[SPARK-16459][SQL] Prevent dropping current database

## What changes were proposed in this pull request?

This PR prevents dropping the current database to avoid errors like the followings.

```scala
scala> sql("create database delete_db")
scala> sql("use delete_db")
scala> sql("drop database delete_db")
scala> sql("create table t as select 1")
org.apache.spark.sql.catalyst.analysis.NoSuchDatabaseException: Database `delete_db` not found;
```

## How was this patch tested?

Pass the Jenkins tests including an updated testcase.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14115 from dongjoon-hyun/SPARK-16459.

1 files changed, 9 insertions, 0 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala
index 7d1f1d1e62..b4294ed7ff 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala
@@ -1270,6 +1270,15 @@ class DDLSuite extends QueryTest with SharedSQLContext with BeforeAndAfterEach {
       "WITH SERDEPROPERTIES ('spark.sql.sources.me'='anything')")
   }
 
+  test("drop current database") {
+    sql("CREATE DATABASE temp")
+    sql("USE temp")
+    val m = intercept[AnalysisException] {
+      sql("DROP DATABASE temp")
+    }.getMessage
+    assert(m.contains("Can not drop current database `temp`"))
+  }
+
   test("drop default database") {
     Seq("true", "false").foreach { caseSensitive =>
       withSQLConf(SQLConf.CASE_SENSITIVE.key -> caseSensitive) {