左连接不 return Null 值

Question

假设我有三个表

剧组

+------------+----------+-------------------------+
| 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     |

员工类型

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

和员工

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

我已经连接了这三个表，但是对于名称为 NULL 的 EmployeeID 7，我没有使用左连接获得 NULL 值

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;

但是我没有在 crew 栏中得到 Null

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

我要在船员栏中 James, John, NULL

Answer 1

我想你打算：

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;

通常在带有 left join 的查询中，您需要继续使用它们。在这种情况下，我认为你想从 crew 开始，因为你是按 FlightNo 聚合的。我不知道您为什么要为不在任何航班上的员工排行。

另一个问题是 group_concat() 忽略了 NULL 值。如果您想查看它们，请使用 coalesce():

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

或者也许：

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

左连接不 return Null 值

Left join doesn't return the Null value

mysql

sql

innodb

join

left-join