How the join engine matches rows in SQL - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the two tables joined on the customer_id.

SQL

SELECT * FROM orders JOIN customers ON orders.[1] = customers.customer_id;

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

Aorder_date

Border_id

Ccustomer_id

Dproduct_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using order_id instead of customer_id in the join condition.

Joining on columns that don't relate the two tables.

The join condition must match the customer_id column from orders to customers to combine related rows.

2fill in blank

medium

Complete the code to perform an INNER JOIN between employees and departments on department_id.

SQL

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

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

AINNER

BRIGHT

CLEFT

DFULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN which includes all employees even without a department.

Using FULL JOIN which includes unmatched rows from both tables.

INNER JOIN returns rows where the join condition matches in both tables.

3fill in blank

hard

Fix the error in the join condition to correctly match orders with customers.

SQL

SELECT * FROM orders JOIN customers ON orders.[1] = customers.[2];

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

Aorder_id

Bcustomer_id

Cproduct_id

Dorder_date

Attempts:

3 left

💡 Hint

Common Mistakes

Using order_id or product_id in the join condition causing no matches.

Using different columns on each side of the join condition.

The join condition must use the customer_id column from both tables to match related rows.

4fill in blank

hard

Fill both blanks to select employee names and their department names using a join.

SQL

SELECT employees.[1], departments.[2] FROM employees JOIN departments ON employees.department_id = departments.department_id;

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

Aname

Bid

Dlocation

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting department id instead of name.

Selecting employee id instead of name.

We select the employee's name and the department's name columns to show both.

5fill in blank

hard

Fill all three blanks to select order id, customer name, and order date from joined tables.

SQL

SELECT orders.[1], customers.[2], orders.[3] FROM orders JOIN customers ON orders.customer_id = customers.customer_id;

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

Aorder_id

Bname

Corder_date

Dcustomer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting customer_id instead of customer name.

Mixing columns from wrong tables.

We select order_id and order_date from orders, and name from customers to show order details with customer names.

Practice

1. What does the SQL JOIN engine use to match rows from two tables?

easy

A. The ON condition specifying matching columns

B. The order of rows in each table

C. The number of columns in each table

D. The table names only

Solution

1. Step 1: Understand the role of the ON condition

   The ON condition defines which columns from each table must have matching values for rows to join.

2. Step 2: Recognize what the join engine uses

   The join engine uses this condition to pair rows correctly, ignoring row order or table names alone.

3. Final Answer:

   The ON condition specifying matching columns -> Option A

4. Quick Check:

   Join engine matches rows using ON condition [OK]

Hint: Remember: JOIN matches rows using ON condition columns [OK]

Common Mistakes:

• Thinking row order affects join matching
• Assuming table names determine matches
• Confusing number of columns with matching criteria

2. Which of the following is the correct syntax to join two tables Employees and Departments on the column DeptID?

easy

A. SELECT * FROM Employees JOIN Departments WHERE Employees.DeptID = Departments.DeptID;

B. SELECT * FROM Employees JOIN Departments USING Employees.DeptID;

C. SELECT * FROM Employees, Departments ON Employees.DeptID = Departments.DeptID;

D. SELECT * FROM Employees JOIN Departments ON Employees.DeptID = Departments.DeptID;

Solution

1. Step 1: Identify correct JOIN syntax

   The correct JOIN syntax uses JOIN ... ON ... to specify the matching condition.

2. Step 2: Check each option

   SELECT * FROM Employees JOIN Departments ON Employees.DeptID = Departments.DeptID; uses JOIN ... ON correctly. SELECT * FROM Employees JOIN Departments WHERE Employees.DeptID = Departments.DeptID; wrongly uses WHERE instead of ON. SELECT * FROM Employees, Departments ON Employees.DeptID = Departments.DeptID; misuses ON with comma join. SELECT * FROM Employees JOIN Departments USING Employees.DeptID; misuses USING syntax with table prefix.

3. Final Answer:

   SELECT * FROM Employees JOIN Departments ON Employees.DeptID = Departments.DeptID; -> Option D

4. Quick Check:

   JOIN syntax requires ON condition [OK]

Hint: JOIN needs ON, not WHERE or comma with ON [OK]

Common Mistakes:

• Using WHERE instead of ON for join condition
• Mixing comma joins with ON clause
• Incorrect USING syntax with table prefixes

3. Given tables Orders and Customers with columns CustomerID, what will be the result of this query?

SELECT Orders.OrderID, Customers.Name FROM Orders JOIN Customers ON Orders.CustomerID = Customers.CustomerID;

Assuming Orders has 3 rows with CustomerIDs 1, 2, 4 and Customers has 2 rows with CustomerIDs 1, 2.

medium

A. 3 rows with OrderIDs 1, 2, 4 and matching customer names for IDs 1 and 2 only

B. 2 rows with OrderIDs 1 and 2 only, matching customer names

C. All 3 rows with customer names, including NULL for CustomerID 4

D. No rows because CustomerID 4 does not exist in Customers

Solution

1. Step 1: Understand INNER JOIN behavior

   INNER JOIN returns only rows where the join condition matches in both tables.

2. Step 2: Apply to given data

   Orders have CustomerIDs 1, 2, 4; Customers have 1, 2. Only CustomerIDs 1 and 2 match, so only those rows appear.

3. Final Answer:

   2 rows with OrderIDs 1 and 2 only, matching customer names -> Option B

4. Quick Check:

   INNER JOIN returns only matching rows [OK]

Hint: INNER JOIN shows only matching rows from both tables [OK]

Common Mistakes:

• Expecting unmatched rows to appear with NULLs
• Confusing INNER JOIN with LEFT JOIN
• Assuming all rows from first table appear

4. You wrote this query but it returns fewer rows than expected:

SELECT * FROM Products JOIN Categories ON Products.CategoryID = Categories.ID;

What is the most likely cause?

medium

A. There are Products with CategoryID values not present in Categories

B. The query needs a WHERE clause to filter rows

C. The JOIN keyword is missing

D. The join condition column names are swapped

Solution

1. Step 1: Analyze INNER JOIN behavior

   INNER JOIN returns only rows where the join condition matches in both tables.

2. Step 2: Consider missing matches

   If some Products have CategoryID values not in Categories, those Products are excluded, reducing rows.

3. Final Answer:

   There are Products with CategoryID values not present in Categories -> Option A

4. Quick Check:

   Missing matches cause fewer rows in INNER JOIN [OK]

Hint: INNER JOIN excludes rows without matching keys [OK]

Common Mistakes:

• Thinking swapped column names cause fewer rows
• Assuming JOIN keyword missing causes fewer rows
• Believing WHERE clause is needed to fix join

5. You want to list all employees and their department names, but some employees have no department assigned. Which join type should you use to ensure all employees appear, even if their department is missing?

hard

A. RIGHT JOIN

B. INNER JOIN

C. LEFT JOIN

D. FULL JOIN

Solution

1. Step 1: Understand join types and their row inclusion

   INNER JOIN includes only matching rows; LEFT JOIN includes all rows from the left table and matches from right; RIGHT JOIN is opposite; FULL JOIN includes all rows from both.

2. Step 2: Apply to employees and departments

   To list all employees even if no department exists, use LEFT JOIN from Employees (left) to Departments (right).

3. Final Answer:

   LEFT JOIN -> Option C

4. Quick Check:

   LEFT JOIN keeps all left table rows [OK]

Hint: Use LEFT JOIN to keep all left table rows [OK]

Common Mistakes:

• Using INNER JOIN and missing employees without departments
• Confusing RIGHT JOIN with LEFT JOIN
• Assuming FULL JOIN is needed when LEFT JOIN suffices