sql学习记录

leetcode上的blog

有下表。

+---------+------------+
| student | class      |
+---------+------------+
| A       | Math       |
| B       | English    |
| C       | Math       |
| D       | Biology    |
| E       | Math       |
| F       | Computer   |
| G       | Math       |
| H       | Math       |
| I       | Math       |
+---------+------------+

统计选课人数大于5的科目。

输出

：

+---------+
| class   |
+---------+
| Math    |
+---------+

解决：

第一种：

第一步：

SELECT
    class, COUNT(DISTINCT student)
FROM
    courses
GROUP BY class
;

output：

| class    | COUNT(student) |
|----------|----------------|
| Biology  | 1              |
| Computer | 1              |
| English  | 1              |
| Math     | 6              |

第二步：

SELECT
    class
FROM
    (SELECT
        class, COUNT(DISTINCT student) AS num
    FROM
        courses
    GROUP BY class) AS temp_table
WHERE
    num >= 5
;

第二种：

Mysql

SELECT
    class
FROM
    courses
GROUP BY class
HAVING COUNT(DISTINCT student) >= 5
;