Right outer join results in all rows from statesTaxRatesDF, including any common in statesPopulationDF and statesTaxRatesDF.

Join the two datasets by the State column as follows:

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

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

scala> joinDF.count
res22: Long = 323

scala> joinDF.show
+--------------------+----+----------+--------------------+-------+
| State|Year|Population| State|TaxRate|
+--------------------+----+----------+--------------------+-------+
| Colorado|2011| 5118360| Colorado| 2.9|
| Colorado|2010| 5048644| Colorado| 2.9|
| null|null| null|Connecticut| 6.35|
| Florida|2016| 20612439| Florida| 6.0|
| Florida|2015| 20244914| Florida| 6.0|
| Florida|2014| 19888741| Florida| 6.0|