Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
MySQL - looking for records in one table that do NOT correspond to records in another table

Q: Can you give an example where I can retrieve all the records of a table "users" where there are no corresponding records in other table "user_role"

A: No (sorry - I'm too busy to set up tables of these names for a demo) but I DO have an example that show you.

We have a table of agents (realtors) and a table of properties they are selling.
• with a regular join, you get all matching records

&bull with a left join, you get all matching records PLUS a row for all the records in the left hand table that don't have anything matching them in the right table

• with a left join and a "where [fieldname] is NULL, you get all the records in the first table that have nothing matching in the second.

Note - you must use is and not = to compare to NULL; NULL is not a value (it is the absence of a value!) so nothing can be equal to it!

mysql> select agent,aid,agid,locate,sid from agents
 join sales on aid=agid;
+--------------------------+-----+------+-------------------+-----+
| agent                    | aid | agid | locate            | sid |
+--------------------------+-----+------+-------------------+-----+
| Kavanaghs                |  10 |   10 | Semington         |   1 |
| Kavanaghs                |  10 |   10 | Melksham          |   2 |
| Kavanaghs                |  10 |   10 | Atworth           |   3 |
| Town and Country Estates |   8 |    8 | Westbury          |   4 |
| Town and Country Estates |   8 |    8 | Trowbridge        |   5 |
| Halifax                  |   6 |    6 | Melksham          |   6 |
| Halifax                  |   5 |    5 | Trowbridge        |   7 |
| Alder King               |   1 |    1 | Trowbridge        |  17 |
| Halifax                  |   5 |    5 | Hilperton         |   8 |
| Jayson Kent              |   4 |    4 | Melksham          |   9 |
| Alder King               |   1 |    1 | Trowbridge        |  18 |
| Jayson Kent              |   4 |    4 | Melksham          |  10 |
| DK Residential           |   3 |    3 | Semington         |  12 |
| DK Residential           |   3 |    3 | Hilperton         |  13 |
| Connells                 |   2 |    2 | Westbury          |  14 |
| Connells                 |   2 |    2 | Trowbridge        |  15 |
| Jayson Kent              |   4 |    4 | Semington         |  20 |
| Connells                 |   2 |    2 | Little Twittering |  21 |
| Greg Pullen              |   7 |    7 | Wenduine          |  22 |
| Jayson Kent              |   4 |    4 | Westbury          |  23 |
+--------------------------+-----+------+-------------------+-----+
20 rows in set (0.07 sec)
 
mysql> select agent,aid,agid,locate,sid from agents 
left join sales on aid=agid; 
+--------------------------+-----+------+-------------------+------+
| agent                    | aid | agid | locate            | sid  |
+--------------------------+-----+------+-------------------+------+
| Alder King               |   1 |    1 | Trowbridge        |   17 |
| Alder King               |   1 |    1 | Trowbridge        |   18 |
| Connells                 |   2 |    2 | Westbury          |   14 |
| Connells                 |   2 |    2 | Trowbridge        |   15 |
| Connells                 |   2 |    2 | Little Twittering |   21 |
| DK Residential           |   3 |    3 | Semington         |   12 |
| DK Residential           |   3 |    3 | Hilperton         |   13 |
| Jayson Kent              |   4 |    4 | Melksham          |    9 |
| Jayson Kent              |   4 |    4 | Melksham          |   10 |
| Jayson Kent              |   4 |    4 | Semington         |   20 |
| Jayson Kent              |   4 |    4 | Westbury          |   23 |
| Halifax                  |   5 |    5 | Trowbridge        |    7 |
| Halifax                  |   5 |    5 | Hilperton         |    8 |
| Halifax                  |   6 |    6 | Melksham          |    6 |
| Greg Pullen              |   7 |    7 | Wenduine          |   22 |
| Town and Country Estates |   8 |    8 | Westbury          |    4 |
| Town and Country Estates |   8 |    8 | Trowbridge        |    5 |
| Davies and Davies        |   9 | NULL | NULL              | NULL |
| Kavanaghs                |  10 |   10 | Semington         |    1 |
| Kavanaghs                |  10 |   10 | Melksham          |    2 |
| Kavanaghs                |  10 |   10 | Atworth           |    3 |
| Andrews                  |  14 | NULL | NULL              | NULL |
| Eddy and Isaac           |  16 | NULL | NULL              | NULL |
| Boris and Boris          |  15 | NULL | NULL              | NULL |
+--------------------------+-----+------+-------------------+------+
24 rows in set (0.00 sec)
 
mysql> select agent,aid,agid,locate,sid from agents 
left join sales on aid=agid where sid is NULL;
+-------------------+-----+------+--------+------+
| agent             | aid | agid | locate | sid  |
+-------------------+-----+------+--------+------+
| Davies and Davies |   9 | NULL | NULL   | NULL |
| Andrews           |  14 | NULL | NULL   | NULL |
| Eddy and Isaac    |  16 | NULL | NULL   | NULL |
| Boris and Boris   |  15 | NULL | NULL   | NULL |
+-------------------+-----+------+--------+------+
4 rows in set (0.01 sec)
 
mysql> 

 
(written 2009-03-31)

 
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)
  [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)


Back to
Why most training fails ...
Previous and next
or
Horse's mouth home
Forward to
Business Networking for Melksham - 23rd April
Some other Articles
Which Version of Java am I running?
Answering a question with a question
Whatever next ... the joys of being an employer
Business Networking for Melksham - 23rd April
MySQL - looking for records in one table that do NOT correspond to records in another table
Why most training fails ...
Funny Amusing, Funny Amazing and The Smarts
How to tweet automatically from a blog
Learning to Twitter / what is Twitter?
Hire Car, from Atlanta Airport
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., 2024: 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/2110_MyS ... table.html • PAGE BUILT: Sun Oct 11 16:07:41 2020 • BUILD SYSTEM: JelliaJamb