scala> df1.join(df2,Seq("k1"),"left").show
+---+---+---+---+---+
| k1| k2| k3| k2| k4|
+---+---+---+---+---+
| a| 1| 2| 2| 2|
| b| 2| 3| 2| 1|
| b| 2| 3| 3| 3|
+---+---+---+---+---+
//左外连接 left_outer可简写为left
scala> df1.join(df2,Seq("k1"),"left_outer").show
+---+---+---+---+---+
| k1| k2| k3| k2| k4|
+---+---+---+---+---+
| a| 1| 2| 2| 2|
| b| 2| 3| 2| 1|
| b| 2| 3| 3| 3|
+---+---+---+---+---+
//左半连接
scala> df1.join(df2,Seq("k1"),"leftsemi").show
+---+---+---+
| k1| k2| k3|
+---+---+---+
| a| 1| 2|
| b| 2| 3|
+---+---+---+
scala> df1.join(df2,Seq("k1","k2"),"left").show
+---+---+---+----+
| k1| k2| k3| k4|
+---+---+---+----+
| a| 1| 2|null|
| b| 2| 3| 1|
+---+---+---+----+
scala> df1.join(df2,Seq("k1"),"right").show
+---+----+----+---+---+
| k1| k2| k3| k2| k4|
+---+----+----+---+---+
| a| 1| 2| 2| 2|
| b| 2| 3| 3| 3|
| b| 2| 3| 2| 1|
| c|null|null| 1| 1|
+---+----+----+---+---+
