按星期几和每个主题的星期总数计算总数

Question

In my table I have recorded various bits of information but the key fields for this problem are the subject_id and lesson_time where I want to group by subject to give a total count of rows for each day of week for each subject and total rows for each subject(weekly total), so from this sample data:

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

I want to count the totals for each subject for monday to friday and week total, so output for the above data example would be:

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

I can get total by subject easy enough as it is just count(*) after group by, what I am struggling with is the count for each individual day, my current (non working) query is

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

Any help would be much appreciated.

Answer 1

try this

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

In mysql COUNT(n) counts every row (+ 1 for every row where n is not NULL ). SUM(n) sums all n values (+ n for every row).

As boolean expression returns 1 or 0 SUM(DAYOFWEEK(lesson_time)=2) will return number of rows where DAYOFWEEK(lesson_time)=2 (it's like summing booleans 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)