As per my analysis, append will re-add the data, even though its available in the table, whereas overwrite Savemode will update existing date if any and will add addition row in the data frame.
val secondCompaniesDF = Seq((100, "comp1"), (101, "comp2"),(103,"comp2"))
.toDF("companyid","name")
secondCompaniesDF.write.mode(SaveMode.Overwrite)
.option("createTableColumnTypes","companyid int , name varchar(100)")
.jdbc(url, "Company", connectionProperties)
If SaveMode is Append, and this program is re-executed company will have 3 rows, whereas in case of Overwrite, if re-execute with any changes or addition row, existing records will be updated and new row will be added
Note: Overwrite drops the table and re-create the table. Is there any way where existing record get updated and new record get inserted something like upsert.