LEFT JOIN preserving all left rows in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the left table and join with the right table.

SQL

SELECT * FROM employees [1] departments ON employees.department_id = departments.id;

Drag options to blanks, or click blank then click option'

AINNER JOIN

BLEFT JOIN

CRIGHT JOIN

DFULL JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN which excludes unmatched rows from the left table.

Using RIGHT JOIN which keeps all rows from the right table instead.

The LEFT JOIN keyword returns all rows from the left table (employees), and the matched rows from the right table (departments). If there is no match, the result is NULL on the right side.

2fill in blank

medium

Complete the code to select employee names and their department names, preserving all employees even if they have no department.

SQL

SELECT employees.name, [1].name AS department_name FROM employees LEFT JOIN departments ON employees.department_id = departments.id;

Drag options to blanks, or click blank then click option'

Aprojects

Bemployees

Cmanagers

Ddepartments

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting employees.name as department_name by mistake.

Selecting a table that is not joined in the query.

We want the department name from the departments table, so we select departments.name.

3fill in blank

hard

Fix the error in the join condition to correctly join employees with departments preserving all employees.

SQL

SELECT employees.name, departments.name FROM employees LEFT JOIN departments ON employees.[1] = departments.id;

Drag options to blanks, or click blank then click option'

Adepartment_id

Bmanager_id

Cdept_id

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using employees.id which is the employee's own id, not the department reference.

Using a column that does not exist in employees.

The join condition must match employees.department_id with departments.id to link employees to their departments.

4fill in blank

hard

Fill both blanks to select employee names and their department names, showing 'No Department' if the department is missing.

SQL

SELECT employees.name, COALESCE(departments.[1], [2]) AS department_name FROM employees LEFT JOIN departments ON employees.department_id = departments.id;

Drag options to blanks, or click blank then click option'

Aname

B'No Department'

C'Unknown'

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong column name from departments.

Not providing a string literal for the default value.

We use departments.name to get the department name. If it is NULL, COALESCE returns the string 'No Department' instead.

5fill in blank

hard

Fill all three blanks to count employees per department, including departments with zero employees.

SQL

SELECT departments.[1], COUNT(employees.[2]) AS employee_count FROM departments LEFT JOIN employees ON employees.[3] = departments.id GROUP BY departments.name;

Drag options to blanks, or click blank then click option'

Aname

Bid

Cdepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Counting department id instead of employee id.

Joining on wrong columns causing incorrect counts.

We select departments.name to show the department name. We count employees.id to count employees. The join condition matches employees.department_id to departments.id.

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