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MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates.
To determine how many years old each of your pets is, compute the difference in the year part of the current date and the birth date, then subtract one if the current date occurs earlier in the calendar year than the birth date. The following query shows, for each pet, the birth date, the current date, and the age in years.
mysql> SELECT name, birth, CURDATE(), -> (YEAR(CURDATE())-YEAR(birth)) -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5)) -> AS age -> FROM pet; +----------+------------+------------+------+ | name | birth | CURDATE() | age | +----------+------------+------------+------+ | Fluffy | 1993-02-04 | 2003-08-19 | 10 | | Claws | 1994-03-17 | 2003-08-19 | 9 | | Buffy | 1989-05-13 | 2003-08-19 | 14 | | Fang | 1990-08-27 | 2003-08-19 | 12 | | Bowser | 1989-08-31 | 2003-08-19 | 13 | | Chirpy | 1998-09-11 | 2003-08-19 | 4 | | Whistler | 1997-12-09 | 2003-08-19 | 5 | | Slim | 1996-04-29 | 2003-08-19 | 7 | | Puffball | 1999-03-30 | 2003-08-19 | 4 | +----------+------------+------------+------+ |
Here, YEAR()
pulls out the year part of a date and RIGHT()
pulls off the rightmost five characters that represent the MM-DD
(calendar year) part of the date. The part of the expression that
compares the MM-DD
values evaluates to 1 or 0, which adjusts the
year difference down a year if CURDATE()
occurs earlier in
the year than birth
. The full expression is somewhat ungainly,
so an alias (age
) is used to make the output column label more
meaningful.
The query works, but the result could be scanned more easily if the rows
were presented in some order. This can be done by adding an ORDER
BY name
clause to sort the output by name:
mysql> SELECT name, birth, CURDATE(), -> (YEAR(CURDATE())-YEAR(birth)) -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5)) -> AS age -> FROM pet ORDER BY name; +----------+------------+------------+------+ | name | birth | CURDATE() | age | +----------+------------+------------+------+ | Bowser | 1989-08-31 | 2003-08-19 | 13 | | Buffy | 1989-05-13 | 2003-08-19 | 14 | | Chirpy | 1998-09-11 | 2003-08-19 | 4 | | Claws | 1994-03-17 | 2003-08-19 | 9 | | Fang | 1990-08-27 | 2003-08-19 | 12 | | Fluffy | 1993-02-04 | 2003-08-19 | 10 | | Puffball | 1999-03-30 | 2003-08-19 | 4 | | Slim | 1996-04-29 | 2003-08-19 | 7 | | Whistler | 1997-12-09 | 2003-08-19 | 5 | +----------+------------+------------+------+ |
To sort the output by age
rather than name
, just use a
different ORDER BY
clause:
mysql> SELECT name, birth, CURDATE(), -> (YEAR(CURDATE())-YEAR(birth)) -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5)) -> AS age -> FROM pet ORDER BY age; +----------+------------+------------+------+ | name | birth | CURDATE() | age | +----------+------------+------------+------+ | Chirpy | 1998-09-11 | 2003-08-19 | 4 | | Puffball | 1999-03-30 | 2003-08-19 | 4 | | Whistler | 1997-12-09 | 2003-08-19 | 5 | | Slim | 1996-04-29 | 2003-08-19 | 7 | | Claws | 1994-03-17 | 2003-08-19 | 9 | | Fluffy | 1993-02-04 | 2003-08-19 | 10 | | Fang | 1990-08-27 | 2003-08-19 | 12 | | Bowser | 1989-08-31 | 2003-08-19 | 13 | | Buffy | 1989-05-13 | 2003-08-19 | 14 | +----------+------------+------------+------+ |
A similar query can be used to determine age at death for animals that have
died. You determine which animals these are by checking whether the
death
value is NULL
. Then, for those with non-NULL
values, compute the difference between the death
and birth
values:
mysql> SELECT name, birth, death, -> (YEAR(death)-YEAR(birth)) - (RIGHT(death,5)<RIGHT(birth,5)) -> AS age -> FROM pet WHERE death IS NOT NULL ORDER BY age; +--------+------------+------------+------+ | name | birth | death | age | +--------+------------+------------+------+ | Bowser | 1989-08-31 | 1995-07-29 | 5 | +--------+------------+------------+------+ |
The query uses death IS NOT NULL
rather than death <> NULL
because NULL
is a special value that cannot be compared using the usual
comparison operators. This is discussed later.
See section Working with NULL
.
What if you want to know which animals have birthdays next month? For this
type of calculation, year and day are irrelevant; you simply want to extract
the month part of the birth
column. MySQL provides several
date-part extraction functions, such as YEAR()
, MONTH()
, and
DAYOFMONTH()
. MONTH()
is the appropriate function here. To
see how it works, run a simple query that displays the value of both
birth
and MONTH(birth)
:
mysql> SELECT name, birth, MONTH(birth) FROM pet; +----------+------------+--------------+ | name | birth | MONTH(birth) | +----------+------------+--------------+ | Fluffy | 1993-02-04 | 2 | | Claws | 1994-03-17 | 3 | | Buffy | 1989-05-13 | 5 | | Fang | 1990-08-27 | 8 | | Bowser | 1989-08-31 | 8 | | Chirpy | 1998-09-11 | 9 | | Whistler | 1997-12-09 | 12 | | Slim | 1996-04-29 | 4 | | Puffball | 1999-03-30 | 3 | +----------+------------+--------------+ |
Finding animals with birthdays in the upcoming month is easy, too. Suppose
the current month is April. Then the month value is 4
and you look
for animals born in May (month 5
) like this:
mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5; +-------+------------+ | name | birth | +-------+------------+ | Buffy | 1989-05-13 | +-------+------------+ |
There is a small complication if the current month is December.
You don't just add one to the month number (12
) and look for animals
born in month 13
, because there is no such month. Instead, you look for
animals born in January (month 1
).
You can even write the query so that it works no matter what the current
month is. That way you don't have to use a particular month number
in the query. DATE_ADD()
allows you to add a time interval to a
given date. If you add a month to the value of CURDATE()
, then extract
the month part with MONTH()
, the result produces the month in which to
look for birthdays:
mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MONTH(DATE_ADD(CURDATE(), INTERVAL 1 MONTH)); |
A different way to accomplish the same task is to add 1
to get the
next month after the current one (after using the modulo function (MOD
)
to wrap around the month value to 0
if it is currently
12
):
mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MOD(MONTH(CURDATE()), 12) + 1; |
Note that MONTH
returns a number between 1
and 12
. And
MOD(something,12)
returns a number between 0
and 11
. So the
addition has to be after the MOD()
, otherwise we would go from
November (11
) to January (1
).
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