Table aliases in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign an alias c to the table customers.

MySQL

SELECT * FROM customers [1];

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

AAS c

Bc

CALIAS c

Dc AS

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the alias before the table name.

Using the word ALIAS which is not valid in MySQL.

Omitting the AS keyword but placing alias incorrectly.

In MySQL, to give a table an alias, you use AS alias_name. So AS c correctly assigns the alias c to customers.

2fill in blank

medium

Complete the code to select the name column from the table employees using alias e.

MySQL

SELECT [1] FROM employees AS e;

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

Aemployees.name

Be.name

Cname

Demployees AS e.name

Attempts:

3 left

💡 Hint

Common Mistakes

Using the full table name instead of the alias.

Omitting the alias and just writing the column name when ambiguous.

Trying to alias the column incorrectly.

When a table has an alias, you refer to its columns using the alias. So e.name selects the name column from the alias e.

3fill in blank

hard

Fix the error in the code by completing the alias for the table orders as o.

MySQL

SELECT o.order_id FROM orders [1];

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

AALIAS o

Bo AS

CAS o

DAS

Attempts:

3 left

💡 Hint

Common Mistakes

Placing the alias before the AS keyword.

Using unsupported keywords.

Omitting the alias name.

The correct syntax to alias a table is AS alias. So AS o correctly aliases orders as o.

4fill in blank

hard

Fill both blanks to alias tables products as p and categories as c in the JOIN.

MySQL

SELECT p.product_name, c.category_name FROM products [1] JOIN categories [2] ON p.category_id = c.id;

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

AAS p

Bp AS

CAS c

Dc AS

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the alias and AS keyword.

Using unsupported keywords.

Forgetting to alias one of the tables.

Each table is aliased using AS alias. So AS p and AS c correctly alias products and categories.

5fill in blank

hard

Fill all three blanks to alias students as s, select s.name as student_name, and filter where s.age is greater than 18.

MySQL

SELECT [1] AS student_name FROM students [2] WHERE [3] > 18;

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

As.name

BAS s

Cs.age

Dstudents AS s

Attempts:

3 left

💡 Hint

Common Mistakes

Not using the alias in the WHERE clause.

Trying to alias the column incorrectly.

Using the full table name instead of the alias.

The table students is aliased as s using AS s. The column s.name is selected and renamed as student_name. The filter uses s.age to check age greater than 18.

Practice

1. What is the main purpose of using table aliases in a SQL query?

easy

A. To delete a table after the query runs

B. To give a short name to a table for easier reference

C. To create a new table from an existing one

D. To rename columns permanently in the database

Solution

1. Step 1: Understand what table aliases do

   Table aliases provide a temporary short name for a table within a query.

2. Step 2: Compare options with this purpose

   Only To give a short name to a table for easier reference correctly describes this purpose; others describe unrelated actions.

3. Final Answer:

   To give a short name to a table for easier reference -> Option B

4. Quick Check:

   Table aliases = short table names [OK]

Hint: Aliases shorten table names to simplify queries [OK]

Common Mistakes:

• Thinking aliases rename tables permanently
• Confusing aliases with creating or deleting tables
• Using aliases only for columns

2. Which of the following is the correct syntax to assign an alias c to a table named customers in a SELECT query?

easy

A. SELECT * FROM customers c c;

B. SELECT * FROM customers c AS;

C. SELECT * FROM AS customers c;

D. SELECT * FROM customers AS c;

Solution

1. Step 1: Recall correct alias syntax

   The correct syntax is: FROM table_name AS alias or FROM table_name alias.

2. Step 2: Check each option

   SELECT * FROM customers AS c; uses 'AS' correctly. Options B, C, and D have syntax errors or misplaced keywords.

3. Final Answer:

   SELECT * FROM customers AS c; -> Option D

4. Quick Check:

   Correct alias syntax = SELECT * FROM customers AS c; [OK]

Hint: Use 'AS' between table name and alias or just space [OK]

Common Mistakes:

• Placing 'AS' after alias name
• Using multiple aliases for one table
• Misordering keywords in FROM clause

3. Given the tables orders and customers, what will the following query return?

SELECT c.name, o.order_id FROM customers AS c JOIN orders AS o ON c.id = o.customer_id;

medium

A. A list of customer names with their order IDs

B. A list of order IDs only

C. A list of customer names only

D. An error due to incorrect alias usage

Solution

1. Step 1: Understand the query structure

   The query joins customers (aliased as c) and orders (aliased as o) on matching IDs.

2. Step 2: Analyze the SELECT clause

   It selects customer names (c.name) and order IDs (o.order_id), so both appear in results.

3. Final Answer:

   A list of customer names with their order IDs -> Option A

4. Quick Check:

   Join with aliases returns combined columns [OK]

Hint: Aliases let you select columns clearly from joined tables [OK]

Common Mistakes:

• Assuming aliases cause errors
• Expecting only one column in output
• Ignoring join condition effects

4. Identify the error in this query using table aliases:

SELECT c.name, o.order_id FROM customers c JOIN orders o ON c.id = orders.customer_id;

medium

A. Missing AS keyword for aliases

B. Aliases cannot be used in JOIN clauses

C. Using 'orders' instead of alias 'o' in ON clause

D. SELECT clause must use full table names

Solution

1. Step 1: Check alias usage in JOIN condition

   The query aliases 'orders' as 'o', but ON clause uses 'orders.customer_id' instead of 'o.customer_id'.

2. Step 2: Understand alias consistency

   Once aliased, the original table name should not be used in the query; use the alias instead.

3. Final Answer:

   Using 'orders' instead of alias 'o' in ON clause -> Option C

4. Quick Check:

   Use aliases consistently after defining them [OK]

Hint: Use alias names everywhere after aliasing tables [OK]

Common Mistakes:

• Mixing original table names with aliases
• Thinking AS keyword is mandatory
• Assuming aliases can't be used in JOIN

5. You want to join the employees table to itself to find pairs of employees who share the same manager. Which query correctly uses table aliases to achieve this?

hard

A. SELECT e1.name, e2.name FROM employees AS e1 JOIN employees AS e2 ON e1.manager_id = e2.manager_id WHERE e1.id != e2.id;

B. SELECT employees.name, employees.name FROM employees JOIN employees ON employees.manager_id = employees.manager_id;

C. SELECT e.name FROM employees e JOIN employees e ON e.manager_id = e.manager_id;

D. SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.id = e2.id;

Solution

1. Step 1: Understand self-join with aliases

   Joining a table to itself requires different aliases to distinguish rows.

2. Step 2: Check join condition and alias usage

   SELECT e1.name, e2.name FROM employees AS e1 JOIN employees AS e2 ON e1.manager_id = e2.manager_id WHERE e1.id != e2.id; correctly uses aliases e1 and e2 and joins on matching manager_id, excluding same employee pairs.

3. Step 3: Review other options for errors

   SELECT employees.name, employees.name FROM employees JOIN employees ON employees.manager_id = employees.manager_id; uses same table names without aliases, causing ambiguity. SELECT e.name FROM employees e JOIN employees e ON e.manager_id = e.manager_id; uses same alias twice, invalid syntax. SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.id = e2.id; joins on employee IDs, not managers.

4. Final Answer:

   SELECT e1.name, e2.name FROM employees AS e1 JOIN employees AS e2 ON e1.manager_id = e2.manager_id WHERE e1.id != e2.id; -> Option A

5. Quick Check:

   Self-join needs distinct aliases and correct join condition [OK]

Hint: Use different aliases for self-join tables [OK]

Common Mistakes:

• Using same alias for both tables
• Joining on wrong columns
• Not excluding same row pairs in self-join