先看两个源数据表的定义：
scala> val df1 = spark.createDataset(Seq(("aaa", 1, 2), ("bbb", 3, 4), ("ccc", 3, 5), ("bbb", 4, 6)) ).toDF("key1","key2","key3")
df1: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 1 more field]
scala> val df2 = spark.createDataset(Seq(("aaa", 2, 2), ("bbb", 3, 5), ("ddd", 3, 5), ("bbb", 4, 6), ("eee", 1, 2), ("aaa", 1, 5), ("fff",5,6))).toDF("key1","key2","key4")
df2: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 1 more field]
scala> df1.printSchema
root
|-- key1: string (nullable = true)
|-- key2: integer (nullable = false)
|-- key3: integer (nullable = false)
scala> df2.printSchema
root
|-- key1: string (nullable = true)
|-- key2: integer (nullable = false)
|-- key4: integer (nullable = false)
scala> df1.show()
+----+----+----+
|key1|key2|key3|
+----+----+----+
| aaa| 1| 2|
| bbb| 3| 4|
| ccc| 3| 5|
| bbb| 4| 6|
+----+----+----+
scala> df2.show()
+----+----+----+
|key1|key2|key4|
+----+----+----+
| aaa| 2| 2|
| bbb| 3| 5|
| ddd| 3| 5|
| bbb| 4| 6|
| eee| 1| 2|
| aaa| 1| 5|
| fff| 5| 6|
+----+----+----+Spark对join的支持很丰富，等值连接，条件连接，自然连接都支持。连接类型包括内连接，外连接，左外连接，右外连接，左半连接以及笛卡尔连接。
下面一一示例，先看内连接
/*
内连接 select * from df1 join df2 on df1.key1=df2.key1
*/
scala> val df3 = df1.join(df2,"key1")
df3: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 3 more fields]
scala> df3.printSchema
root
|-- key1: string (nullable = true)
|-- key2: integer (nullable = false)
|-- key3: integer (nullable = false)
|-- key2: integer (nullable = false)
|-- key4: integer (nullable = false)
scala> df3.show
+----+----+----+----+----+
|key1|key2|key3|key2|key4|
+----+----+----+----+----+
| aaa| 1| 2| 1| 5|
| aaa| 1| 2| 2| 2|
| bbb| 3| 4| 4| 6|
| bbb| 3| 4| 3| 5|
| bbb| 4| 6| 4| 6|
| bbb| 4| 6| 3| 5|
+----+----+----+----+----+
/*
还是内连接，这次用joinWith。和join的区别是连接后的新Dataset的schema会不一样，注意和上面的对比一下。
*/
scala> val df4=df1.joinWith(df2,df1("key1")===df2("key1"))
df4: org.apache.spark.sql.Dataset[(org.apache.spark.sql.Row, org.apache.spark.sql.Row)] = [_1: struct<key1: string, key2: int ... 1 more field>, _2: struct<key1: string, key2: int ... 1 more field>]
scala> df4.printSchema
root
|-- _1: struct (nullable = false)
| |-- key1: string (nullable = true)
| |-- key2: integer (nullable = false)
| |-- key3: integer (nullable = false)
|-- _2: struct (nullable = false)
| |-- key1: string (nullable = true)
| |-- key2: integer (nullable = false)
| |-- key4: integer (nullable = false)
scala> df4.show
+---------+---------+
| _1| _2|
+---------+---------+
|[aaa,1,2]|[aaa,1,5]|
|[aaa,1,2]|[aaa,2,2]|
|[bbb,3,4]|[bbb,4,6]|
|[bbb,3,4]|[bbb,3,5]|
|[bbb,4,6]|[bbb,4,6]|
|[bbb,4,6]|[bbb,3,5]|
+---------+---------+然后是外连接：
/*
select * from df1 outer join df2 on df1.key1=df2.key1
*/
scala> val df5 = df1.join(df2,df1("key1")===df2("key1"), "outer")
df5: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 4 more fields]
scala> df5.show
+----+----+----+----+----+----+
|key1|key2|key3|key1|key2|key4|
+----+----+----+----+----+----+
|null|null|null| ddd| 3| 5|
| ccc| 3| 5|null|null|null|
| aaa| 1| 2| aaa| 2| 2|
| aaa| 1| 2| aaa| 1| 5|
| bbb| 3| 4| bbb| 3| 5|
| bbb| 3| 4| bbb| 4| 6|
| bbb| 4| 6| bbb| 3| 5|
| bbb| 4| 6| bbb| 4| 6|
|null|null|null| fff| 5| 6|
|null|null|null| eee| 1| 2|
+----+----+----+----+----+----+下面是左外连接，右外连接和左半连接:
/*
左外连接
*/
scala> val df6 = df1.join(df2,df1("key1")===df2("key1"), "left_outer")
df6: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 4 more fields]
scala> df6.show
+----+----+----+----+----+----+
|key1|key2|key3|key1|key2|key4|
+----+----+----+----+----+----+
| aaa| 1| 2| aaa| 1| 5|
| aaa| 1| 2| aaa| 2| 2|
| bbb| 3| 4| bbb| 4| 6|
| bbb| 3| 4| bbb| 3| 5|
| ccc| 3| 5|null|null|null|
| bbb| 4| 6| bbb| 4| 6|
| bbb| 4| 6| bbb| 3| 5|
+----+----+----+----+----+----+
/*
右外连接
*/
scala> val df7 = df1.join(df2,df1("key1")===df2("key1"), "right_outer")
df7: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 4 more fields]
scala> df7.show
+----+----+----+----+----+----+
|key1|key2|key3|key1|key2|key4|
+----+----+----+----+----+----+
| aaa| 1| 2| aaa| 2| 2|
| bbb| 4| 6| bbb| 3| 5|
| bbb| 3| 4| bbb| 3| 5|
|null|null|null| ddd| 3| 5|
| bbb| 4| 6| bbb| 4| 6|
| bbb| 3| 4| bbb| 4| 6|
|null|null|null| eee| 1| 2|
| aaa| 1| 2| aaa| 1| 5|
|null|null|null| fff| 5| 6|
+----+----+----+----+----+----+
/*
左半连接
*/
scala> val df8 = df1.join(df2,df1("key1")===df2("key1"), "leftsemi")
df8: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 1 more field]
scala> df8.show
+----+----+----+
|key1|key2|key3|
+----+----+----+
| aaa| 1| 2|
| bbb| 3| 4|
| bbb| 4| 6|
+----+----+----+笛卡尔连接不太常用，毕竟现在用spark玩的表都大得很，做这种全连接成本太大了。
/*
笛卡尔连接
*/
scala> val df9 = df1.crossJoin(df2)
df9: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 4 more fields]
scala> df9.count
res17: Long = 28
/* 就显示前10条结果吧 */
scala> df9.show(10)
+----+----+----+----+----+----+
|key1|key2|key3|key1|key2|key4|
+----+----+----+----+----+----+
| aaa| 1| 2| aaa| 2| 2|
| aaa| 1| 2| bbb| 3| 5|
| aaa| 1| 2| ddd| 3| 5|
| aaa| 1| 2| bbb| 4| 6|
| aaa| 1| 2| eee| 1| 2|
| aaa| 1| 2| aaa| 1| 5|
| aaa| 1| 2| fff| 5| 6|
| bbb| 3| 4| aaa| 2| 2|
| bbb| 3| 4| bbb| 3| 5|
| bbb| 3| 4| ddd| 3| 5|
+----+----+----+----+----+----+
only showing top 10 rows下面这个例子还是个等值连接，区别之前的等值连接是去调用两个表的重复列，就像自然连接一样：
/*
基于两个公共字段key1和key的等值连接
*/
scala> val df10 = df1.join(df2, Seq("key1","key2"))
df10: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 2 more fields]
scala> df10.show
+----+----+----+----+
|key1|key2|key3|key4|
+----+----+----+----+
| aaa| 1| 2| 5|
| bbb| 3| 4| 5|
| bbb| 4| 6| 6|
+----+----+----+----+条件连接在spark的低版本好像是不支持的，反正现在是ok啦~
/*
select df1.*,df2.* from df1 join df2
on df1.key1=df2.key1 and df1.key2>df2.key2
*/
scala> val df11 = df1.join(df2, df1("key1")===df2("key1") && df1("key2")>df2("key2"))
df11: org.apache.spark.sql.DataFrame = [key1: string, key2: int ... 4 more fields]
scala> df11.show
+----+----+----+----+----+----+
|key1|key2|key3|key1|key2|key4|
+----+----+----+----+----+----+
| bbb| 4| 6| bbb| 3| 5|
+----+----+----+----+----+----+