// 2. Formulate a query to select users who do not have a EUR wallet.

Suppose there are two tables: users and wallets, where each wallet has a currency field (for example, 'EUR'). You need to select users who do not have a wallet with the EUR currency.

Example SQL query:

SELECT u.*
FROM users u
WHERE NOT EXISTS (
    SELECT 1 FROM wallets w
    WHERE w.user_id = u.id AND w.currency = 'EUR'
);

This query selects all users for whom there is no record in the wallets table with the EUR currency.

If using Node.js with a database library (for example, pg), the query would look like this:

const { Client } = require('pg');
const client = new Client({ /* connection parameters */ });

async function getUsersWithoutEURWallet() {
  await client.connect();
  const res = await client.query(`
    SELECT u.*
    FROM users u
    WHERE NOT EXISTS (
      SELECT 1 FROM wallets w
      WHERE w.user_id = u.id AND w.currency = 'EUR'
    );
  `);
  await client.end();
  return res.rows;
}

Thus, we get a list of users without an EUR wallet.