MySQL time and date calculation function application

Calculate the number of years
If you want to calculate the age of the person by birth, you can use the following statement:

SELECT Date_format (From_days to_days (now ())-To_days (@dateofbirth)), '%Y ') + 0;

2. Two difference in time
Gets the difference of two datetime values. Assuming that DT1 and DT2 are datetime types, and their format is ' Yyyy-mm-dd hh:mm:ss ', then the difference in seconds between them is: Unix_timestamp (DT2)-Unix_timestamp (DT1) divided by 60 is the bad The number of minutes divided by 3600 is the difference between the number of hours, divided by 24 is the difference in the number of days.
3. Show the value of n times a column appears
The code is as follows:

SELECT ID
From TBL
GROUP by ID
Having COUNT (*) = N;

4. Calculation of working days between two days
The so-called working day is the addition of Saturday Sundays and holidays.
The code is as follows:

SELECT COUNT (*)
From calendar
WHERE d BETWEEN Start and Stop
and DayOfWeek (d) not in (1,7)
and holiday=0;