擅长:python、mysql、java
<pre><code># raise preferred lines to the top
df2 = df2.sort_values(by='Flag', ascending=False)
# deduplicate
df2 = df2.drop_duplicates(subset=['A','B'], keep='first')
# merge
pd.merge(df1, df2, on=['A','B'])
A B Flag C
0 a 1 1 80
1 b 2 1 40
2 c 3 0 60
</code></pre>