使用同一表中的父id选择父名称

Question

我有一个带有tag_categories的表，我将它与其他一些表连接起来，这样我就可以获得文件标记的类别，在该表中，我有一列parent_id，它只是同一表中的另一个标记类别。在构建如下查询时，它确实检索到了parent_id，我不知道如何获取父名称：

查询：

select 
    tags.id, 
    tags.name,
    tag_categories.name,
    tag_categories.parent_id,
    count(files.id) 
from 
    "files" 
    inner join "file_tags_join" on "files"."public_id" = "file_tags_join"."file_public_id" 
    inner join "tags" on "file_tags_join"."tag_id" = "tags"."id" 
    inner join "tag_categories" on "tags"."category_id" = "tag_categories"."id" 
where 
    "tags"."category_id" is not null 
group by 
    tags.id, 
    tags.name,
    tag_categories.name,
    tag_categories.parent_id

除了我已经得到的之外，我希望得到的是与类别的parent_id匹配的类别的名称：

id  name    name    parent_id   count   parent_name
2   tag2    tagCategory2    1   1         tagCategory1
1   tag1    tagCategory1    (null)  1     null    

这是sql fiddle：

Answer 1

您可以在下面尝试

演示

select 
    tags.id, 
    tags.name,
    tag_categories.name,
    tag_categories.parent_id,
    b.name as parentname,
    count(files.id) 
    from 
    "files" 
    inner join "file_tags_join" on "files"."public_id" = "file_tags_join"."file_public_id" 
    inner join "tags" on "file_tags_join"."tag_id" = "tags"."id" 
    inner join "tag_categories" on "tags"."category_id" = "tag_categories"."id" 
    left join  "tag_categories" b on tag_categories.parent_id = b.id
    where 
    "tags"."category_id" is not null 
    group by 
    tags.id, 
    tags.name,
    tag_categories.name,
    tag_categories.parent_id,b.name

输出：

id  name    name         parent_id  parentname  count
2   tag2    tagCategory2    1       tagCategory1    1
1   tag1    tagCategory1                            1