Knex as query builder alternative in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Knex query?

Consider this Knex query to select users with age over 30. What will be the resulting SQL query string?

Express

const query = knex('users').select('id', 'name').where('age', '>', 30).toString();
console.log(query);

A SELECT `id`, `name` FROM `users` WHERE `age` > 30

B SELECT id, name FROM users WHERE age > 30

C select `id`, `name` from `users` where `age` > 30

D select id, name from users where age > 30

💡 Hint

Knex outputs SQL with backticks and lowercase keywords by default.

Knex generates SQL with backticks around identifiers and lowercase SQL keywords when calling toString().

❓ component_behavior

intermediate

What happens when you chain .insert() and .returning() in Knex?

Given this Knex code inserting a new user, what will be the value of 'result' after execution?

Express

const result = await knex('users').insert({name: 'Alice', age: 25}).returning('id');

A [{ id: 1 }]

B [1]

C 1

D undefined

💡 Hint

The returning method returns an array of inserted ids.

Knex returns an array of inserted ids when using .returning('id') with insert.

🔧 Debug

advanced

Why does this Knex query throw an error?

This code throws an error. What is the cause?

Express

knex('users').select('name').where({age: '> 30'}).then(console.log);

A The where clause uses an object with a string '> 30' instead of a comparison operator.

B The then method is not awaited.

C The select method is missing parentheses.

D The table name 'users' is invalid.

💡 Hint

Check how the where clause expects conditions.

Knex expects the where object values to be exact values, not comparison strings. Use .where('age', '>', 30) instead.

📝 Syntax

advanced

Which option correctly updates a user's age using Knex?

Choose the correct Knex syntax to update the age of user with id 5 to 40.

A knex('users').where('id', 5).update({age: 40});

B knex('users').set({age: 40}).where('id', 5);

C knex.update('users').set({age: 40}).where('id', 5);

D knex('users').update({age: 40}).where('id', 5);

💡 Hint

Knex update queries start with the table, then where, then update.

The correct order is knex('table').where(...).update(...). Options A, B, and C use invalid methods or incorrect order.

❓ state_output

expert

What is the final state of the database after this transaction?

Given this Knex transaction code, what rows remain in the 'products' table after commit?

Express

await knex.transaction(async trx => {
  await trx('products').where('stock', '<', 5).del();
  await trx('products').insert({name: 'New Product', stock: 10});
});

A All original products including those with stock < 5 plus the new product

B Only the new product with stock 10

C No products remain

D All products with stock >= 5 plus the new product with stock 10

💡 Hint

The transaction deletes low stock products then adds one new product.

The transaction deletes products with stock less than 5, then inserts a new product with stock 10. So remaining are products with stock >= 5 plus the new one.

Practice

1. What is the main advantage of using Knex as a query builder in an Express app?

easy

A. It allows writing database queries using JavaScript instead of raw SQL.

B. It automatically creates database tables without any code.

C. It replaces Express middleware for handling requests.

D. It compiles JavaScript into SQL code for faster execution.

Solution

1. Step 1: Understand Knex's purpose

   Knex is designed to let developers write database queries in JavaScript instead of SQL.

2. Step 2: Compare options

   Options B, C, and D describe features Knex does not provide. It does not create tables automatically, replace Express middleware, or compile JS into SQL.

3. Final Answer:

   It allows writing database queries using JavaScript instead of raw SQL. -> Option A

4. Quick Check:

   Knex = JS query builder [OK]

Hint: Knex lets you write queries in JS, not raw SQL [OK]

Common Mistakes:

• Thinking Knex creates tables automatically
• Confusing Knex with Express middleware
• Believing Knex compiles JS to SQL code

2. Which of the following is the correct way to select all rows from a table named users using Knex?

easy

A. knex.tables('users').select('*')

B. knex.from('users').selects()

C. knex.query('SELECT * FROM users')

D. knex.select('*').from('users')

Solution

1. Step 1: Review Knex select syntax

   The common pattern is knex.select('*').from('tableName') to get all rows.

2. Step 2: Check each option

   knex.select('*').from('users') matches the correct syntax. knex.from('users').selects() uses invalid selects() method. knex.tables('users').select('*') uses knex.tables which is not standard Knex syntax. knex.query('SELECT * FROM users') uses raw SQL string which is not the Knex query builder method.

3. Final Answer:

   knex.select('*').from('users') -> Option D

4. Quick Check:

   Correct select syntax = knex.select('*').from('users') [OK]

Hint: Use knex.select('*').from('table') to get all rows [OK]

Common Mistakes:

• Using knex.tables() instead of knex.select().from()
• Using selects() instead of select()
• Trying to pass raw SQL string to knex

3. What will the following Knex query return?

knex('products').where('price', '>', 100).select('id', 'name')

medium

A. All products with price less than or equal to 100, showing id and name.

B. All products with price greater than 100, showing all columns.

C. All products with price greater than 100, showing id and name.

D. Syntax error due to incorrect method chaining.

Solution

1. Step 1: Analyze the where clause

   The query filters products where price is greater than 100.

2. Step 2: Analyze the select clause

   It selects only the 'id' and 'name' columns to return.

3. Final Answer:

   All products with price greater than 100, showing id and name. -> Option C

4. Quick Check:

   where('price', '>', 100) + select('id', 'name') = All products with price greater than 100, showing id and name. [OK]

Hint: where filters rows; select chooses columns [OK]

Common Mistakes:

• Confusing > with < in where clause
• Assuming all columns are returned
• Thinking method chaining causes syntax error

4. Identify the error in this Knex query:

knex('orders').where('status' = 'pending').select()

medium

A. Missing table name in knex call.

B. Using '=' instead of ',' inside where method.

C. select() requires column names as arguments.

D. where method cannot be chained after knex call.

Solution

1. Step 1: Check where method syntax

   The where method expects arguments separated by commas, e.g., where('status', 'pending').

2. Step 2: Identify the error

   The query uses '=' inside where which is invalid syntax in JavaScript function calls.

3. Final Answer:

   Using '=' instead of ',' inside where method. -> Option B

4. Quick Check:

   where('status' = 'pending') is invalid [OK]

Hint: Use commas, not '=' inside where() arguments [OK]

Common Mistakes:

• Using '=' instead of ',' in method arguments
• Thinking select() must have columns
• Believing where cannot be chained

5. You want to update the email of a user with id = 5 using Knex. Which query correctly performs this update?

hard

A. knex('users').where('id', 5).update({ email: '[e-mail]' })

B. knex('users').set('email', '[e-mail]').where('id', 5)

C. knex.update('users').set('email', '[e-mail]').where('id', 5)

D. knex('users').update({ email: '[e-mail]' }).where('id' = 5)

Solution

1. Step 1: Recall correct update syntax

   Knex updates usually chain where() before update() to specify which rows to change.

2. Step 2: Evaluate each option

   knex('users').where('id', 5).update({ email: '[e-mail]' }) correctly chains where('id', 5) before update({ email: ... }). knex('users').update({ email: '[e-mail]' }).where('id' = 5) uses '=' instead of ',' causing syntax error. knex('users').set('email', '[e-mail]').where('id', 5) uses invalid set() method. knex.update('users').set('email', '[e-mail]').where('id', 5) uses invalid syntax.

3. Final Answer:

   knex('users').where('id', 5).update({ email: '[e-mail]' }) -> Option A

4. Quick Check:

   where() before update() is correct pattern [OK]

Hint: Chain where() before update() to target rows [OK]

Common Mistakes:

• Using '=' instead of ',' in where arguments
• Using non-existent set() method
• Using invalid knex.update('users') syntax