子查询在 WHERE 中？

SQL 中的数据处理

Mona Khalil

Data Scientist, Greenhouse Software

什么是子查询？

嵌套在另一个查询中的查询

SELECT column
FROM (SELECT column
      FROM table) AS subquery;

适用于中间变换

子查询能做什么？

可出现在查询的任意部分
- SELECT、FROM、WHERE、GROUP BY
可返回多种信息
- 标量（3.14159、-2、0.001）
- 列表（id = (12, 25, 392, 401, 939)）
- 表

为什么用子查询？

将分组与汇总值比较
- 当年利物浦与英超平均表现相比如何？
重塑数据
- 德甲每月进球数的最高平均值是多少？
合并无法直接连接的数据
- 如何把主队和客队名称加入比赛结果表？

简单子查询

可独立于外层查询计算

SELECT home_goal
FROM match
WHERE home_goal > (
    SELECT AVG(home_goal) 
    FROM match);

SELECT AVG(home_goal) FROM match;

1.56091291478423

简单子查询

在整个语句中只处理一次

SELECT home_goal
FROM match
WHERE home_goal > (
    SELECT AVG(home_goal) 
    FROM match);

WHERE 子句中的子查询

2012/2013 赛季中，哪些比赛的主队进球高于总体平均值？

SELECT AVG(home_goal) FROM match;

1.56091291478423

SELECT date, hometeam_id, awayteam_id, home_goal, away_goal
FROM match
WHERE season = '2012/2013'
      AND home_goal > 1.56091291478423;

WHERE 子句中的子查询

2012/2013 赛季中，哪些比赛的主队进球高于总体平均值？

SELECT date, hometeam_id, awayteam_id, home_goal, away_goal
FROM match
WHERE season = '2012/2013'
      AND home_goal > (SELECT AVG(home_goal)
                     FROM match);

| date       | hometeam_id | awayteam_id | home_goal | away_goal |
|------------|-------------|-------------|-----------|-----------|
| 2012-07-28 | 9998        | 1773        | 5         | 2         |
| 2012-07-29 | 9987        | 9984        | 3         | 3         |
| 2012-10-05 | 9993        | 9991        | 2         | 2         |

用 IN 的子查询筛选列表

哪些球队属于波兰联赛？

SELECT
  team_long_name,
  team_short_name AS abbr
FROM team
WHERE 
  team_api_id IN 
  (SELECT hometeam_id
   FROM match
   WHERE country_id = 15722);

| team_long_name | abbr |
|----------------|------|
| Ruch Chorzów   | CHO  |
| Jagiellonia    | BIA  |
| Lech Poznań    | POZ  |
| P. Warszawa    | PWA  |
| Cracovia       | CKR  |
| Górnik Łęczna  | LEC  |
| Polonia Bytom  | GOR  |
| Zagłębie Lubin | ZAG  |
| Pogoń Szczecin | POG  |
| Widzew Łódź    | WID  |
| Śląsk Wrocław  | SLA  |

Vamos praticar!

SQL 中的数据处理