这是我的项目表：
Column | Type | Modifiers | Storage | Stats target | Description
------------+-----------------------------+-------------------------------------------------------+----------+--------------+-------------
id | integer | not null default nextval('projects_id_seq'::regclass) | plain | |
name | character varying(255) | | extended | |
repo_id | integer | | plain | |
created_at | timestamp without time zone | | plain | |
updated_at | timestamp without time zone | | plain | |
user_id | integer | | plain | |
data_path | character varying(255) | | extended | |
private | boolean | default false | plain | |
uniqueurl | character varying(255) | | extended | |
urlbase | character varying(255) | | extended | |
ancestry | character varying(255) | | extended | |
deleted_at | timestamp without time zone | | plain | |
Indexes:
"projects_pkey" PRIMARY KEY, btree (id)
"index_projects_on_name_and_user_id" UNIQUE, btree (name, user_id)
"index_projects_on_ancestry" btree (ancestry)
"index_projects_on_deleted_at" btree (deleted_at)
Has OIDs: no
这是我的rating_cache表：
Column | Type | Modifiers | Storage | Stats target | Description
----------------+-----------------------------+------------------------------------------------------------+----------+--------------+-------------
id | integer | not null default nextval('rating_caches_id_seq'::regclass) | plain | |
cacheable_id | integer | | plain | |
cacheable_type | character varying(255) | | extended | |
avg | double precision | not null | plain | |
qty | integer | not null | plain | |
dimension | character varying(255) | | extended | |
created_at | timestamp without time zone | | plain | |
updated_at | timestamp without time zone | | plain | |
Indexes:
"rating_caches_pkey" PRIMARY KEY, btree (id)
"index_rating_caches_on_cacheable_id_and_cacheable_type" btree (cacheable_id, cacheable_type)
Has OIDs: no
我正在使用以下两个表进行左外连接：
SELECT "projects".* FROM "projects" LEFT OUTER JOIN rating_caches
ON rating_caches.cacheable_id = projects.id
WHERE "projects"."deleted_at" IS NULL ORDER BY rating_caches.avg desc
这正确地命令项目（最高平均值先到先）但是在rating_caches表中没有匹配记录的项目甚至在最高平均值之前。例如： 项目：
id name
1 A
2 B
3 C
rating_caches：
id cacheable_id avg
1 3 3.0
2 2 2.5
查询结果如下：
id name
1 A
3 C
2 B
id = 1的项目不应该持续吗？
答案 0 :(得分：2)
只需使用NULLS LAST：
ORDER BY rating_caches.avg desc NULLS LAST
作为备注，documentation指定：
NULLS FIRST和NULLS LAST选项可用于确定 是否在排序中的非空值之前或之后出现空值 排序。默认情况下，null值排序为大于任何非null值 值;也就是说，NULLS FIRST是DESC订单的默认设置，否则为NULLS LAST。