实用函数概览

PostgreSQL 汇总统计与窗口函数

Michel Semaan

Data Scientist

到处都是 null

查询

SELECT
  Country, Medal, COUNT(*) AS Awards
FROM summer_medals
WHERE
  Year = 2008 AND Country IN ('CHN', 'RUS')
GROUP BY ROLLUP(Country, Medal)
ORDER BY Country ASC, Medal ASC;

null 表示分组汇总

结果

| Country | Medal  | Awards |
|---------|--------|--------|
| CHN     | Bronze | 57     |
| CHN     | Gold   | 74     |
| CHN     | Silver | 53     |
| CHN     | null   | 184    |
| RUS     | Bronze | 56     |
| RUS     | Gold   | 43     |
| RUS     | Silver | 44     |
| RUS     | null   | 143    |
| null    | null   | 327    |

认识 COALESCE

COALESCE() 接收一组值，从左到右返回第一个非 null 的值
COALESCE(null, null, 1, null, 2) → 1
适用于会返回 null 的 SQL 操作
- ROLLUP 和 CUBE
- 透视（Pivot）
- LAG 和 LEAD

消灭 null

查询

SELECT
  COALESCE(Country, 'Both countries') AS Country,
  COALESCE(Medal, 'All medals') AS Medal,
  COUNT(*) AS Awards
FROM summer_medals
WHERE
  Year = 2008 AND Country IN ('CHN', 'RUS')
GROUP BY ROLLUP(Country, Medal)
ORDER BY Country ASC, Medal ASC;

结果

| Country        | Medal      | Awards |
|----------------|------------|--------|
| Both countries | All medals | 327    |
| CHN            | All medals | 184    |
| CHN            | Bronze     | 57     |
| CHN            | Gold       | 74     |
| CHN            | Silver     | 53     |
| RUS            | All medals | 143    |
| RUS            | Bronze     | 56     |
| RUS            | Gold       | 43     |
| RUS            | Silver     | 44     |

压缩数据

之前

| Country | Rank |
|---------|------|
| CHN     | 1    |
| RUS     | 2    |
| USA     | 3    |

Rank 冗余，因为名次已隐含

之后

CHN, RUS, USA

更精炼且信息齐全，因为名次已隐含

认识 STRING_AGG

STRING_AGG(column, separator) 将列中的所有值用 separator 连接成一个字符串

STRING_AGG(Letter, ', ') 会把这个…

| Letter |
|--------|
| A      |
| B      |
| C      |

…转换为这个

A, B, C

查询与结果

之前

WITH Country_Medals AS (
  SELECT
    Country, COUNT(*) AS Medals
  FROM Summer_Medals
  WHERE Year = 2012
    AND Country IN ('CHN', 'RUS', 'USA')
    AND Medal = 'Gold'
    AND Sport = 'Gymnastics'
  GROUP BY Country),

  SELECT
    Country,
    RANK() OVER (ORDER BY Medals DESC) AS Rank
  FROM Country_Medals
  ORDER BY Rank ASC;

之后

WITH Country_Medals AS (...),

  Country_Ranks AS (...)

  SELECT STRING_AGG(Country, ', ')
  FROM Country_Medals;

结果

CHN, RUS, USA

Passons à la pratique !

PostgreSQL 汇总统计与窗口函数