Why LEFT JOIN preserving all left rows in SQL? - Purpose & Use Cases

The Big Idea

What if you could see every friend's gift status without missing a single one, even if they didn't bring anything?

The Scenario

Imagine you have two lists: one with all your friends and another with the gifts they gave you. You want to see every friend and the gift they gave, if any. Doing this by hand means checking each friend one by one and matching gifts, which is slow and confusing.

The Problem

Manually matching each friend to their gift is slow and easy to mess up. You might forget some friends who didn't give gifts, or mix up who gave what. It's hard to keep track and update when new friends or gifts appear.

The Solution

Using a LEFT JOIN in SQL automatically keeps all friends on the left list and adds gift info if it exists. This way, you never miss a friend, even if they didn't give a gift. It's fast, clear, and always up to date.

Before vs After

✗ Before

for friend in friends:
    gift = find_gift(friend)
    print(friend, gift or 'No gift')

✓ After

SELECT friends.name, gifts.item
FROM friends
LEFT JOIN gifts ON friends.id = gifts.friend_id;

What It Enables

It lets you combine data from two sources while keeping every item from the main list, even if there's no matching data on the side.

Real Life Example

A store wants to list all customers and any orders they placed. Using LEFT JOIN, the store sees every customer, including those who haven't bought anything yet.

Key Takeaways

LEFT JOIN keeps all rows from the left table.

It adds matching rows from the right table or shows NULL if none.

This helps show complete data without losing unmatched items.

Practice

1. What does a LEFT JOIN do in SQL?

easy

A. Deletes rows from the left table that have no match in the right table.

B. Keeps only rows that have matches in both tables.

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

D. Keeps all rows from the left table and adds matching rows from the right table or NULL if no match.

Solution

1. Step 1: Understand LEFT JOIN behavior

   A LEFT JOIN returns all rows from the left table regardless of matches in the right table.

2. Step 2: Check what happens with unmatched rows

   If there is no matching row in the right table, the result shows NULL for right table columns.

3. Final Answer:

   Keeps all rows from the left table and adds matching rows from the right table or NULL if no match. -> Option D

4. Quick Check:

   LEFT JOIN = all left rows kept [OK]

Hint: Remember: LEFT JOIN keeps all left rows, fills right with NULL if no match [OK]

Common Mistakes:

• Confusing LEFT JOIN with INNER JOIN
• Thinking it keeps all right table rows
• Assuming unmatched rows are dropped

2. Which of the following is the correct syntax for a LEFT JOIN in SQL?

easy

A. SELECT * FROM table1 LEFT ON JOIN table2 WHERE table1.id = table2.id;

B. SELECT * FROM table1 JOIN LEFT table2 ON table1.id = table2.id;

C. SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id;

D. SELECT * FROM table1 LEFT JOIN table2 WHERE table1.id = table2.id;

Solution

1. Step 1: Review correct LEFT JOIN syntax

   The correct syntax is: SELECT columns FROM left_table LEFT JOIN right_table ON condition;

2. Step 2: Identify syntax errors in other options

   Options A, B, and D misuse keywords or omit ON clause, causing syntax errors.

3. Final Answer:

   SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id; -> Option C

4. Quick Check:

   LEFT JOIN syntax = SELECT ... LEFT JOIN ... ON ... [OK]

Hint: LEFT JOIN always uses ON to specify join condition [OK]

Common Mistakes:

• Swapping JOIN and LEFT keywords
• Using WHERE instead of ON for join condition
• Omitting ON clause

3. Given these tables:

Employees
id | name
1 | Alice
2 | Bob
3 | Carol

Sales
emp_id | amount
1 | 100
3 | 200

What is the result of:
SELECT Employees.name, Sales.amount FROM Employees LEFT JOIN Sales ON Employees.id = Sales.emp_id;

medium

A. [('Alice', 100), ('Bob', 0), ('Carol', 200)]

B. [('Alice', 100), ('Bob', NULL), ('Carol', 200)]

C. [('Alice', 100), ('Carol', 200)]

D. [('Bob', NULL)]

Solution

1. Step 1: Match Employees with Sales using LEFT JOIN

   All Employees rows appear. For matching emp_id in Sales, amount is shown; else NULL.

2. Step 2: Map each employee to sales amount or NULL

   Alice (id=1) matches 100, Bob (id=2) no match so NULL, Carol (id=3) matches 200.

3. Final Answer:

   [('Alice', 100), ('Bob', NULL), ('Carol', 200)] -> Option B

4. Quick Check:

   LEFT JOIN keeps all left rows with NULL for no match [OK]

Hint: LEFT JOIN shows NULL for unmatched right rows, not zero [OK]

Common Mistakes:

• Replacing NULL with zero
• Omitting unmatched rows
• Confusing LEFT JOIN with INNER JOIN

4. Consider this query:

SELECT a.id, b.value FROM A LEFT JOIN B ON a.id = b.a_id WHERE b.value > 10;

What is the problem with this query if you want to keep all rows from A?

medium

A. The WHERE clause filters out rows where b.value is NULL, losing some left rows.

B. The ON clause is missing a join condition.

C. LEFT JOIN should be replaced with INNER JOIN for correct results.

D. The SELECT statement is missing table aliases.

Solution

1. Step 1: Understand effect of WHERE on LEFT JOIN

   WHERE filters after join, so rows with NULL b.value are removed, losing left rows.

2. Step 2: Identify how to fix to keep all left rows

   Move condition to ON clause or use WHERE b.value > 10 OR b.value IS NULL to preserve unmatched rows.

3. Final Answer:

   The WHERE clause filters out rows where b.value is NULL, losing some left rows. -> Option A

4. Quick Check:

   WHERE after LEFT JOIN can remove unmatched rows [OK]

Hint: Put filters on right table in ON, not WHERE, to keep all left rows [OK]

Common Mistakes:

• Assuming WHERE doesn't affect LEFT JOIN results
• Confusing ON and WHERE clauses
• Replacing LEFT JOIN with INNER JOIN unnecessarily

5. You have two tables:

Products
product_id | name
1 | Pen
2 | Pencil
3 | Eraser

Sales
product_id | quantity
1 | 10
1 | 5
3 | 7

Write a query using LEFT JOIN to get each product's total sales quantity, showing 0 if no sales exist.

hard

A. SELECT p.name, COALESCE(SUM(s.quantity), 0) AS total FROM Products p LEFT JOIN Sales s ON p.product_id = s.product_id GROUP BY p.name;

B. SELECT p.name, SUM(s.quantity) AS total FROM Products p INNER JOIN Sales s ON p.product_id = s.product_id GROUP BY p.name;

C. SELECT p.name, SUM(s.quantity) AS total FROM Products p LEFT JOIN Sales s ON p.product_id = s.product_id WHERE s.quantity > 0 GROUP BY p.name;

D. SELECT p.name, SUM(s.quantity) AS total FROM Sales s LEFT JOIN Products p ON s.product_id = p.product_id GROUP BY p.name;

Solution

1. Step 1: Use LEFT JOIN to keep all products

   LEFT JOIN ensures all products appear even if no sales exist.

2. Step 2: Use COALESCE with SUM to show 0 for no sales

   SUM returns NULL if no matching rows; COALESCE converts NULL to 0.

3. Final Answer:

   SELECT p.name, COALESCE(SUM(s.quantity), 0) AS total FROM Products p LEFT JOIN Sales s ON p.product_id = s.product_id GROUP BY p.name; -> Option A

4. Quick Check:

   LEFT JOIN + COALESCE(SUM()) = total sales with zeros [OK]

Hint: Use COALESCE(SUM()) with LEFT JOIN to replace NULL totals with zero [OK]

Common Mistakes:

• Using INNER JOIN losing products with no sales
• Not handling NULL sums with COALESCE
• Filtering in WHERE removing unmatched rows