Left join doesn't return the Null value

Question

Suppose I have three tables

crew

+------------+----------+-------------------------+
| EmployeeID | FlightNo | DepartureTime           |
+------------+----------+-------------------------+
|         2  | AA123    | 2018-12-30 09:30:45     |
|         7  | AA123    | 2018-12-30 09:30:45     |
|         8  | AA123    | 2018-12-30 09:30:45     |

employeetype

+----------------+-----------------+
| EmployeeTypeID | Name            |
+----------------+-----------------+
|              1 | Pilot           |
|              2 | First Officer   |
|              3 | Stewardess      |

and employee

+------------+-----------+----------------+
| EmployeeID | Name      | EmployeeTypeID |
+------------+-----------+----------------+
|          2 | James     |              2 |
|          3 | Alexandra |              3 |
|          4 | Alina     |              2 |
|          6 | Peter     |              2 |
|          7 | NULL      |              3 |
|          8 | John      |              1 |
|          9 | Frank     |              1 |

I have joined these three tables but for EmployeeID 7 where the Name is NULL I don't get the NULL value using left join

select crew.FlightNo, group_concat(distinct(crew.DepartureDateAndTimeUTC) separator ',') as time, group_concat(employee.Name separator ',') as crew,group_concat(distinct(employeetype.Name)separator ',') as type
from employee 
left join crew on employee.EmployeeID = crew.EmployeeID 
inner join employeetype 
on employeetype.EmployeeTypeID = employee.EmployeeTypeID group by crew.FlightNo;

But I don't get the Null in the crew column

+----------+---------------------+---------------------------------+----------------------------------+
| FlightNo | time                | crew                            | type                             |
+----------+---------------------+---------------------------------+----------------------------------+
| AA123    | 2018-12-30 09:30:45 | James,John                      | Pilot,First Officer, Stewardess  |

I want in the crew column James, John, NULL

Answer 1

I think you intend:

select c.FlightNo,
       group_concat(distinct c.DepartureDateAndTimeUTC) separator ',') as time,
       group_concat(e.Name separator ',') as crew,
       group_concat(distinct et.Name) separator ',') as type
from crew c left join
     employee e
     on e.EmployeeID = c.EmployeeID left join
     employeetype et
     on et.EmployeeTypeID = e.EmployeeTypeID
group by c.FlightNo;

Usually in a query with left joins, you need to keep using them. In this case, I think you want to start with crew, because you are aggregating by FlightNo. I don't know why you would want a row for employees that are not on any flights.

The other issue is that group_concat() ignores NULL values. If you want to see them, use coalesce():

   group_concat(coalesce(e.Name, '') separator ',') as crew,

or perhaps:

   group_concat(coalesce(e.Name, '<NULL>') separator ',') as crew,