Outer join results in all rows from statesPopulationDF and statesTaxRatesDF.

Join the two datasets by the State column as follows:

val joinDF = statesPopulationDF.join(statesTaxRatesDF, statesPopulationDF("State") === statesTaxRatesDF("State"), "fullouter")

%sql
val joinDF = spark.sql("SELECT * FROM statesPopulationDF FULL OUTER JOIN statesTaxRatesDF ON statesPopulationDF.State = statesTaxRatesDF.State")

scala> joinDF.count
res22: Long = 351

scala> joinDF.show
+--------------------+----+----------+--------------------+-------+
| State|Year|Population| State|TaxRate|
+--------------------+----+----------+--------------------+-------+
| Delaware|2010| 899816| null| null|
| Delaware|2011| 907924| null| null|
| West Virginia|2010| 1854230| West Virginia| 6.0|
| West Virginia|2011| 1854972| West Virginia| 6.0|
| Missouri|2010| 5996118| Missouri| 4.225|
| null|null| null|  Connecticut|   6.35|