[英]Left join doesn't return the Null value
假設我有三個桌子
船員
+------------+----------+-------------------------+
| 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 |
員工類型
+----------------+-----------------+
| EmployeeTypeID | Name |
+----------------+-----------------+
| 1 | Pilot |
| 2 | First Officer |
| 3 | Stewardess |
和員工
+------------+-----------+----------------+
| EmployeeID | Name | EmployeeTypeID |
+------------+-----------+----------------+
| 2 | James | 2 |
| 3 | Alexandra | 3 |
| 4 | Alina | 2 |
| 6 | Peter | 2 |
| 7 | NULL | 3 |
| 8 | John | 1 |
| 9 | Frank | 1 |
我已經加入了這三個表,但是對於名稱為NULL的EmployeeID 7,我沒有使用左連接獲得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;
但是我在乘員列中沒有得到Null
+----------+---------------------+---------------------------------+----------------------------------+
| FlightNo | time | crew | type |
+----------+---------------------+---------------------------------+----------------------------------+
| AA123 | 2018-12-30 09:30:45 | James,John | Pilot,First Officer, Stewardess |
我想要在劇組專欄中James,John,NULL
我認為您打算:
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;
通常在帶有left join
s的查詢中,您需要繼續使用它們。 在這種情況下,我認為您想從crew
入手,因為您正在按FlightNo
進行匯總。 我不知道您為什么要為不在任何航班上的員工排位。
另一個問題是group_concat()
忽略NULL
值。 如果要查看它們,請使用coalesce()
:
group_concat(coalesce(e.Name, '') separator ',') as crew,
也許:
group_concat(coalesce(e.Name, '<NULL>') separator ',') as crew,
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