---
title: "dataframe-join (tc)"
description: DataFrame 的 join() 以索引黏合 — 5 個例子。
url: "https://aaronchan.com/zh/kb/dataframe-join/"
updated: "2026-09-15"
category: "Code-Python (tc)"
tags:
  - python
---

# dataframe-join (tc)

- **concat** — pandas 層級：單純把資料黏在一起，無任何條件
- **merge** — pandas 層級：以共同的欄位或索引層級黏合
- **join** — DataFrame 層級：只以「索引」黏合

## Example 1：基本 join — 以索引合併兩個 DataFrame

 Copy

```
import pandas as pd

df1 = pd.DataFrame({
    "Age": [25,30,22]
}, index=["Alice", "Bob",  "Cat"])

df2 = pd.DataFrame({
    "City": ["HK", "Tokyo", "Sydney", "Taiwan"]
}, index=["Alice", "Bob",  "Cat", "David"])

df1.join(df2)
```

**output:**

```
	Age	City
Alice	25	HK
Bob	30	Tokyo
Cat	22	Sydney
```

---

## Example 2：索引不完全對應時 join — 缺的一邊補上 NaN

 Copy

```
import pandas as pd

df1 = pd.DataFrame({
    "Age": [25,30,22, 40]
}, index=["Alice", "Bob", "Cat", "Peter"])

df2 = pd.DataFrame({
    "City": ["HK", "Tokyo", "Sydney", "New York"]
}, index=["Alice", "Bob",  "Cat", "David"])

df1.join(df2)
```

**output:**

```
	   Age City
Alice	25	HK
Bob	30	Tokyo
Cat	22	Sydney
Peter	40	NaN
```

---

## Example 3：索引不匹配時 join — Bob 補上 NaN

 Copy

```
#When index not match
import pandas as pd

df1 = pd.DataFrame({
    "Age": [25,30]
}, index=["Alice", "Bob"])

df2 = pd.DataFrame({
    "City": ["HK"]
}, index=["Alice"])

df1.join(df2)
```

**output:**

```
	Age	City
Alice	25	HK
Bob	30	NaN
```

---

## Example 4：一次 join 多個 DataFrame — 傳入 list

 Copy

```
import pandas as pd

df1 = pd.DataFrame({
    "Age": [25,30]
}, index=["Alice", "Bob"])

df2 = pd.DataFrame({
    "City": ["HK", "Taiwan"]
}, index=["Alice", "Bob"])

df3 = pd.DataFrame({
    "Score": [90, 50]
}, index=["Alice", "Bob"])

df1.join([df2, df3])
```

**output:**

```
	Age	City	Score
Alice	25	HK	90
Bob	30	Taiwan	50
```

---

## Example 5：how="inner" join — 先將索引轉為小寫以匹配

 Copy

```
names = pd.DataFrame({"age":[30,40]}, index=["alice","ben"])
cities = pd.DataFrame({"city":["HK","Tokyo"]}, index=["Alice","Ben"])

names.index = names.index.str.lower()
cities.index = cities.index.str.lower()

print(names.join(cities, how="inner"))
```

**output:**

```
       age   city
alice   30     HK
ben     40  Tokyo
```