Some time function records commonly used by mysql

1. no parameter required

   1. NOW()| CURRENT_TIMESTAMP
      Return current time (date + hour, minute and second): 2021-12-27 12:12:12

   2. CURDATE()| CURRENT_DATE()| CURRENT_DATE
      Return to the current date: 2021-12-27

   3. CURTIME()| CURRENT_TIME
      Return current time: 12:12:12

2. time conversion

   1. UNIX_TIMESTAMP(date), the date is converted to a timestamp
   2. FROM_UNIXTIME(date), the timestamp is converted to a date
   3. YEAR(date), returns the year
   4. WEEK(date), returns the week of the year
   5. HOUR(time), returns the hour value
   6. MINUTE(time), returns the minute value
   7. MONTHNAME(date), returns the month name of the date, such as December
   8. Select various parts of a datetime: date, time, year, quarter, month, day, hour, minute, second, microsecond (commonly used)

      SELECT now(),date(now()); -- 日期
      SELECT now(),time(now()); -- 时间
      SELECT now(),year(now()); -- 年
      SELECT now(),quarter(now()); -- 季度
      SELECT now(),month(now()); -- 月
      SELECT now(),week(now()); -- 周
      SELECT now(),day(now()); -- 日
      SELECT now(),hour(now()); -- 小时
      SELECT now(),minute(now()); -- 分钟
      SELECT now(),second(now()); -- 秒
      SELECT now(),microsecond(now()); -- 微秒
      

3. time calculation

   1. Add (add) or subtract (sub) an interval value expr to the given date date respectively

      interval is the interval type keyword
      expr is an expression corresponding to the following type

      DATE_ADD(date, INTERVAL expr unit);
      DATE_SUB(date, INTERVAL expr unit);

      unit is the time interval unit, and the time interval types are as follows

   unit	describe
   HOUR	Hour
   MINUTE	point
   SECOND	Second
   MICROSECOND	millisecond
   YEAR	Year
   MONTH	moon
   DAY	Day
   WEEK	week
   QUARTER	the quarter
   YEAR_MONTH	year and month
   DAY_HOUR	day and hour
   DAY_MINUTE	day and minute
   DAY_SECOND	day and second
   HOUR_MINUTE	hours and minutes
   HOUR_SECOND	hours and seconds
   MINUTE_SECOND	minutes and seconds

   example

   -- 获取上个月第一天和最后一天
   SELECT DATE_FORMAT(DATE_SUB(now(),INTERVAL 1 month), '%Y-%m-01'), DATE_SUB(DATE_FORMAT(now(), '%Y-%m-01'), INTERVAL 1 day);
   

   2. Get the time difference or the difference in days between two times

   1. TIMEDIFF(expr1, expr2): Returns the time difference between two dates subtracted (expr1 − expr2) (both parameter types must be the same)
   2. DATEDIFF(expr1, expr2): Returns the number of days between the subtraction of two dates (expr1 − expr2)

4. personalized display

   1. The return date is the day of the week, January, year

      1. Day of the week:dayofweek(date)
      2. Day of January:dayofmonth(date)
      3. Day of the year:dayofyear(date)

   2. Returns the day of the week and month name for a date

      The name is Chinese or English controlled by the system variable lc_time_names (default value is 'en_US')

      mysql> show variables like 'lc_time_names';
      +---------------+-------+
      | Variable_name | Value |
      +---------------+-------+
      | lc_time_names | en_US |
      +---------------+-------+
      1 row in set (0.00 sec)
      
      mysql> select dayname(now()),monthname(now());
      +----------------+------------------+
      | dayname(now()) | monthname(now()) |
      +----------------+------------------+
      | Monday         | January          |
      +----------------+------------------+
      1 row in set (0.00 sec)
      
      mysql> set lc_time_names='zh_CN';
      Query OK, 0 rows affected (0.00 sec)
      
      mysql> select dayname(now()),monthname(now());
      +----------------+------------------+
      | dayname(now()) | monthname(now()) |
      +----------------+------------------+
      | 星期一         | 一月             |
      +----------------+------------------+
      1 row in set (0.00 sec)