PG：左連接上表中把第一零記錄

Question

這裏是我的項目表：

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 

，這裏是我的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 

我做左外連接的兩個表：

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 

這個正確的訂單項目（最高平均是第一位的），但是在rating_c沒有匹配的記錄項目疼痛表甚至在最高平均值之前出現。對於例如： 項目：

id name 
1 A 
2 B 
3 C 

rating_caches：查詢

id cacheable_id avg 
1 3   3.0 
2 2   2.5 

結果如下：

id name 
1 A 
3 C 
2 B 

不宜與ID項目= 1放在最後？

Answer 1

只需使用NULLS LAST：

ORDER BY rating_caches.avg desc NULLS LAST 

作爲一個說明，該documentation規定：

的NULLS FIRST和NULLS LAST選項可以用來確定 空是否之前或之後非空值出現在排序 排序。默認情況下，空值的排序方式好像大於任何非空值 值;即NULLS FIRST是DESC訂單的默認值，NULLS LAST是其他訂單的默認值。