Fixed "join_use_nulls" mode #1544

dbms/src/Interpreters/ExpressionActions.h

@@ -51,7 +51,6 @@ public:
             */
         ARRAY_JOIN,
 
-        /// INNER|LEFT JOIN.
         JOIN,
 
         /// Reorder and rename the columns, delete the extra ones. The same column names are allowed in the result.

dbms/src/Interpreters/ExpressionAnalyzer.cpp

@@ -15,6 +15,7 @@
 
 #include <DataTypes/DataTypeSet.h>
 #include <DataTypes/DataTypeArray.h>
+#include <DataTypes/DataTypeNullable.h>
 #include <DataTypes/DataTypeTuple.h>
 #include <DataTypes/DataTypeExpression.h>
 #include <DataTypes/DataTypeNested.h>
@@ -2838,7 +2839,9 @@ void ExpressionAnalyzer::collectJoinedColumns(NameSet & joined_columns, NamesAnd
             && !joined_columns.count(col.name)) /// Duplicate columns in the subquery for JOIN do not make sense.
         {
             joined_columns.insert(col.name);
-            joined_columns_name_type.emplace_back(col.name, col.type);
+
+            bool make_nullable = settings.join_use_nulls && (table_join.kind == ASTTableJoin::Kind::Left || table_join.kind == ASTTableJoin::Kind::Full);
+            joined_columns_name_type.emplace_back(col.name, make_nullable ? makeNullable(col.type) : col.type);
         }
     }
 }

dbms/tests/queries/0_stateless/00549_join_use_nulls.reference

@@ -0,0 +1,2 @@
+0	0	\N
+1	1	1

dbms/tests/queries/0_stateless/00549_join_use_nulls.sql

@@ -0,0 +1,30 @@
+SET join_use_nulls = 1;
+
+DROP TABLE IF EXISTS test.null;
+CREATE TABLE test.null (k UInt64, a String, b Nullable(String)) ENGINE = Log;
+
+INSERT INTO test.null SELECT
+    k,
+    a,
+    b
+FROM
+(
+    SELECT
+        number AS k,
+        toString(number) AS a
+    FROM system.numbers
+    LIMIT 2
+)
+ANY LEFT JOIN
+(
+    SELECT
+        number AS k,
+        toString(number) AS b
+    FROM system.numbers
+    LIMIT 1, 2
+) USING (k)
+ORDER BY k ASC;
+
+SELECT * FROM test.null ORDER BY k, a, b;
+
+DROP TABLE test.null;