDDL(struct) == "`metaData` STRUCT<`eventId`: STRING>")
+  }
 }