I am using DATE() function to calculate the difference between two dates in MySQL
value of SYSDATE() function is following
select SYSDATE();
2020-07-15 12:16:07.0
When I am using date from same month, it is giving correct result
select DATE(SYSDATE())- DATE('2020-07-13');
2
But when I am using date from last month it is giving difference as 86 instead of 16;
select DATE(SYSDATE())- DATE('2020-06-29');
86
Edit: I am aware that we can use DATEDIFF() but I want to verify why DATE() function is giving results like this since we are already using this in code
MySQL doesn't support subtracting one date from another. The code
SELECT DATE '2020-07-15' - DATE '2020-06-29';
should hence result in an error, but MySQL silently converts this to this instead:
SELECT 20200715 - 20200629;
Seeing that you want to subtract two values, it assumes that you want to work with numbers. Dates are not numbers, but their internal representation yyyymmdd can be represented numerically. So, while CAST(DATE '2020-07-15 ' AS int) fails with a syntax error, as it should, MySQL is not consistent, when it comes to subtraction. It generates the numbers 20200715 and 20200629 and works with these.
I consider this a bug. MySQL should either raise an exception or return an INTERVAL when subtracting one DATE from another.