在父表和子表之间加入

Question

I've 2 tables: Parent and Child with following data

Now when I execute the following queries:

delete from Parent where Id in(2,3,4)
delete from Child

The only record is left Parent table

Now when I execute the following query I don't get any records

select p.Id AS [ParentId],p.Name AS [ParentName], c.Id, c.Name from Parent p
Left join Child c on p.Id = c.ParentId 
where p.IsActive = 1 and c.IsActive = 1

And when I remove and c.IsActive = 1 from above query I get the record in Package table, but I want to apply both the active checks. How to achieve this?

Answer 1

Your where clause turns your left join into an inner join . Use

select p.Id AS [ParentId],p.Name AS [ParentName], c.Id, c.Name 
from Parent p
Left join Child c on p.Id = c.ParentId 
                 and c.IsActive = 1
where p.IsActive = 1 

All filters of the left joined table need to be in the on clause.

Answer 2

另一个解决方案是在where子句条件中处理这两种情况（当c.IsActive为null时 - 在子表中缺少行 - 或者为1）

where p.IsActive = 1 and COALESCE(c.IsActive, 1) = 1