fetchOne is similar to fetch() but will return a tuple [id, entity] of the first entity in the result set or null if the result set is empty. For example:

await client.insert('employees', { id: 'Fry', name: 'Philip J. Fry' });
await client.insert('employees', { id: 'Leela', name: 'Turanga Leela' });
const query = client.query('employees').build();
const result = await client.fetchOne(query, options); // ['Fry', { name: 'Philip J. Fry' }]
const queryEmpty = client
  .where('name', '=', 'Bender')
const resultEmpty = await client.fetchOne(queryEmpty); // null

This is a convenient shorthand for using the limit parameter .limit(1) and extracting the result from the result set.