Zero foundation self-study SQL course | complete collection of date functions in SQL

Hello everyone , I'm Ning Yi .
Today is our first 21 course ：SQL The date function in .
MySQL Built in date function in , Include acquisition date 、 Date formatting 、 Date calculation , Let's have a look at .
1、 Get date function
MySQL Built in date and time acquisition function ：

SELECT    NOW() AS ' The current date + Time ',  CURDATE() AS ' The current date ',  CURTIME() AS ' current time ';

Get the specific month, year and day function ：

SELECT    YEAR("2022-04-11 15:44:28") AS ' year ',  MONTH("2022-04-11 15:44:28") AS ' month ',  DAY("2022-04-11 15:44:28") AS ' Japan ',  HOUR("2022-04-11 15:44:28") AS ' Hours ',  MINUTE("2022-04-11 15:44:28") AS ' minute ',  SECOND("2022-04-11 15:44:28") AS ' second ',  DAYNAME("2022-04-11 15:44:28") AS ' What day ',  MONTHNAME("2022-04-11 15:44:28") AS ' How many months ';

example ： stay Students In the table , Find out birthday Sage by 1995 Student records for .

SELECT *FROM StudentsWHERE YEAR(Sage) = 1995

We have done this before , use BETWEEN…AND… Realized , You can click on the home page to find the fifth lecture --WHERE Review the conditional clause ~

2、 Format date function

We mainly use DATE_FORMAT、TIME_FORMAT Function to format the date and time , Let's take a look at the specific usage .

SELECT  NOW() AS " present time ",  DATE_FORMAT(NOW(), '%Y.%m.%d') AS ' Format date ',  TIME_FORMAT(NOW(), '%h:%i:%s') AS ' Format time '

The available formats are ：

3、 Calculate date function

In real business , We often have to calculate the date and time , For example, add or subtract one day from the date , Or calculate the date interval , Let's take a look at the specific usage .
Increase or decrease the number of days based on the date ：

SELECT NOW() AS " present time ", DATE_ADD(NOW(), INTERVAL 1 DAY) AS " increase 1 God ", DATE_SUB(NOW(), INTERVAL 1 DAY) AS " Reduce 1 God "

It can also increase the number of teenagers 、 month 、 Hours 、 minute ：

SELECT  NOW() AS " present time ",  DATE_ADD(NOW(), INTERVAL 1 YEAR) AS " increase 1 year ",  DATE_SUB(NOW(), INTERVAL 1 MONTH) AS " Reduce 1 God ",  DATE_SUB(NOW(), INTERVAL 1 HOUR) AS " Reduce 1 Hours "

Calculate the number of days between two dates ：

SELECT  DATEDIFF('2022-04-11','2021-04-11') AS " Days between ",  DATEDIFF('2022-04-11 01:00','2022-04-10 23:00') AS " Days between "

Be careful DATEDIFF Function ignores the time part , Calculate only date variance , such as , above SQL In the sentence 04-10 23:00 To 04-11 01:00 It was only two hours away , But with DATEDIFF The function calculates a time interval of 1 God .
Homework ： stay Students In the table , By student birthday Sage, Calculate the age of each student , Finally show today's date 、 The student's name 、 Student age 3 Column .
Homework analysis ： adopt DATEDIFF function , Calculate today and students' birthdays Sage Days of difference , Divide 365 Get years , Re pass FLOOR Numerical function , Take a large integer smaller than age .

SELECT  DATE_FORMAT(NOW(),'%Y-%m-%d') AS ' Today's date ',  Sname AS ' The student's name ',  FLOOR(DATEDIFF(NOW(),Sage)/365) AS " Student age "FROM Students;

Note that these functions are not standard SQL sentence , It is MySQL Functions built into the software , stay Oracle、 perhaps SQL Server Not necessarily common in .
Next time we will talk about IF function .
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author ： Meow Ningyi

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