Ensure only one single record is returned by Eloquent query

first() returns the a Model instance matching whatever went before in the Eloquent/Query Builder - it returns null if nothing is found.

firstOrFail() will throw a ModelNotFoundException if nothing is found.

Exactly one record can be returned only if at least one record exists!

You can use ->first()

Like Enployee::where('salary',2000)->first();

No, the idea here is different.

Example #1. There is 1 employee with 'salary' = 1000 in the database. Employee::where('salary', 1000)->firstOrFail() will return this employee.

Example #2. There are 2 employees with 'salary' = 1000 in the database. Employee::where('salary', 1000)->firstOrFail() will return one of them.

Example #3. There are no employees with 'salary' = 1000 in the database. In this case Employee::where('salary', 1000)->firstOrFail() will raise an exception.

The idea of method I'm talking about (singleOrFail()) is that it will raise an exception not only in example #3 (because there are no matching users), but also in example #2 (because there are many matching users).

In that case, extend the query builder with a macro (in a ServiceProvider)

\Illuminate\Database\Query\Builder::macro('singleOrFail', function () {
    $result = $this->get();
    if ($result->count() == 1) {
        return $result->first();
    }
    throw new \Exception; 
});

Employee::where('salary', 1000)
->groupBy('salary')
->havingRaw('COUNT(salary) = 1')
->first();

Just to close off this six-year-old thread, the method being saught here is sole().

$query->sole() will return exactly one record. If no records match, an exception will be thrown. If more than one record matches, also an exception will be thrown.