Github user liancheng commented on a diff in the pull request:
https://github.com/apache/spark/pull/10333#discussion_r47871420
--- Diff:
sql/core/src/test/scala/org/apache/spark/sql/DataFrameJoinSuite.scala ---
@@ -48,11 +48,13 @@ class DataFrameJoinSuite extends QueryTest with
SharedSQLContext {
checkAnswer(
df.join(df2, Seq("int", "str"), "left"),
- Row(1, 2, "1", null) :: Row(2, 3, "2", null) :: Row(3, 4, "3", null)
:: Nil)
+ Row(1, 2, "1", null, null, null) :: Row(2, 3, "2", null, null, null)
::
+ Row(3, 4, "3", null, null, null) :: Nil)
checkAnswer(
df.join(df2, Seq("int", "str"), "right"),
- Row(null, null, null, 2) :: Row(null, null, null, 3) :: Row(null,
null, null, 4) :: Nil)
+ Row(null, null, null, 1, 2, "2") :: Row(null, null, null, 2, 3, "3")
::
+ Row(null, null, null, 3, 4, "4") :: Nil)
--- End diff --
We probably shouldn't show join keys multiple times in the result set. For
`LEFT/RIGHT JOIN USING` queries, both PostgreSQL and MySQL show join keys only
once. ScalaDoc of this overloaded `DataFrame.join` method also has similar
description:
```scala
/**
* Equi-join with another [[DataFrame]] using the given columns.
*
* Different from other join functions, the join columns will only appear
once in the output,
* i.e. similar to SQL's `JOIN USING` syntax.
...
*/
```
The following example comes from [PostgreSQL docs][1] (section 7.2.1.1):
```sql
CREATE TABLE t1 (num INT, name TEXT);
INSERT INTO t1 VALUES (1, 'a');
INSERT INTO t1 VALUES (2, 'b');
INSERT INTO t1 VALUES (3, 'c');
CREATE TABLE t2 (num INT, value TEXT);
INSERT INTO T2 VALUES (1, 'xxx');
INSERT INTO t2 VALUES (3, 'yyy');
INSERT INTO t2 VALUES (5, 'zzz');
SELECT * FROM t1 LEFT JOIN t2 USING (num);
```
PostgreSQL results in:
```
num | name | value
-----+------+-------
1 | a | xxx
2 | b |
3 | c | yyy
(3 rows)
```
and MySQL results in:
```
+------+------+-------+
| num | name | value |
+------+------+-------+
| 1 | a | xxx |
| 2 | b | NULL |
| 3 | c | yyy |
+------+------+-------+
3 rows in set (0.01 sec)
```
[1]:
http://www.postgresql.org/docs/9.4/static/queries-table-expressions.html
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