SQL(count)

• 逗号,多条件计数方式

  SELECT
  	DATE_FORMAT( start_time, '%Y%m' ) MONTH,
  	ROUND( COUNT( DISTINCT uid, DATE_FORMAT( start_time, '%Y%M%d' ))/ COUNT( DISTINCT uid ), 2 ),
  	COUNT( DISTINCT uid ) 
  FROM
  	exam_record 
  WHERE
  	score IS NOT NULL 
  	AND DATE_FORMAT( start_time, '%Y' )= '2021' 
  GROUP BY
  	DATE_FORMAT(
  	start_time,
  	'%Y%m')
  

• count计数方式可以and 表示多条件计数

	SELECT
	COUNT( start_time ) AS total_pv,#有开始作答时间可视为一次作答
	COUNT( submit_time ) AS complete_pv,#有交卷时间可视为完成一次作答
	-- and 计数方式
	count( DISTINCT exam_id AND score IS NOT NULL ) AS complete_exam_cnt #将试卷去重，同时将筛选完成的试卷
	FROM
	    exam_record;

count判断条件计数方式，不能直接在count中用等号=，此时显示的是对是否等于1的结果的计数，并不是等于1时的计数。可以用sum-0-1的方式代替。

SELECT
	uid,
	sum(IF( submit_time IS NULL, 1, 0 )) `0`,
	sum(IF( submit_time IS NULL, 0, 1 )) `1`,
	count( IF ( submit_time IS NULL, 0, 1 )= 1 ) `total` 
FROM
	exam_record 
GROUP BY
	uid

结果展示：

也可以用case when 的方式

SELECT
	uid,
	COUNT( CASE WHEN submit_time IS NULL THEN 1 ELSE NULL END) im ,
	COUNT( 1 )- COUNT( CASE WHEN submit_time IS NOT NULL THEN 1 ELSE NULL END ) im,
	COUNT( CASE WHEN submit_time IS NOT NULL THEN 1 ELSE NULL END ) co
FROM
	exam_record
GROUP BY
	uid 

结果展示：

建表语句

drop table if exists exam_record;
CREATE TABLE  exam_record (
    id int PRIMARY KEY AUTO_INCREMENT COMMENT '自增ID',
    uid int NOT NULL COMMENT '用户ID',
    exam_id int NOT NULL COMMENT '试卷ID',
    start_time datetime NOT NULL COMMENT '开始时间',
    submit_time datetime COMMENT '提交时间',
    score tinyint COMMENT '得分'
)CHARACTER SET utf8 COLLATE utf8_general_ci;


INSERT INTO exam_record(uid,exam_id,start_time,submit_time,score) VALUES
(1001, 9001, '2021-07-02 09:01:01', '2021-07-02 09:21:01', 80),
(1002, 9001, '2021-09-05 19:01:01', '2021-09-05 19:40:01', 81),
(1002, 9002, '2021-09-02 12:01:01', null, null),
(1002, 9003, '2021-09-01 12:01:01', null, null),
(1002, 9001, '2021-07-02 19:01:01', '2021-07-02 19:30:01', 82),
(1002, 9002, '2021-07-05 18:01:01', '2021-07-05 18:59:02', 90),
(1003, 9002, '2021-07-06 12:01:01', null, null),
(1003, 9003, '2021-09-07 10:01:01', '2021-09-07 10:31:01', 86),
(1004, 9003, '2021-09-06 12:01:01', null, null),
(1002, 9003, '2021-09-01 12:01:01', '2021-09-01 12:31:01', 81),
(1005, 9001, '2021-09-01 12:01:01', '2021-09-01 12:31:01', 88),
(1005, 9002, '2021-09-01 12:01:01', '2021-09-01 12:31:01', 88),
(1006, 9002, '2021-09-02 12:11:01', '2021-09-02 12:31:01', 89);