Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
Optimising and caching your MySQL enquiries

If you want to get a list of all the different values in a column, but don't care how many times each occurs, you can apply the DISTINCT keyword on your SELECT. See [here]. The DISTINCTROW is a synonym of DISTINCT, and the default is ALL which you may also state.

MySQL Caches queries - so that if it receives an identical query for a second time, it doesn't have to do all the hard work again. MySQL is aware of changes that may be made to the tables, so that it can identify when the results would be changed and so it needs to re'calculate', so most of the time you can simply say "Thank you" for the facility and write code that repeats the same query many times over, secure in the knowledge that MySQL will spot your inefficiencies (it does this across sessions too - see [here]). There are, however, times that you don't want caching to be running - the most common example is if you're benchmarking your application and you want to see if a change that you've made has made things faster or slower; in such a situation, cached results will cause major variations in the times you get which are not related to the software changes that you're making. You can apply the SQL_NO_CACHE keyword to your select statement to ensure that the results you generate are not saved in the cache ... so that subsequent requests will not refer back to this results set. [[You can set SQL_CACHE if you explicitly want caching on; it's the default]]

Aside - although MySQL will help you by using the cache when it's sure that it can, you should not rely on it as a crutch for your own sloppy, repeated coding. It will err on the side of caution and recalculate if there's any possible doubt, and when you think of it, that's a far better solution than risking wrong results!


Other keywords that you can apply directly after the SELECT are:

HIGH_PRIORITY - Do the SELECT in preference to any outstanding updates

STRAIGHT_JOIN - For optimizing joins (to help the built in optimiser)

SQL_SMALL_RESULT, SQL_BIG_RESULT, SQL_BUFFER_RESULT - To help MySQL optimise its performance based on the anticipated size of the result set

SQL_CALC_FOUND_ROWS - If you're doing a query and only asking for some of the results back through LIMIT, this keyword will tell MySQL go go ahead and find ALL the results not just the few you want, in so far as it will be able to tell you how big the result set would have been without the LIMIT.

The extra keywords are MySQL extensions to the SQL standard, and if you use them you'll restrict the portability of your code (that may not be a problem to you ... but it's worth bearing in mind if you may use a different database engine in the future!)

(written 2010-02-22, updated 2010-03-03)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
S157 - More MySQL commands
  [158] MySQL - LEFT JOIN and RIGHT JOIN, INNER JOIN and OUTER JOIN - (2004-12-20)
  [159] MySQL - Optimising Selects - (2004-12-21)
  [279] Getting a list of unique values from a MySQL column - (2005-04-14)
  [449] Matching in MySQL - (2005-09-24)
  [494] MySQL - a score of things to remember - (2005-11-12)
  [502] SELECT in MySQL - choosing the rows you want - (2005-11-22)
  [513] MySQL - JOIN or WHERE to link tables correctly? - (2005-12-01)
  [515] MySQL - an FAQ - (2005-12-03)
  [517] An occasional chance, and reducing data to manageable levels - (2005-12-04)
  [567] Combining similar rows from a MySQL database select - (2006-01-17)
  [572] Giving the researcher power over database analysis - (2006-01-22)
  [581] Saving a MySQL query results to your local disc for Excel - (2006-01-29)
  [591] Key facts - SQL and MySQL - (2006-02-04)
  [673] Helicopter views and tartans - (2006-04-06)
  [1213] MySQL - the order of clauses and the order of actions - (2007-06-01)
  [1235] Outputting numbers as words - MySQL with Perl or PHP - (2007-06-17)
  [1331] MySQL joins revisited - (2007-09-03)
  [1574] Joining MySQL tables revisited - finding nonmatching records, etc - (2008-03-15)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1904] Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23)
  [2110] MySQL - looking for records in one table that do NOT correspond to records in another table - (2009-03-31)
  [2259] Grouping rows for a summary report - MySQL and PHP - (2009-06-27)
  [2448] MySQL - efficiency and other topics - (2009-10-10)
  [2643] Relating tables with joins in MySQL - (2010-02-21)
  [2644] Counting rows in joined MySQL tables - (2010-02-22)
  [2647] Removing duplicates from a MySQL table - (2010-02-22)
  [3061] Databases - why data is split into separate tables, and how to join them - (2010-11-20)
  [3270] SQL - Data v Metadata, and the various stages of data selection - (2011-04-29)
  [4481] Extracting data from backups to restore selected rows from MySQL tables - (2015-05-01)


Back to
Counting rows in joined MySQL tables
Previous and next
or
Horse's mouth home
Forward to
Compile but do not run PHP - syntax check only
Some other Articles
Length, size or capacity in Java?
Java arrays - are they true arrays or not?
Compile but do not run PHP - syntax check only
Optimising and caching your MySQL enquiries
What does a web application look like under Tomcat?
Object Oriented Programming in PHP
How is your tax pound spent?
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page


This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2021: 48 Spa Road • Melksham, Wiltshire • United Kingdom • SN12 7NY
PH: 01144 1225 708225 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/2645_Opt ... iries.html • PAGE BUILT: Sun Oct 11 16:07:41 2020 • BUILD SYSTEM: JelliaJamb