Sql 从联接查询中获取每个查询的最新值 系统中大约有100项资产 每项资产都有许多购买行为 我想了解每项资产的最新购买情况

我尝试以下查询：

CREATE TABLE asset (
  asset_name VARCHAR(255) NOT NULL PRIMARY KEY
);

CREATE TABLE asset_purchase (
  asset_name         VARCHAR(255) REFERENCES asset,
  purchase_value     INT NOT NULL,
  purhcase_timestamp INT NOT NULL
);

但它只返回一个条目

SELECT * 
FROM 
asset 
JOIN asset_purchase USING (asset_name) ORDER BY purhcase_timestamp LIMIT 1;

，或

，或

，或（在PostgreSQL 9.3+中）

---

用

postgresql

重新标记为

psql

应该用于解决工具的问题，而不是一般的Postgres问题。当然，它只返回一行：您已经告诉Postgres，使用

LIMIT 1

非常感谢！我唯一能听懂的就是这一点。哪个性能更好？@samol：这取决于您的数据分布和索引。查询2和查询4基本相同，只是4是9.3及以上版本的首选语法。

SELECT  *
FROM    (
        SELECT  ap.*,
                ROW_NUMBER() OVER (PARTITION BY a.asset_name ORDER BY purchase_timestamp DESC) rn
        FROM    asset a
        LEFT JOIN
                asset_purchase ap
        ON      ap.asset_name = a.asset_name
        ) q
WHERE   rn = 1

SELECT  (a).*, (ap).*
FROM    (
        SELECT  asset.a,
                (
                SELECT  ap
                FROM    asset_purchase ap
                WHERE   ap.asset_name = a.asset_name
                ORDER BY
                        purchase_timestamp DESC
                LIMIT 1
                )
         ) q

SELECT  DISTINCT ON (a.asset_name)
        *
FROM    asset a
LEFT JOIN
        asset_purchase ap
ON      ap.asset_name = a.asset_name
ORDER BY
        a.asset_name, ap.purchase_timestamp DESC

SELECT  *
FROM    asset
LEFT JOIN
        LATERAL
        (
        SELECT  *
        FROM    asset_purchase ap
        WHERE   ap.asset_name = a.asset_name
        ORDER BY
                purchase_timestamp DESC
        LIMIT 1
        ) ap
ON      TRUE