support NVL sql function (#7965)

* sql nvl

* add nvl in sql doc

docs/content/querying/sql.md

@@ -278,7 +278,8 @@ simplest way to write literal timestamps in other time zones is to use TIME_PARS
 |`CASE WHEN boolean_expr1 THEN result1 \[ WHEN boolean_expr2 THEN result2 ... \] \[ ELSE resultN \] END`|Searched CASE.|
 |`NULLIF(value1, value2)`|Returns NULL if value1 and value2 match, else returns value1.|
 |`COALESCE(value1, value2, ...)`|Returns the first value that is neither NULL nor empty string.|
-|`BLOOM_FILTER_TEST(<expr>, <serialized-filter>)`|Returns true if the value is contained in the base64 serialized bloom filter. See [bloom filter extension](../development/extensions-core/bloom-filter.html) documentation for additional details.
+|`NVL(expr,expr-for-null)`|Returns 'expr-for-null' if 'expr' is null (or empty string for string type).|
+|`BLOOM_FILTER_TEST(<expr>, <serialized-filter>)`|Returns true if the value is contained in the base64 serialized bloom filter. See [bloom filter extension](../development/extensions-core/bloom-filter.html) documentation for additional details.|
 ### Unsupported features
 
 Druid does not support all SQL features, including:

sql/src/main/java/org/apache/druid/sql/calcite/planner/DruidConvertletTable.java

@@ -25,6 +25,7 @@ import org.apache.calcite.rex.RexNode;
 import org.apache.calcite.sql.SqlCall;
 import org.apache.calcite.sql.SqlKind;
 import org.apache.calcite.sql.SqlOperator;
+import org.apache.calcite.sql.fun.OracleSqlOperatorTable;
 import org.apache.calcite.sql.fun.SqlStdOperatorTable;
 import org.apache.calcite.sql2rel.SqlRexContext;
 import org.apache.calcite.sql2rel.SqlRexConvertlet;
@@ -67,6 +68,7 @@ public class DruidConvertletTable implements SqlRexConvertletTable
           .add(SqlStdOperatorTable.TIMESTAMP_DIFF)
           .add(SqlStdOperatorTable.UNION)
           .add(SqlStdOperatorTable.UNION_ALL)
+          .add(OracleSqlOperatorTable.NVL)
           .build();
 
   private final Map<SqlOperator, SqlRexConvertlet> table;

sql/src/test/java/org/apache/druid/sql/calcite/CalciteQueryTest.java

@@ -7945,7 +7945,6 @@ public class CalciteQueryTest extends BaseCalciteQueryTest
     );
   }
 
-
   @Test
   public void testMultiValueStringWorksLikeStringGroupBy() throws Exception
   {
@@ -8085,4 +8084,42 @@ public class CalciteQueryTest extends BaseCalciteQueryTest
         )
     );
   }
+
+  @Test
+  public void testNvlColumns() throws Exception
+  {
+    testQuery(
+        "SELECT NVL(dim2, dim1), COUNT(*) FROM druid.foo GROUP BY NVL(dim2, dim1)\n",
+        ImmutableList.of(
+            GroupByQuery.builder()
+                        .setDataSource(CalciteTests.DATASOURCE1)
+                        .setInterval(querySegmentSpec(Filtration.eternity()))
+                        .setGranularity(Granularities.ALL)
+                        .setVirtualColumns(
+                            expressionVirtualColumn(
+                                "v0",
+                                "case_searched(notnull(\"dim2\"),\"dim2\",\"dim1\")",
+                                ValueType.STRING
+                            )
+                        )
+                        .setDimensions(dimensions(new DefaultDimensionSpec("v0", "v0", ValueType.STRING)))
+                        .setAggregatorSpecs(aggregators(new CountAggregatorFactory("a0")))
+                        .setContext(QUERY_CONTEXT_DEFAULT)
+                        .build()
+        ),
+        NullHandling.replaceWithDefault() ?
+        ImmutableList.of(
+            new Object[]{"10.1", 1L},
+            new Object[]{"2", 1L},
+            new Object[]{"a", 2L},
+            new Object[]{"abc", 2L}
+        ) :
+        ImmutableList.of(
+            new Object[]{"", 1L},
+            new Object[]{"10.1", 1L},
+            new Object[]{"a", 2L},
+            new Object[]{"abc", 2L}
+        )
+    );
+  }
 }