Adding column with values from one dataframe to another based on same datetime

Question

I have a big dataframe of datetime and values so I will just be taking a sample dataframe called df1

DateTime            Values
2020-12-26 14:00:00 439.0
2020-12-26 14:00:00 441.0
2020-12-26 14:00:00 461.0
2020-12-27 08:00:00 536.0
2020-12-27 08:00:00 547.0
2020-12-27 08:00:00 484.0
2020-12-27 08:00:00 497.0
2020-12-27 14:00:00 491.0
2020-12-27 14:00:00 512.0
2020-12-27 14:00:00 529.0
2020-12-27 14:00:00 436.0

and I have another full dataframe df 2

DateTime            Trend   
2020-12-18 14:00    no trend    
2020-12-19 08:00    no trend    
2020-12-19 14:00    no trend    
2020-12-21 08:00    no trend        
2020-12-21 14:00    no trend    
2020-12-22 08:00    no trend    
2020-12-22 14:00    decreasing  
2020-12-23 08:00    no trend    
2020-12-23 14:00    no trend    
2020-12-24 08:00    no trend    
2020-12-24 14:00    no trend    
2020-12-25 08:00    no trend    
2020-12-25 14:00    decreasing  
2020-12-26 08:00    no trend
2020-12-26 14:00    decreasing  
2020-12-27 08:00    no trend    
2020-12-27 14:00    no trend    

I am trying to create a new column df1[Trend] and assign the trend values based on the right date time

The result should be like this

DateTime            Values    Trend
2020-12-26 14:00:00 439.0     decreasing
2020-12-26 14:00:00 441.0     decreasing
2020-12-26 14:00:00 461.0     decreasing
2020-12-27 08:00:00 536.0     no trend
2020-12-27 08:00:00 547.0     no trend
2020-12-27 08:00:00 484.0     no trend
2020-12-27 08:00:00 497.0     no trend
2020-12-27 14:00:00 491.0     no trend
2020-12-27 14:00:00 512.0     no trend
2020-12-27 14:00:00 529.0     no trend
2020-12-27 14:00:00 436.0     no trend

Is there anyway I could it?

Answer 1

Use pandas.DataFrame.merge :

df1.merge(df2)

              DateTime  Values       Trend
0  2020-12-26 14:00:00   439.0  decreasing
1  2020-12-26 14:00:00   441.0  decreasing
2  2020-12-26 14:00:00   461.0  decreasing
3  2020-12-27 08:00:00   536.0    no trend
4  2020-12-27 08:00:00   547.0    no trend
5  2020-12-27 08:00:00   484.0    no trend
6  2020-12-27 08:00:00   497.0    no trend
7  2020-12-27 14:00:00   491.0    no trend
8  2020-12-27 14:00:00   512.0    no trend
9  2020-12-27 14:00:00   529.0    no trend
10 2020-12-27 14:00:00   436.0    no trend

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EDIT

You're getting

ValueError: You are trying to merge on datetime64[ns] and object columns. If you wish to proceed you should use pd.concat

because df1["DateTime"] is a datetime column but df2["DateTime"] is a str column then you can't merge. To fix it convert df2["DateTime"] to datetime with pandas.to_datetime :

df2['DateTime'] = pd.to_datetime(df2.DateTime)