PostgreSQL：合并 Count 和 DISTINCT ON

Question

鉴于此 table

| id | name  | created_at                 |
| 1  | test  | 2015-02-24 11:13:28.605968 |
| 2  | other | 2015-02-24 13:04:56.968004 |
| 3  | test  | 2015-02-24 11:14:24.670765 |
| 4  | test  | 2015-02-24 11:15:05.293904 |

而此查询 return 只有行 ID 2 和 ID 4。

SELECT DISTINCT ON (documents.name) documents.*
FROM "documents"  
ORDER BY documents.name, documents.created_at DESC

如何 return 影响行数？像

SELECT COUNT(DISTINCT ON (documents.name) documents.*) FROM "documents"

Answer 1

您可以使用外部查询：

SELECT COUNT(1)
FROM (
    SELECT DISTINCT ON (name) *
    FROM documents
    ORDER BY name, created_at DESC
    ) alias

PostgreSQL：合并 Count 和 DISTINCT ON

PostgreSQL: Combine Count and DISTINCT ON

postgresql

count

distinct