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Detailed explanation of the usage of date function in MySQL learning

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Release： 2022-08-15 20:19:07

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This article brings you relevant knowledge about mysql, which mainly introduces MySQL’s date functions, including obtaining system time functions, date formatting functions, etc. Let’s learn about it together Take a look, hope it helps everyone.

Detailed explanation of the usage of date function in MySQL learning

Recommended learning: mysql video tutorial

Get the system time function

"NOW()" function can Get the current system date and time in the following format: "YYYY-MM-DD hh:mm:ss" (the hour unit here is 24-hour system)

The "CURDATE()" function can get the current system date and time Date, the format is as follows: "YYYY-MM-DD"

"CURTIME()" function can get the current system time, the format is as follows: "hh:mm:ss" (24-hour format)

SELECT NOW();            -- 返回结果 "2020-06-10 17:22:51"  (示例)

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SELECT CURDATE();        -- 返回结果 "2020-06-10"

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SELECT CURTIME();        -- 返回结果 "17:22:52"

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Date formatting function

The "DATE_FORMAT()" function is used to format dates and can help us extract very useful date information

The syntax is as follows:

DATE_FORMAT(date, expression)

SELECT ename, DATE_FORMAT(hiredate,"%Y") AS "入职日期" FROM t_emp;

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##PlaceholderFunctionPlaceholderFunction%YYear%mMonth%dDate%wWeek (number) - (0 is Sunday)%WWeek (name) - (English)%jDay of the year%UThe number of weeks in this year%Hhours(24)%hHours (12)%iMinutes%sSeconds%rTime(24)%T Time(12)

Date function practice

Use the "date function" to query the day of the week when your birthday is.

SELECT DATE_FORMAT("2018-01-01","%W");        -- 返回结果为 "Monday"

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Date function exercise

Use "date function" to query how many employees joined the company in the first half of 1981?

SELECT COUNT(*)
FROM t_emp
WHERE DATE_FORMAT(hiredate,"%Y") = 1981
AND DATE_FORMAT(hiredate,"%m") <= 6;

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Notes on date calculation

In MySQL, two dates cannot be added or subtracted directly; at the same time, Dates cannot be added or subtracted from numbers.

The reason is that date is a special calculation unit, and the bases are not like ordinary decimals.

Although we use date to perform the operation of "1" without causing a syntax error, the result obtained is a pure number, not the result in the date format we want. (Examples are as follows)

SELECT ename, hiredate, hiredate+1 FROM t_emp;

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Date offset calculation

DATE_ADD() function can implement date offset calculation, and in the processing of time units, Relatively flexible.

The syntax is as follows:

SELECT DATE_ADD("原始日期", INTERVAL, 偏移量, 时间单位)        -- INTERVAL 是 关键字

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"DATE_ADD() function" The demonstration case is as follows

SELECT DATE_ADD(NOW(), INTERVAL 10 DAY);         -- 得到的结果为 10 天 之后的日期时间

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SELECT DATE_ADD(NOW(), INTERVAL -500 MINUTE);         -- 得到的结果为 500 分钟 之前的日期时间

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SELECT DATE_ADD(DATE_ADD(NOW(), INTERVAL -6 MONTH), INTERVAL -3 DAY);         -- 得到的结果为 6 个月 3 天 之前的日期时间

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SELECT 
DATE_FORMAT(
DATE_ADD(DATE_ADD(NOW(), INTERVAL -6 MONTH), INTERVAL -3 DAY) , "%Y-%m-%d") AS DATE;     -- 返回 "%Y-%m-%d" 格式结果

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Calculate the number of days between dates

DATEDIFF() function is used To calculate the number of days between two dates, the syntax is as follows:

DATEDIFF("date", "date")

Query 10 departments with annual income exceeding 15,000 Information about employees with more than 20 years of service.

SELECT empno, ename, sal, hiredate
FROM t_emp
WHERE deptno = 10 
AND (sal + IFNULL(comm,0)) * 12 >=15000
AND DATEDIFF(NOW(),hiredate)/365 >= 20

-- IFNULL(expr1,expr2)：IFNULL 函数的语法，当第一个参数的值为null 的时候，则返回第二个参数的值
-- DATEDIFF(expr1,expr2)：DATEDIFF 函数的语法，计算第一个日期与第二个日期的偏差时间差
-- NOW()：NOW 函数可以获得当前日期

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Recommended learning:

mysql video tutorial

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