Merge two column based on two conditions with pandas merge

Question

This question has been answered with only one condition. How can I join two column with two conditions, in my case date and sku?

Thank you.

Example1
date sku loc flag  
2017 122  61 True 
2018 122  61 True
2017 113  62 True 
2018 113  62 True 


Example2 
date  sku dept 
2017  122 a
2018  122 b
2017  113 c
2018  113 d



 Example3
date  sku loc flag   dept  
2017  122  61 True   a
2018  122  61 True   b
2017  113  62 True   c
2018  113  62 True   d

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