SQL 生成日历

生成日历是SQL语言中的一个经典问题。下面我将介绍如何使用SQL生成日历。

创建日历表

首先，我们需要先创建一个日历表。以下是创建日历表的SQL代码：

CREATE TABLE calendar (
  `date` date NOT NULL PRIMARY KEY,
  year int(4) NOT NULL,
  month int(2) NOT NULL,
  day int(2) NOT NULL,
  quarter int(1) NOT NULL,
  week int(2) NOT NULL,
  dayofweek int(1) NOT NULL,
  dayofyear int(3) NOT NULL,
  holiday varchar(50),
  weekend int(1) NOT NULL,
  lunar_year int(4),
  lunar_month int(2),
  lunar_day int(2),
  solar_term varchar(10)
);

插入数据

然后，我们需要插入数据来填充日历表。以下是插入数据的SQL代码：

--生成数据1900-01-01到2100-12-31
SET @start_date = '1900-01-01';
SET @end_date = '2100-12-31';

WHILE @start_date <= @end_date DO
  INSERT INTO calendar (
    `date`,
    year,
    month,
    day,
    quarter,
    week,
    dayofweek,
    dayofyear,
    weekend,
    holiday,
    lunar_year,
    lunar_month,
    lunar_day,
    solar_term
  ) VALUES (
    @start_date,
    YEAR(@start_date),
    MONTH(@start_date),
    DAY(@start_date),
    QUARTER(@start_date),
    WEEKOFYEAR(@start_date),
    DAYOFWEEK(@start_date),
    DAYOFYEAR(@start_date),
    IF(weekday(@start_date)=5 OR weekday(@start_date)=6, 1, 0),
    NULL,
    NULL,
    NULL,
    NULL
  );

  SET @start_date = DATE_ADD(@start_date, INTERVAL 1 DAY);
END WHILE;

--添加节日
UPDATE calendar SET holiday = '元旦节' WHERE month = 1 AND day = 1;
UPDATE calendar SET holiday = '春节' WHERE month = 2 AND day = 5;
UPDATE calendar SET holiday = '清明节' WHERE month = 4 AND day = 4;
UPDATE calendar SET holiday = '劳动节' WHERE month = 5 AND day = 1;
UPDATE calendar SET holiday = '端午节' WHERE month = 6 AND day = 2;
UPDATE calendar SET holiday = '中秋节' WHERE month = 9 AND day = 15;
UPDATE calendar SET holiday = '国庆节' WHERE month = 10 AND day = 1;

查询数据

有了日历表的数据，我们就可以使用SQL语句轻松地生成日历了。以下是两个具体的实例：

实例1：按周生成日历

我们可以按周生成一个日历。如下所示：

SELECT
  year,
  week,
  CONCAT(MAX(CASE WHEN dayofweek = 2 THEN `date` END), ',',
         MAX(CASE WHEN dayofweek = 3 THEN `date` END), ',',
         MAX(CASE WHEN dayofweek = 4 THEN `date` END), ',',
         MAX(CASE WHEN dayofweek = 5 THEN `date` END), ',',
         MAX(CASE WHEN dayofweek = 6 THEN `date` END), ',',
         MAX(CASE WHEN dayofweek = 7 THEN `date` END)) AS week_range
FROM calendar
GROUP BY year, week;

该查询会生成以年和周为分组的日历，其中包含每周的日期范围。

实例2：按月生成日历

我们也可以按月生成一个日历。如下所示：

SELECT 
  *,
  CONCAT(YEAR(`date`), '-', LPAD(MONTH(`date`), 2, '0'), '-', LPAD(DAY(`date`), 2, '0')) AS `full_date`
FROM
  calendar
WHERE 
  year = 2021 AND month = 9
ORDER BY `date` ASC;

该查询会生成一个以2021年9月为基础的月份日历，并按日期升序排序。

总之，使用以上的方法，我们可以轻松地使用SQL生成日历表。