How to combine several Dataframes together in scala? - scala

Question

I have several dataframes which contains single column in them. Let's say I have 4 such dataframe all with one column. How can I form a single dataframe by combining all of them?
val df = xmldf.select(col("UserData.UserValue._valueRef"))
val df2 = xmldf.select(col("UserData.UserValue._title"))
val df3 = xmldf.select(col("author"))
val df4 = xmldf.select(col("price"))
To combine, I am trying this, but it doesn't work:
var newdf = df
newdf = newdf.withColumn("col1",df1.col("UserData.UserValue._title"))
newdf.show()
It errors out saying that field of one column are not present in another. I am not sure how can I combine these 4 dataframes together. They don't have any common column.
df2 looks like this:
+---------------+
| _title|
+---------------+
|_CONFIG_CONTEXT|
|_CONFIG_CONTEXT|
|_CONFIG_CONTEXT|
+---------------+
and df looks like this:
+-----------+
|_valuegiven|
+-----------+
| qwe|
| dfdfrt|
| dfdf|
+-----------+
df3 and df4 are also in same format. I want like below dataframe:
+-----------+---------------+
|_valuegiven| _title|
+-----------+---------------+
| qwe|_CONFIG_CONTEXT|
| dfdfrt|_CONFIG_CONTEXT|
| dfdf|_CONFIG_CONTEXT|
+-----------+---------------+
I used this:
val newdf = xmldf.select(col("UserData.UserValue._valuegiven"),col("UserData.UserValue._title") )
newdf.show()
But I am getting column name on the go and as such, I would need to append on the go, due to which I don't know exactly how many columns I will get. Which is why I cannot use the above command.

Answer 1

It's a little unclear of your goal. If asking to join these dataframes, but perhaps you just want to select those 4 columns.
val newdf = xmldf.select($"UserData.UserValue._valueRef", $"UserData.UserValue._title", 'author,'price")
newdf.show
If you really want to join all these dataframes, you'll need to join them all and select the appropriate fields.

Answer 2

If the goal is to get 4 columns from xmldf into a new dataframe you shouldn't be splitting it into 4 dataframes in the first place.
You can select multiple columns from a dataframe by providing additional column names in the select function.
val newdf = xmldf.select(
col("UserData.UserValue._valueRef"),
col("UserData.UserValue._title"),
col("author"),
col("price"))
newdf.show()

Answer 3

So I looked at various ways and finally Ram Ghadiyaram's answer in Solution 2 does what I wanted to do. Using this approach, you can combine any number of columns on the go. Basically, you need to create indexes by which you can join the dataframes together and after joining, drop the index column altogether.