php - 在 mysql 中应用联接后如何获取字段为 null 或空的所有列

Question

我的第一个表结构和数据如下(允许):

-------------------------
|Sno   |   allwnce      |
------------------------|
|1     |    HRA         |
------------------------|
|2     |    CA          |
------------------------|
|3     |    TA          |
------------------------|
|4     |    SA          |
------------------------

第二个表结构和数据如下(save_employee_allowance):

----------------------------------------------
|Sno   |   EmpID     | allowanceName|  amount |
----------------------------------------------|
|1     |    EM-1001  |     HRA      |  1200   |
----------------------------------------------|
|2     |    EM-1001  |      CA      |  800    |
----------------------------------------------|

我想要这样的:

----------------------------------------------
|Sno   |   EmpID     | allowanceName|  amount |
----------------------------------------------|
|1     |    EM-1001  |     HRA      |  1200   |
----------------------------------------------|
|2     |    EM-1001  |      CA      |  800    |
----------------------------------------------|
|3     |    EM-1001  |     TA       |  NULL   |
----------------------------------------------|
|4     |    EM-1001  |      SA      |  NULL   |
----------------------------------------------|

我正在尝试像这样的左连接:

Select * 
From save_employee_allowance 
LEFT JOIN allowance 
ON allowance.allwnce = save_employee_allowance.allowanceName 
Where EmpID = 'EM-1001'

但它只给出匹配津贴名称的结果

Answer 1

如果您想要allowance 中的所有行，请将其作为LEFT JOIN 中的第一个表。此外，您还需要将 WHERE 条件移至 ON 子句:

Select a.Sno, coalesce(em.EmpID, 'EM-1001') as EmpId, a.allowanceName, ea.amount
From allowance a LEFT JOIN
     save_employee_allowance ea
     ON a.allwnce = ea.allowanceName and ea.EmpID = 'EM-1001';