用pandas实现表格的内连接、外连接、左连接和右连接

基于pd.merge函数，实现两个表格的内连接、外连接、左连接和右连接，代码示例如下

import pandas as pd
df1 = pd.DataFrame({'year':[2020,2020,2021,2021], 
                    'company':['apple','google','apple','google'], 
                    'profit':[100,200,150,300]})
df1

df2 = pd.DataFrame({'year':[2020,2020,2021,2021,2021], 
                    'company':['apple','google','apple','google','tesla'], 
                    'employee':[20000,50000,22000,60000,30000]})
df2

pd.merge(df1,df2,how='inner',on=['year','company'])

pd.merge(df1,df2,how='outer',on=['year','company'])

pd.merge(df1,df2,how='left',on=['year','company'])

pd.merge(df1,df2,how='right',on=['year','company'])