Explanation
todayTransactionsDf.union(yesterdayTransactionsDf)
Correct. The union command appends rows of yesterdayTransactionsDf to the rows of todayTransactionsDf, ignoring that both DataFrames have different column names. The resulting DataFrame will have the column names of DataFrame todayTransactionsDf.
todayTransactionsDf.unionByName(yesterdayTransactionsDf)
No. unionByName specifically tries to match columns in the two DataFrames by name and only appends values in columns with identical names across the two DataFrames. In the form presented above, the command is a great fit for joining DataFrames that have exactly the same columns, but in a different order. In this case though, the command will fail because the two DataFrames have different columns.
todayTransactionsDf.unionByName(yesterdayTransactionsDf, allowMissingColumns=True) No. The unionByName command is described in the previous explanation. However, with the allowMissingColumns argument set to True, it is no longer an issue that the two DataFrames have different column names. Any columns that do not have a match in the other DataFrame will be filled with null where there is no value. In the case at hand, the resulting DataFrame will have 7 or more columns though, so it this command is not the right answer.
union(todayTransactionsDf, yesterdayTransactionsDf)
No, there is no union method in pyspark.sql.functions.
todayTransactionsDf.concat(yesterdayTransactionsDf)
Wrong, the DataFrame class does not have a concat method.
More info: pyspark.sql.DataFrame.union - PySpark 3.1.2 documentation,
pyspark.sql.DataFrame.unionByName - PySpark 3.1.2 documentation
Static notebook | Dynamic notebook: See test 3