Left and right joins in Data Analysis Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a left join in data analysis?

A left join keeps all rows from the left table and adds matching rows from the right table. If no match is found, it fills with missing values (like NaN).

beginner

What does a right join do?

A right join keeps all rows from the right table and adds matching rows from the left table. If no match is found, it fills with missing values.

beginner

In pandas, which function and parameter do you use to perform a left join?

Use pd.merge(left_df, right_df, how='left', on='key_column') to do a left join on the key column.

beginner

What happens to rows in the right table that do not match any row in the left table during a left join?

They are dropped because the left join keeps all rows from the left table only.

intermediate

How is a right join different from an inner join?

A right join keeps all rows from the right table, even if no match exists in the left table. An inner join keeps only rows with matches in both tables.

Which join keeps all rows from the left table and matches from the right?

A Left join

B Right join

C Inner join

D Outer join

A left join keeps all rows from the left table and adds matching rows from the right.

In pandas, how do you specify a right join?

A how='left'

B how='right'

C how='inner'

D how='outer'

Use how='right' in pd.merge() to perform a right join.

What happens to unmatched rows in the right table during a left join?

A They are dropped

B They are kept with missing values

C They become duplicates

D They are merged with default values

Unmatched rows in the right table are dropped in a left join.

Which join keeps all rows from both tables, filling missing matches with NaN?

A Left join

B Right join

C Inner join

D Outer join

An outer join keeps all rows from both tables.

If you want only rows with matching keys in both tables, which join do you use?

A Left join

B Right join

C Inner join

D Outer join

Inner join returns only rows with matching keys in both tables.

Explain in your own words what a left join does and give a real-life example.

Think about keeping all your friends and adding info about their pets if available.

Describe the difference between a left join and a right join with a simple analogy.

Imagine two lists: one is your shopping list (left), the other is what’s in the fridge (right).

Practice

1. What does a left join do when combining two tables in data analysis?

easy

A. Keeps all rows from both tables, matching or not.

B. Keeps all rows from the left table and adds matching rows from the right table.

C. Keeps only rows that match in both tables.

D. Keeps all rows from the right table and adds matching rows from the left table.

Solution

1. Step 1: Understand the role of the left table in a left join

   A left join keeps every row from the left table regardless of matches in the right table.

2. Step 2: Understand how matching rows from the right table are added

   Only rows from the right table that match the left table's keys are added; unmatched right rows are ignored.

3. Final Answer:

   Keeps all rows from the left table and adds matching rows from the right table. -> Option B

4. Quick Check:

   Left join = all left rows + matching right rows [OK]

Hint: Left join keeps all left rows, adds right matches [OK]

Common Mistakes:

• Confusing left join with right join
• Thinking left join keeps unmatched right rows
• Believing left join only keeps matching rows

2. Which of the following is the correct syntax for a left join using pandas in Python?

easy

A. pd.merge(df1, df2, how='left', on='key')

B. pd.merge(df1, df2, how='right', on='key')

C. pd.join(df1, df2, how='left')

D. df1.left_join(df2, on='key')

Solution

1. Step 1: Recall pandas merge syntax for joins

   In pandas, the function to join tables is pd.merge(), specifying the join type with the 'how' parameter.

2. Step 2: Identify the correct parameters for a left join

   Using how='left' and on='key' correctly performs a left join on the specified key column.

3. Final Answer:

   pd.merge(df1, df2, how='left', on='key') -> Option A

4. Quick Check:

   pd.merge + how='left' = left join [OK]

Hint: Use pd.merge with how='left' for left join [OK]

Common Mistakes:

• Using pd.join instead of pd.merge
• Wrong 'how' parameter like 'right' or missing
• Calling non-existent left_join method

3. Given these two DataFrames:

df1 = pd.DataFrame({'id': [1, 2, 3], 'value': ['A', 'B', 'C']})
df2 = pd.DataFrame({'id': [2, 3, 4], 'score': [90, 80, 70]})

What is the result of pd.merge(df1, df2, how='left', on='id')?

medium

A. [{'id':1,'value':'A','score':NaN},{'id':2,'value':'B','score':90},{'id':3,'value':'C','score':80}]

B. [{'id':2,'value':'B','score':90},{'id':3,'value':'C','score':80},{'id':4,'value':NaN,'score':70}]

C. [{'id':1,'value':'A','score':NaN},{'id':2,'value':'B','score':90},{'id':3,'value':'C','score':80},{'id':4,'value':NaN,'score':70}]

D. [{'id':1,'value':'A','score':NaN},{'id':2,'value':'B','score':NaN},{'id':3,'value':'C','score':NaN}]

Solution

1. Step 1: Identify the left and right tables

   df1 is the left table with ids 1, 2, 3; df2 is the right table with ids 2, 3, 4.

2. Step 2: Perform left join on 'id'

   All rows from df1 appear. For id=1, no match in df2, so score is NaN. For ids 2 and 3, scores 90 and 80 are added. id=4 from df2 is ignored.

3. Final Answer:

   [{'id':1,'value':'A','score':NaN},{'id':2,'value':'B','score':90},{'id':3,'value':'C','score':80}] -> Option A

4. Quick Check:

   Left join keeps all df1 rows + matching df2 rows [OK]

Hint: Left join keeps all left rows, unmatched right rows ignored [OK]

Common Mistakes:

• Including unmatched right rows in output
• Replacing unmatched left values with NaN incorrectly
• Confusing left and right join results

4. You run this code but get an error:

pd.merge(df1, df2, how='right', key='id')

What is the error and how to fix it?

medium

A. Error: DataFrames must have same columns. Fix by renaming columns.

B. Error: 'how' must be 'left' or 'inner'. Fix by changing 'right' to 'left'.

C. Error: 'key' is not a valid argument. Fix by using 'on='id'' instead.

D. No error, code runs fine.

Solution

1. Step 1: Identify the incorrect parameter name

   The parameter to specify join keys in pd.merge is 'on', not 'key'.

2. Step 2: Correct the parameter name

   Replace 'key='id'' with 'on='id'' to fix the error.

3. Final Answer:

   Error: 'key' is not a valid argument. Fix by using 'on='id'' instead. -> Option C

4. Quick Check:

   Use 'on' for join keys in pd.merge [OK]

Hint: Use 'on' not 'key' for join columns in pd.merge [OK]

Common Mistakes:

• Using 'key' instead of 'on' parameter
• Changing 'how' incorrectly
• Assuming no error occurs

5. You have two DataFrames:

df_customers = pd.DataFrame({'customer_id': [1, 2, 3], 'name': ['Ann', 'Bob', 'Cara']})
df_orders = pd.DataFrame({'order_id': [101, 102], 'customer_id': [2, 4], 'amount': [250, 300]})

You want to list all orders with customer names, including orders without matching customers. Which join should you use and why?

hard

A. Full outer join to keep all customers and all orders.

B. Left join on 'customer_id' to keep all customers and add matching orders.

C. Inner join on 'customer_id' to keep only matching customers and orders.

D. Right join on 'customer_id' to keep all orders and add matching customer names.

Solution

1. Step 1: Identify which table to keep all rows from

   You want all orders listed, even if no matching customer exists, so keep all rows from df_orders.

2. Step 2: Choose join type to keep all right table rows

   df_orders is the right table in the join. Using a right join keeps all rows from df_orders and adds matching customer names from df_customers.

3. Final Answer:

   Right join on 'customer_id' to keep all orders and add matching customer names. -> Option D

4. Quick Check:

   Right join keeps all right table rows [OK]

Hint: Use right join to keep all orders, add matching customers [OK]

Common Mistakes:

• Using left join keeps all customers, not all orders
• Using inner join drops unmatched orders
• Using full join adds unmatched customers unnecessarily