每个主题按星期几和周总计计算总计

Question

在我的表格中，我记录了各种信息，但此问题的关键字段是 subject_id 和 course_time对于每个主题（每周总计），因此来自此示例数据：

id    subject_id   lesson_time
1        4         2015-04-28
2        4         2015-04-28
3        3         2015-04-28
4        1         2015-04-28
5        4         2015-04-27

我想计算周一到周五和每周的每个主题的总数，因此上述数据示例的输出将是：

subject_id    monday_total   tuesday_total   wednesday_total  ... week_total
  1               0                1               0                  1
  3               0                1               0                  1
  4               1                2               0                  3

我可以很容易地按主题计算总数，因为它只是分组后的计数（*），我正在努力的是每一天的计数，我当前（非工作）查询是

SELECT
    subject_id,
    COUNT( DAYOFWEEK(lesson_time)=2) AS monday_total,
    COUNT( DAYOFWEEK(lesson_time)=3) AS tuesday_total,
    COUNT( DAYOFWEEK(lesson_time)=4) AS wednesday_total,
    COUNT( DAYOFWEEK(lesson_time)=5) AS thursday_total,
    COUNT( DAYOFWEEK(lesson_time)=6) AS friday_total,
    COUNT(*) AS week_total
FROM
    tbl_lessons
GROUP BY
    subject_id

任何帮助将不胜感激。

Answer 1

试试这个

SELECT
    subject_id,
    SUM(DAYOFWEEK(lesson_time)=2) AS monday_total,
    SUM(DAYOFWEEK(lesson_time)=3) AS tuesday_total,
    SUM(DAYOFWEEK(lesson_time)=4) AS wednesday_total,
    SUM(DAYOFWEEK(lesson_time)=5) AS thursday_total,
    SUM(DAYOFWEEK(lesson_time)=6) AS friday_total,
    COUNT(*) AS week_total
FROM
    tbl_lessons
GROUP BY
    subject_id

在 mysql 中，COUNT(n) 计算每一行（n 不是 NULL 的每一行都加 1）。 SUM(n) 对所有 n 值求和（每行 + n）。

当布尔表达式返回 1 或 0 SUM(DAYOFWEEK(lesson_time)=2) 将返回 DAYOFWEEK(lesson_time)=2 所在的行数（这就像对布尔值求和 1+0+0+1+1+1+0+0+1+...）

Answer 2

SELECT date(lesson_time) AS lesson_time, count( * ) AS count FROM tbl_lessons GROUP BY date(lesson_time)