如何根据不同的条件连接或合并两个数据帧？

import pandas as pd

# 创建示例数据帧
df1 = pd.DataFrame({
    'key': ['A', 'B', 'C', 'D'],
    'value': [1, 2, 3, 4]
})

df2 = pd.DataFrame({
    'key': ['B', 'D', 'E', 'F'],
    'value': [5, 6, 7, 8]
})

# 内连接
inner_join = pd.merge(df1, df2, on='key', how='inner')
print("Inner Join:")
print(inner_join)

# 外连接
outer_join = pd.merge(df1, df2, on='key', how='outer')
print("\nOuter Join:")
print(outer_join)

# 左连接
left_join = pd.merge(df1, df2, on='key', how='left')
print("\nLeft Join:")
print(left_join)

# 右连接
right_join = pd.merge(df1, df2, on='key', how='right')
print("\nRight Join:")
print(right_join)

merged_df = pd.merge(df1, df2, on='key', suffixes=('_left', '_right'))

filled_df = merged_df.fillna(0)  # 用0填充NaN值