MySQL complex date calculation in inner join - mysql

Question

I am reading the MySql tutorial in the docs and have the following tables and SQL statements:
Event table:
+----------+------------+----------+------------------------------+
| name | date | type | remark |
+----------+------------+----------+------------------------------+
| Fluffy | 1995-05-15 | litter | 4 kittens, 3 females, 1 male |
| Buffy | 1993-06-23 | litter | 5 puppies, 2 female, 3 male |
| Buffy | 1994-06-19 | litter | 3 puppies, 3 female |
| Chirpy | 1999-03-21 | vet | needed beak streightened |
| Slim | 1997-08-03 | vet | broken rib |
| Bowser | 1991-10-12 | kennel | NULL |
| Fang | 1991-10-12 | kennel | NULL |
| Fang | 1998-08-28 | birthday | Gave him new chew toy |
| Claws | 1998-03-17 | birthday | Gave him a flea collar |
| Whistler | 1998-12-09 | birthday | First birthday |
+----------+------------+----------+------------------------------+
Pet table:
+----------+--------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+----------+--------+---------+------+------------+------------+
| Fluffy | Harold | cat | f | 1993-02-04 | NULL |
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
| Fang | Benny | dog | m | 1990-08-27 | NULL |
| Bowser | Diane | dog | m | 1989-03-31 | 1995-07-29 |
| Chirpy | Gwen | bird | f | 1998-09-11 | NULL |
| Whistler | Gwen | bird | NULL | 1997-12-09 | NULL |
| Slim | Benny | snake | m | 1996-04-29 | NULL |
| Puffball | Diane | hamster | f | 1999-03-30 | NULL |
| Jenny | Robert | dog | f | 2004-01-01 | 2014-05-04 |
+----------+--------+---------+------+------------+------------+
SQL:
select pet.name,
( YEAR(date) - YEAR(birth) ) - ( RIGHT(date,5) < RIGHT(birth,5) ) AS age, remark
from pet inner join event
on pet.name = event.name
where event.type = 'litter';
I understand the SQL statement except for this one:
( YEAR(date) - YEAR(birth) ) - ( RIGHT(date,5) < RIGHT(birth,5) )
A step by step explanation would greatly help. I know that the YEAR() function is used to extract the year from a date.

Answer 1

As you've mentioned YEAR() gets the year from the date.
We will use Fluffy as an example with birth = 1993-02-04 and date = 1995-05-15
Step 1:
Subtract the extracted year from both dates ( YEAR(date) - YEAR(birth) )
You now have 1995 - 1993 which is equal to 2
Step 2:
( RIGHT(date,5) < RIGHT(birth,5) ) this will actually read the date and birth string 5 paces from right to left, so if you perform RIGHT(date,5) you will get the value 0, and if you perform RIGHT(birth,5) you will also get a value of 0.
Step 3:
Now we get on to the < operator, this returns a boolean value of 1 or 0 if it satisfies the condition. Since 0 = 0, the statement is false, so it will return to 0.
The whole function actually checks if the day part of your date is less than the day on your birthdate which will determine if you have a sort of remaining days before a whole year. And if you do, it will return 1 which will be subtracted from the current year - year operation you performed earlier.
But in our case, since the < will return 0, we can definitely say that Fluffy's age is 2 - 0, which is 2.
If, however, Fluffy's birthdate is say, 1993-12-04, this will yield a 1 value for the < operation meaning that the year is not yet complete which will bring a result of 2 - 1, which is 1.
Sorry if its a bit messy.

Answer 2

If you were born in 1980 and I know that right now it's 2015, then I I can almost compute your age by computing YEAR(date) - YEAR(birth) = 2015 - 1980 = 35. The sticking point is that your birthday might not have happened yet. So how can I tell if your birthday has happened? Lets say you were born on July 20, 1980. Then in MySQL date format your birthday would look like 2015-06-20. If I look at the last 5 characters of your birthday, the right-most 5 characters, or RIGHT(birth, 5) I'd get 06-20. If the the last 5 characters of today's date, 06-07 is less than the last 5 characters of your birthday then your birthday hasn't happened yet. Here, by "less than" we mean only that it would "sort before" in normal string order. So, if RIGHT(date, 5) < RIGHT(birth, 5) then your birthday hasn't happened yet. And, luckily, MySQL treats a "true" as the number 1 and a "false" as the number 0. So, the above will subtract 1 from our date computation only if your birthday hasn't happened yet.

Answer 3

It's a clever(?) way of checking if the month and day of the date of birth happened before or after the date from the event (litter) in order to properly calculate the number of years between the events (as the year part in itself isn't enough).
In this part:
( YEAR(date) - YEAR(birth) ) - ( RIGHT(date,5) < RIGHT(birth,5) )
the RIGHT(date,5) returns the month and day part like (03-30) and does a boolean less than comparison which returns either 0 or 1 depending on the result. This is then subtracted from the YEAR(date) - YEAR(birth) calculation so that the years between the events get adjusted correctly.