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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.
MySQL - the order of clauses and the order of actions

You specify your select command in the following order:
1. The columns you want to choose (SELECT)
2. Where you want to send the results (INTO)
3. The tables that contain the data (FROM)
4. How you want to connect those tables (JOIN, ON)
5. Which individual rows you want to choose (WHERE)
6. Bunching of individual rows together (GROUP)
7. Which bunches you want to choose (HAVING)
8. How to sort the records (ORDER)
9. How many output records you want (LIMIT)

The order that MySQL performs these tasks is as follows:
1. Joining the tables
2. Selecting individual records
3. Sorting the records
4. Bunching
5. Selecting bunches
6. How many output records you want
7. Choosing the columns to display

Here's an example that shows some of the less common clauses in use:

drop table if exists calltimes;
drop table if exists townnames;
create table calltimes (
  cid int primary key not null auto_increment,
  pcode text,
  calls text);
create table townnames (
  tid int primary key not null auto_increment,
  pcode text,
  fullname text);
insert into calltimes values
  (1, "MKM", "06:44"),
  (2, "MKM", "07:17"),
  (3, "MKM", "19:08"),
  (4, "MKM", "19:50"),
  (7, "CPM", "16:25"),
  (5, "CPM", "16:55"),
  (6, "CPM", "17:25"),
  (8, "TRO", "23:42"),
  (9, "PNG", "15:10");
insert into townnames values
  (1, "MKM", "Melksham"),
  (2, "CPM", "Chippenham"),
  (3, "BTH", "Bath");
select calltimes.pcode, townnames.fullname, count(cid)
  from calltimes left join townnames
  on calltimes.pcode = townnames.pcode
  group by calltimes.pcode
  order by fullname;


And the results:
+-------+------------+------------+
| pcode | fullname | count(cid) |
+-------+------------+------------+
| TRO | NULL | 1 |
| PNG | NULL | 1 |
| CPM | Chippenham | 3 |
| MKM | Melksham | 4 |
+-------+------------+------------+

(written 2007-06-01)

 
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)
  [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)
  [2645] Optimising and caching your MySQL enquiries - (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)


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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.

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