sql - 在同一表中使用父 ID 选择父名称

Question

我有一个带有 tag_categories 的表，我正在将它与其他一些表连接起来，这样我就可以获得文件标签的类别，在该表中我有一个 parent_id 列，它只是同一个表中的另一个标签类别。在构建如下查询时，它确实检索了 parent_id，但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，我期望得到的是与类别的 parent_id 匹配的类别名称:

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

你可以试试下面

DEMO

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