查询最高版本

Question

我有下表（剥离到基本要素）：（脚本在 postgresql 中）

CREATE TABLE t_version (
  name VARCHAR(64) NOT NULL,
  major INTEGER NOT NULL,
  minor INTEGER NOT NULL,
  attr VARCHAR(64)
);

ALTER TABLE t_version
  ADD CONSTRAINT PK_VERSION
  PRIMARY KEY (name, major, minor);

以及一些测试数据：

INSERT INTO t_version (name, major, minor, attr) VALUES ('n1', 1, 0, 'a1');
INSERT INTO t_version (name, major, minor, attr) VALUES ('n1', 1, 1, 'a2');
INSERT INTO t_version (name, major, minor, attr) VALUES ('n1', 2, 0, 'a3');

我想弄清楚，如何从中选择最高版本，即一个可以给我的选择

'n1', 2, 0, 'a3'

其结果。

我知道如何做到这一点，当只需要最高的次要版本时：

SELECT A.name, A.major, A.minor, A.attr
  FROM t_version AS A
    JOIN (
           SELECT name, major, 
             max(minor) AS minor
           FROM t_version
           GROUP BY name, major
         ) LATEST
      ON A.name = LATEST.name
         AND A.major = LATEST.major
         AND A.minor = LATEST.minor;

但我对更普遍的问题不知所措。

顺便说一句，该解决方案不仅适用于 sql server 的 postgresql。

Answer 1

每组最大 n 个问题通常使用窗口函数来解决。

以下是标准 SQL，也适用于 SQL Server：

select name, major, minor, attr
from (
  select name, major, minor, attr, 
         row_number() over (partition by name order by major desc, minor desc) as rn
  from t_version
) t
where rn = 1
order by name;

对于 Postgres，使用它通常会更快distinct on ()：

select distinct on (name) name, major, minor, attr
from t_version
order by name, major desc, minor desc;