在 SELECT 中使用子查询

SQL 中的数据处理

Mona Khalil

Data Scientist, Greenhouse Software

SELECT 到底选什么？

返回单个值
- 可包含聚合值，与个体值对比

用于数学计算
- 与平均值的差异

在 SELECT 中使用子查询

计算所有赛季的比赛总数

SELECT COUNT(id) FROM match;

在 SELECT 中使用子查询

SELECT
  season,
  COUNT(id) AS matches,
  12837 as total_matches
FROM match
GROUP BY season;

| season    | matches | total_matches |
|-----------|---------|---------------|
| 2011/2012 | 3220    | 12837         |
| 2012/2013 | 3260    | 12837         |
| 2013/2014 | 3032    | 12837         |
| 2014/2015 | 3325    | 12837         |

在 SELECT 中使用子查询

SELECT
  season,
  COUNT(id) AS matches,
  (SELECT COUNT(id) FROM match) as total_matches
FROM match
GROUP BY season;

| season    | matches | total_matches |
|-----------|---------|---------------|
| 2011/2012 | 3220    | 12837         |
| 2012/2013 | 3260    | 12837         |
| 2013/2014 | 3032    | 12837         |
| 2014/2015 | 3325    | 12837         |

用于数学计算的 SELECT 子查询

SELECT AVG(home_goal + away_goal) 
FROM match
WHERE season = '2011/2012';

2.72

SELECT
  date,
  (home_goal + away_goal) AS goals,
  (home_goal + away_goal) - 2.72 AS diff
FROM match
WHERE season = '2011/2012';

在 SELECT 中使用子查询

SELECT
  date,
  (home_goal + away_goal) AS goals,
  (home_goal + away_goal) - 
     (SELECT AVG(home_goal + away_goal) 
      FROM match
      WHERE season = '2011/2012') AS diff
FROM match
WHERE season = '2011/2012';

| date       | goals | diff              |
|------------|-------|-------------------|
| 2011-07-29 | 3     | 0.28354037267081  |
| 2011-07-30 | 2     | -0.71645962732919 |
| 2011-07-30 | 4     | 1.28354037267081  |
| 2011-07-30 | 1     | -1.71645962732919 |

SELECT 子查询——注意事项

必须返回单个值
- 否则会报错
确保过滤条件位置正确
- 正确过滤主查询和子查询两者！

SELECT
    date,
    (home_goal + away_goal) AS goals,
    (home_goal + away_goal) - 
       (SELECT AVG(home_goal + away_goal) 
        FROM match
        WHERE season = '2011/2012') AS diff
FROM match
WHERE season = '2011/2012';

¡Vamos a practicar!

SQL 中的数据处理