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    1. General Information
    2. MySQL Installation
    3. Tutorial Introduction
    4. Database Administration
    5. MySQL Optimisation
    6. MySQL Language Reference
    7. MySQL Table Types
    8. MySQL APIs
    9. Extending MySQL
    Chapter 3:  Tutorial Introduction 157 You can sort on multiple columns.  For example, to sort by type of animal, then by birth date within animal type with youngest animals rst, use the following query: mysql> SELECT name, species, birth FROM pet ORDER BY species, birth DESC; +----------+---------+------------+ | name | species | birth | +----------+---------+------------+ | Chirpy | bird | 1998-09-11 | | Whistler | bird | 1997-12-09 | | Claws | cat | 1994-03-17 | | Fluffy | cat | 1993-02-04 | | Fang | dog | 1990-08-27 | | Bowser | dog | 1989-08-31 | | Buffy | dog | 1989-05-13 | | Puffball | hamster | 1999-03-30 | | Slim | snake | 1996-04-29 | +----------+---------+------------+ Note  that  the  DESC  keyword  applies  only  to  the  column  name  immediately  preceding  it (birth); species values are still sorted in ascending order. 3.3.4.5  Date Calculations 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 di erence 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, CURRENT_DATE, -> (YEAR(CURRENT_DATE)-YEAR(birth)) -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5)) -> AS age -> FROM pet; +----------+------------+--------------+------+ | name | birth | CURRENT_DATE | age   | +----------+------------+--------------+------+ | Fluffy | 1993-02-04 | 2001-08-29 | 8 | | Claws | 1994-03-17 | 2001-08-29 | 7 | | Buffy | 1989-05-13 | 2001-08-29 | 12 | | Fang | 1990-08-27 | 2001-08-29 | 11 | | Bowser | 1989-08-31 | 2001-08-29 | 11 | | Chirpy | 1998-09-11 | 2001-08-29 | 2 | | Whistler | 1997-12-09 | 2001-08-29 | 3 | | Slim | 1996-04-29 | 2001-08-29 | 5 | | Puffball | 1999-03-30 | 2001-08-29 | 2 | +----------+------------+--------------+------+ Here,  YEAR()  pulls  out  the  year  part  of  a  date  and  RIGHT()  pulls  o  the  rightmost   ve characters  that  represent  the  MM-DD  (calendar  year)  part  of  the  date.    The  part  of  the
     

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