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.