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Joining MySQL tables revisited - finding nonmatching records, etc

A join lets me connect two tables one to the right of the other. Here's an example. First table - estate agents:

mysql> select agent,town,aid from agents;
+--------------------------+------------------+-----+
| agent | town | aid |
+--------------------------+------------------+-----+
| Alder King | Trowbridge | 1 |
| Connells | Trowbridge | 2 |
| DK Residential | Trowbridge | 3 |
| Jayson Kent | Melksham | 4 |
| Halifax | Trowbridge | 5 |
| Halifax | Melksham | 6 |
| Greg Pullen | Devizes | 7 |
| Town and Country Estates | Westbury | 8 |
| Davies and Davies | Bradford on Avon | 9 |
| Kavanaghs | Melksham | 10 |
+--------------------------+------------------+-----+
10 rows in set (0.17 sec)


And second table - homes for sale:

mysql> select agid,locate,asking,sid from sales;
+------+------------+--------+-----+
| agid | locate | asking | sid |
+------+------------+--------+-----+
| 10 | Semington | 225000 | 1 |
| 10 | Melksham | 195000 | 2 |
| 10 | Atworth | 237500 | 3 |
| 8 | Westbury | 152000 | 4 |
| 8 | Trowbridge | 205000 | 5 |
| 6 | Melksham | 229950 | 6 |
| 5 | Trowbridge | 335000 | 7 |
| 1 | Trowbridge | 96950 | 17 |
| 5 | Hilperton | 279950 | 8 |
| 4 | Melksham | 149950 | 9 |
| 1 | Trowbridge | 220000 | 18 |
| 4 | Melksham | 127500 | 10 |
| 11 | Melksham | 465000 | 11 |
| 3 | Semington | 222000 | 12 |
| 3 | Hilperton | 465000 | 13 |
| 2 | Westbury | 140000 | 14 |
| 2 | Trowbridge | 116000 | 15 |
| 11 | Melksham | 275000 | 16 |
| 4 | Semington | 250000 | 20 |
+------+------------+--------+-----+
19 rows in set (0.17 sec)


By default, a join connects all records in the first table with all records in the second table , so with 19 rows in one and 10 rows in the other, a joine gives me no less that 190 records!

(If the following outputs clip or folds - see here for an alternative display)

mysql> select agid,locate,asking,sid,agent,town,aid 
from sales join agents;

+------+------------+--------+-----+--------------------------+------------------+-----+
| agid | locate | asking | sid | agent | town | aid |
+------+------------+--------+-----+--------------------------+------------------+-----+
| 10 | Semington | 225000 | 1 | Alder King | Trowbridge | 1 |
| 10 | Melksham | 195000 | 2 | Alder King | Trowbridge | 1 |
| 10 | Atworth | 237500 | 3 | Alder King | Trowbridge | 1 |
| 8 | Westbury | 152000 | 4 | Alder King | Trowbridge | 1 |
| 8 | Trowbridge | 205000 | 5 | Alder King | Trowbridge | 1 |
| 6 | Melksham | 229950 | 6 | Alder King | Trowbridge | 1 |
| 5 | Trowbridge | 335000 | 7 | Alder King | Trowbridge | 1 |
| 1 | Trowbridge | 96950 | 17 | Alder King | Trowbridge | 1 |
| 5 | Hilperton | 279950 | 8 | Alder King | Trowbridge | 1 |
| 4 | Melksham | 149950 | 9 | Alder King | Trowbridge | 1 |
| 1 | Trowbridge | 220000 | 18 | Alder King | Trowbridge | 1 |
| 4 | Melksham | 127500 | 10 | Alder King | Trowbridge | 1 |
(etc)
| 3 | Hilperton | 465000 | 13 | Kavanaghs | Melksham | 10 |
| 2 | Westbury | 140000 | 14 | Kavanaghs | Melksham | 10 |
| 2 | Trowbridge | 116000 | 15 | Kavanaghs | Melksham | 10 |
| 11 | Melksham | 275000 | 16 | Kavanaghs | Melksham | 10 |
| 4 | Semington | 250000 | 20 | Kavanaghs | Melksham | 10 |
+------+------------+--------+-----+--------------------------+------------------+-----+
190 rows in set (0.36 sec)


Clearly that's not what I want - I usually want so see what connects to what, so I add an on clause to give me all the records that match as appropriate:

mysql> select agid,locate,asking,sid,agent,town,aid 
from sales join agents on aid = agid;

+------+------------+--------+-----+--------------------------+------------+-----+
| agid | locate | asking | sid | agent | town | aid |
+------+------------+--------+-----+--------------------------+------------+-----+
| 10 | Semington | 225000 | 1 | Kavanaghs | Melksham | 10 |
| 10 | Melksham | 195000 | 2 | Kavanaghs | Melksham | 10 |
| 10 | Atworth | 237500 | 3 | Kavanaghs | Melksham | 10 |
| 8 | Westbury | 152000 | 4 | Town and Country Estates | Westbury | 8 |
| 8 | Trowbridge | 205000 | 5 | Town and Country Estates | Westbury | 8 |
| 6 | Melksham | 229950 | 6 | Halifax | Melksham | 6 |
| 5 | Trowbridge | 335000 | 7 | Halifax | Trowbridge | 5 |
| 1 | Trowbridge | 96950 | 17 | Alder King | Trowbridge | 1 |
| 5 | Hilperton | 279950 | 8 | Halifax | Trowbridge | 5 |
| 4 | Melksham | 149950 | 9 | Jayson Kent | Melksham | 4 |
| 1 | Trowbridge | 220000 | 18 | Alder King | Trowbridge | 1 |
| 4 | Melksham | 127500 | 10 | Jayson Kent | Melksham | 4 |
| 3 | Semington | 222000 | 12 | DK Residential | Trowbridge | 3 |
| 3 | Hilperton | 465000 | 13 | DK Residential | Trowbridge | 3 |
| 2 | Westbury | 140000 | 14 | Connells | Trowbridge | 2 |
| 2 | Trowbridge | 116000 | 15 | Connells | Trowbridge | 2 |
| 4 | Semington | 250000 | 20 | Jayson Kent | Melksham | 4 |
+------+------------+--------+-----+--------------------------+------------+-----+
17 rows in set (0.18 sec)


Which gives me just 17 out of 190 records.

I can even ask for the other 173 records if I wish by switching to a not equals operator and getting a list of who is NOT selling what - the Estate agents you can walk into and enquire about a specific house to be told "Sorry - that one's NOT on our books". Sounds silly, doesn't it - but the query IS possible:

mysql> select agid,locate,asking,sid,agent,town,aid 
from sales join agents on aid != agid;

+------+------------+--------+-----+--------------------------+------------+-----+
| agid | locate | asking | sid | agent | town | aid |
+------+------------+--------+-----+--------------------------+------------+-----+
| 10 | Semington | 225000 | 1 | Kavanaghs | Melksham | 10 |
| 10 | Melksham | 195000 | 2 | Kavanaghs | Melksham | 10 |
| 10 | Atworth | 237500 | 3 | Kavanaghs | Melksham | 10 |
(etc}

| 2 | Trowbridge | 116000 | 15 | Kavanaghs | Melksham | 10 |
| 11 | Melksham | 275000 | 16 | Kavanaghs | Melksham | 10 |
| 4 | Semington | 250000 | 20 | Kavanaghs | Melksham | 10 |
+------+------------+--------+-----+--------------------------+------------------+-----+
173 rows in set (0.35 sec)


You may have noticed that we had 19 properties for sale, but only 17 of them came up on the "reasonable" join. That's because my data includes two properties which are not listed with any agent at all - they're for sale by owner. If I want to include extra records to ensure that I have one output row for EVERY incoming row in the leftmost of the tables in my join, I can generate the extra records by specifiying a LEFT JOIN rather than a JOIN, and the extra records are generated, NULL filled. There's a full explanation and example of left and right joins [here] and that page also shows you how to find ONLY those records which don't match - the "for sale by owner" orphans, and the estate agents with nothing for sale.
(written 2008-03-15)

 
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)
  [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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