Related
I have a MySQL database.
I want to update a column(in my case title column in bms_title table) in a table using the values from concat columns in other tables.
SELECT * FROM(SELECT distinct t.id, t.title as Textbook,
GROUP_CONCAT(concat(ci.discipline_code, ci.code, " (" , ci.type , ")") SEPARATOR ', ') as CourseCode FROM
tms_local.bms_material m,
tms_local.bms_title t,
tms_local.bms_course c,
tms_local.bms_courseinfo ci
where t.id > 1 AND t.id = m.book_id
and c.id = m.course_id
and ci.id = c.id
and isbn != 'NA'
GROUP BY t.id) AS temporary_table;
UPDATE tms_local.bms_title
SET tms_local.bms_title.thumbnail = temporary_table.CourseCode
WHERE tms_local.bms_title.title=temporary_table.Textbook;
But I got the error: Unknow temporary_table.Textbook in where clause.
How could I update the tms_local.bms_title.thumbnail column using CourseCode column from the selected table?
enter image description here
I have tried
CREATE TEMPORARY TABLE IF NOT EXISTS temporary_table AS (SELECT distinct t.id, t.title as Textbook,
GROUP_CONCAT(concat(ci.discipline_code, ci.code, " (" , ci.type , ")") SEPARATOR ', ') as CourseCode FROM
tms_local.bms_material m,
tms_local.bms_title t,
tms_local.bms_course c,
tms_local.bms_courseinfo ci
where t.id > 1 AND t.id = m.book_id
and c.id = m.course_id
and ci.id = c.id
and isbn != 'NA'
GROUP BY t.id);
UPDATE tms_local.bms_title
SET tms_local.bms_title.thumbnail = temporary_table.CourseCode
WHERE tms_local.bms_title.title=temporary_table.Textbook;
But got the same error.
you need to join the select statement.
As seen below:
UPDATE tms_local.bms_title t0
INNER JOIN
(SELECT
*
FROM
(SELECT DISTINCT
t.id,
t.title AS Textbook,
GROUP_CONCAT(CONCAT(ci.discipline_code, ci.code, ' (', ci.type, ')')
SEPARATOR ', ') AS CourseCode
FROM
tms_local.bms_material m, tms_local.bms_title t, tms_local.bms_course c, tms_local.bms_courseinfo ci
WHERE
t.id > 1 AND t.id = m.book_id
AND c.id = m.course_id
AND ci.id = c.id
AND isbn != 'NA'
GROUP BY t.id) AS temporary_table) t1 ON t0.title = t1.Textbook
SET
t0.thumbnail = t1.ourseCode;
Your temporary_table is being lost between the first statement and the second.
I find the WITH ... AS SQL structure to be helpful to get these together and is far more readable:
WITH temporary_table AS(
SELECT *
FROM(SELECT distinct t.id,
t.title as Textbook,
GROUP_CONCAT(concat(ci.discipline_code,
ci.code,
" (" ,
ci.type ,
")")
SEPARATOR ', '
) as CourseCode
FROM tms_local.bms_material m,
tms_local.bms_title t,
tms_local.bms_course c,
tms_local.bms_courseinfo ci
WHERE t.id > 1 AND t.id = m.book_id
and c.id = m.course_id
and ci.id = c.id
and isbn != 'NA'
GROUP BY t.id)
UPDATE tms_local.bms_title
SET tms_local.bms_title.thumbnail = temporary_table.CourseCode
WHERE tms_local.bms_title.title=temporary_table.Textbook;
I'm trying to get some data from my db.
It kinda looks like this
GROUPS
groups_id, groups_name, groups_description, groups_active, groups_hash, groups_entry_date, user_id, groups_email, groups_sms
CUSTOMERS_GROUPS
customers_hash, groups_hash
CUSTOMERS
customers_id, customers_first_name, customers_surname, customers_telephone, customers_email, customers_telephone_active, customers_email_active, client_type, customers_hash, customers_entry_date
I want customers.groups_hash and groups.groups_name in a concat form. Here is my attempt ...
SELECT * , GROUP_CONCAT( DISTINCT customers_groups.groups_hash
SEPARATOR '/' ) , GROUP_CONCAT( groups.groups_name
SEPARATOR '/' )
FROM customers
INNER JOIN customers_groups ON ( customers.customers_hash = customers_groups.customers_hash )
LEFT JOIN groups ON ( customers_groups.customers_hash = groups.groups_hash )
WHERE groups.groups_active ='1' GROUP BY customers.customers_entry_date
but it gives me back a zero set ...
The problem is your where clause. It must be part of the on clause:
SELECT * , GROUP_CONCAT( DISTINCT customers_groups.groups_hash
SEPARATOR '/' ) , GROUP_CONCAT( groups.groups_name
SEPARATOR '/' )
FROM customers
INNER JOIN customers_groups ON ( customers.customers_hash = customers_groups.customers_hash )
LEFT JOIN groups ON ( customers_groups.customers_hash = groups.groups_hash ) AND groups.groups_active ='1'
GROUP BY customers.customers_entry_date
The problem is here:
LEFT JOIN groups ON ( customers_groups.customers_hash = groups.groups_hash )
which should probably be
LEFT JOIN groups ON ( customers_groups.groups_hash = groups.groups_hash )
(While it remains unclear what the hashes actually represent and why there is no bridge table linking the tables' IDs instead. I've asked that question in the comment section to your request.)
I use view has subquery and concat in mysql. Normally, query works rapidly, but if query has subquery works very slowly.
This code is running quickly (approximate 1 seconds)
CREATE OR REPLACE VIEW ilceler AS (
SELECT I.id, I.modulid, I.id as icerikidsi,
MAX(IF(D.alanid=2,D.textkisa,NULL)) AS ilceadi,
MAX(IF(D.alanid=2,D.id,NULL)) AS ilceadi_i,
I.seo_description,
I.seo_h1,
I.seo_h2,
I.seo_h3,
I.seo_h4,
I.seo_imgalt,
I.seo_imgtitle,
I.seo_keywords,
I.seo_pagetitle,
I.seo_url,
I.seo_urltitle
FROM datalar as D LEFT JOIN icerikler as I ON D.icerikid=I.id WHERE D.modulid='3' GROUP BY D.icerikid ORDER BY sehiradi asc )
But this code is working very very slowly (approximate 20 seconds)
CREATE OR REPLACE VIEW ilceler AS (
SELECT I.id, I.modulid, I.id as icerikidsi,
MAX(IF(D.alanid=2,D.textkisa,NULL)) AS ilceadi,
MAX(IF(D.alanid=2,D.id,NULL)) AS ilceadi_i,
( SELECT CONVERT ( GROUP_CONCAT(D2.id SEPARATOR ' ₋ ' ) USING UTF8 )FROM datalar as D1
LEFT JOIN datalar as D2 ON D1.iliskialanid=D2.id WHERE D1.modulid='3' AND D1.alanid='3' AND D1.icerikid=icerikidsi ) as sehiradi_a ,
( SELECT GROUP_CONCAT(iliskiid SEPARATOR ' ₋ ') FROM datalar WHERE alanid='3' AND modulid='3' AND icerikid=icerikidsi ) as sehiradi_i,
( SELECT GROUP_CONCAT(D2.textkisa SEPARATOR ' ₋ ' ) FROM datalar as D1 LEFT JOIN datalar as D2 ON D1.iliskialanid=D2.id
WHERE D1.modulid='3' AND D1.alanid='3' AND D1.icerikid=icerikidsi ) as sehiradi ,
I.seo_description,
I.seo_h1,
I.seo_h2,
I.seo_h3,
I.seo_h4,
I.seo_imgalt,
I.seo_imgtitle,
I.seo_keywords,
I.seo_pagetitle,
I.seo_url,
I.seo_urltitle
FROM datalar as D LEFT JOIN icerikler as I ON D.icerikid=I.id WHERE D.modulid='3' GROUP BY D.icerikid ORDER BY sehiradi asc )
Why? Where do I make mistake?
I am waiting for your help.
Your sub queries rely on the values of the select, hence each of those 3 sub queries needs to be performed for each returned row. With a small number of rows this isn't an issue but with lots of rows this can rapidly add up.
Normal solution is to join against the sub query (hence it is done once for all rows and you just join the results).
For example:-
SELECT I.id,
I.modulid,
I.id as icerikidsi,
MAX(IF(D.alanid=2,D.textkisa,NULL)) AS ilceadi,
MAX(IF(D.alanid=2,D.id,NULL)) AS ilceadi_i,
sub1.sehiradi_a ,
sub2.sehiradi_i,
sub1.sehiradi ,
I.seo_description,
I.seo_h1,
I.seo_h2,
I.seo_h3,
I.seo_h4,
I.seo_imgalt,
I.seo_imgtitle,
I.seo_keywords,
I.seo_pagetitle,
I.seo_url,
I.seo_urltitle
FROM datalar as D
LEFT JOIN icerikler as I ON D.icerikid=I.id
LEFT OUTER JOIN
(
SELECT D1.icerikid, CONVERT ( GROUP_CONCAT(D2.id SEPARATOR ' ₋ ' ) USING UTF8 ) AS sehiradi_a, GROUP_CONCAT(D2.textkisa SEPARATOR ' ₋ ' ) AS sehiradi
FROM datalar as D1
LEFT JOIN datalar as D2 ON D1.iliskialanid=D2.id
WHERE D1.modulid='3'
AND D1.alanid='3'
GROUP BY D1.icerikid
) sub1
ON sub1.icerikid = I.id
LEFT OUTER JOIN
(
SELECT icerikid, GROUP_CONCAT(iliskiid SEPARATOR ' ₋ ') AS sehiradi_i
FROM datalar
WHERE alanid='3'
AND modulid='3'
GROUP BY icerikid
) sub2
ON sub2.icerikid = I.id
WHERE D.modulid='3'
GROUP BY D.icerikid
ORDER BY sehiradi asc
Or depending on your actual database design you might be able to simplify it to
SELECT I.id,
I.modulid,
I.id as icerikidsi,
MAX(IF(D.alanid=2,D.textkisa,NULL)) AS ilceadi,
MAX(IF(D.alanid=2,D.id,NULL)) AS ilceadi_i,
sub1.sehiradi_a ,
sub1.sehiradi_i,
sub1.sehiradi ,
I.seo_description,
I.seo_h1,
I.seo_h2,
I.seo_h3,
I.seo_h4,
I.seo_imgalt,
I.seo_imgtitle,
I.seo_keywords,
I.seo_pagetitle,
I.seo_url,
I.seo_urltitle
FROM datalar as D
LEFT JOIN icerikler as I ON D.icerikid=I.id
LEFT OUTER JOIN
(
SELECT D1.icerikid,
CONVERT ( GROUP_CONCAT(D2.id SEPARATOR ' ₋ ' ) USING UTF8 ) AS sehiradi_a,
GROUP_CONCAT(D2.textkisa SEPARATOR ' ₋ ' ) AS sehiradi,
GROUP_CONCAT(DISTINCT D1.iliskiid SEPARATOR ' ₋ ') AS sehiradi_i
FROM datalar as D1
LEFT JOIN datalar as D2 ON D1.iliskialanid=D2.id
WHERE D1.modulid='3'
AND D1.alanid='3'
GROUP BY D1.icerikid
) sub1
ON sub1.icerikid = I.id
WHERE D.modulid='3'
GROUP BY D.icerikid
ORDER BY sehiradi asc
However there is a minor issue here. In MySQL a view cannot contain a FROM that takes data from a sub query. As such to use this syntax in your view you would need to split the sub queries off into their own views. Then you could join against the view rather than the sub query.
Is anyone aware of a T-SQL script that can detect redundant indexes across an entire database? An example of a redundant index in a table would be as follows:
Index 1: 'ColumnA', 'ColumnB', 'ColumnC'
Index 2: 'ColumnA', 'ColumnB'
Ignoring other considerations, such as the width of columns and covering indexes, Index 2 would be redundant.
Thanks.
There are situations where the redundancy doesn't hold. For example, say ColumnC was a huuge field, but you'd sometimes have to retrieve it quickly. Your index 1 would not require a key lookup for:
select ColumnC from YourTable where ColumnnA = 12
On the other hand index 2 is much smaller, so it can be read in memory for queries that require an index scan:
select * from YourTable where ColumnnA like '%hello%'
So they're not really redundant.
If you're not convinced by my above argument, you can find "redundant" indexes like:
;with ind as (
select a.object_id
, a.index_id
, cast(col_list.list as varchar(max)) as list
from (
select distinct object_id
, index_id
from sys.index_columns
) a
cross apply
(
select cast(column_id as varchar(16)) + ',' as [text()]
from sys.index_columns b
where a.object_id = b.object_id
and a.index_id = b.index_id
for xml path(''), type
) col_list (list)
)
select object_name(a.object_id) as TableName
, asi.name as FatherIndex
, bsi.name as RedundantIndex
from ind a
join sys.sysindexes asi
on asi.id = a.object_id
and asi.indid = a.index_id
join ind b
on a.object_id = b.object_id
and a.object_id = b.object_id
and len(a.list) > len(b.list)
and left(a.list, LEN(b.list)) = b.list
join sys.sysindexes bsi
on bsi.id = b.object_id
and bsi.indid = b.index_id
Bring cake for your users in case performance decreases "unexpectedly" :-)
Inspired by Paul Nielsen, I wrote this query to find/distinguish:
Duplicates (ignoring include order)
Redundant (different include columns)
Overlapping (different index columns)
And also record their usage
(One might also want to use is_descending_key, but I don't need it.)
WITH IndexColumns AS
(
SELECT I.object_id AS TableObjectId, OBJECT_SCHEMA_NAME(I.object_id) + '.' + OBJECT_NAME(I.object_id) AS TableName, I.index_id AS IndexId, I.name AS IndexName
, (IndexUsage.user_seeks + IndexUsage.user_scans + IndexUsage.user_lookups) AS IndexUsage
, IndexUsage.user_updates AS IndexUpdates
, (SELECT CASE is_included_column WHEN 1 THEN NULL ELSE column_id END AS [data()]
FROM sys.index_columns AS IndexColumns
WHERE IndexColumns.object_id = I.object_id
AND IndexColumns.index_id = I.index_id
ORDER BY index_column_id, column_id
FOR XML PATH('')
) AS ConcIndexColumnNrs
,(SELECT CASE is_included_column WHEN 1 THEN NULL ELSE COL_NAME(I.object_id, column_id) END AS [data()]
FROM sys.index_columns AS IndexColumns
WHERE IndexColumns.object_id = I.object_id
AND IndexColumns.index_id = I.index_id
ORDER BY index_column_id, column_id
FOR XML PATH('')
) AS ConcIndexColumnNames
,(SELECT CASE is_included_column WHEN 1 THEN column_id ELSE NULL END AS [data()]
FROM sys.index_columns AS IndexColumns
WHERE IndexColumns.object_id = I.object_id
AND IndexColumns.index_id = I.index_id
ORDER BY column_id
FOR XML PATH('')
) AS ConcIncludeColumnNrs
,(SELECT CASE is_included_column WHEN 1 THEN COL_NAME(I.object_id, column_id) ELSE NULL END AS [data()]
FROM sys.index_columns AS IndexColumns
WHERE IndexColumns.object_id = I.object_id
AND IndexColumns.index_id = I.index_id
ORDER BY column_id
FOR XML PATH('')
) AS ConcIncludeColumnNames
FROM sys.indexes AS I
LEFT OUTER JOIN sys.dm_db_index_usage_stats AS IndexUsage
ON IndexUsage.object_id = I.object_id
AND IndexUsage.index_id = I.index_id
AND IndexUsage.Database_id = db_id()
)
SELECT
C1.TableName
, C1.IndexName AS 'Index1'
, C2.IndexName AS 'Index2'
, CASE WHEN (C1.ConcIndexColumnNrs = C2.ConcIndexColumnNrs) AND (C1.ConcIncludeColumnNrs = C2.ConcIncludeColumnNrs) THEN 'Exact duplicate'
WHEN (C1.ConcIndexColumnNrs = C2.ConcIndexColumnNrs) THEN 'Different includes'
ELSE 'Overlapping columns' END
-- , C1.ConcIndexColumnNrs
-- , C2.ConcIndexColumnNrs
, C1.ConcIndexColumnNames
, C2.ConcIndexColumnNames
-- , C1.ConcIncludeColumnNrs
-- , C2.ConcIncludeColumnNrs
, C1.ConcIncludeColumnNames
, C2.ConcIncludeColumnNames
, C1.IndexUsage
, C2.IndexUsage
, C1.IndexUpdates
, C2.IndexUpdates
, 'DROP INDEX ' + C2.IndexName + ' ON ' + C2.TableName AS Drop2
, 'DROP INDEX ' + C1.IndexName + ' ON ' + C1.TableName AS Drop1
FROM IndexColumns AS C1
INNER JOIN IndexColumns AS C2
ON (C1.TableObjectId = C2.TableObjectId)
AND (
-- exact: show lower IndexId as 1
(C1.IndexId < C2.IndexId
AND C1.ConcIndexColumnNrs = C2.ConcIndexColumnNrs
AND C1.ConcIncludeColumnNrs = C2.ConcIncludeColumnNrs)
-- different includes: show longer include as 1
OR (C1.ConcIndexColumnNrs = C2.ConcIndexColumnNrs
AND LEN(C1.ConcIncludeColumnNrs) > LEN(C2.ConcIncludeColumnNrs))
-- overlapping: show longer index as 1
OR (C1.IndexId <> C2.IndexId
AND C1.ConcIndexColumnNrs <> C2.ConcIndexColumnNrs
AND C1.ConcIndexColumnNrs like C2.ConcIndexColumnNrs + ' %')
)
ORDER BY C1.TableName, C1.ConcIndexColumnNrs
I created the following query that gives me a lot of good information to identify duplicate and near-duplicate indexes. It also includes other information like how many pages of memory an index takes, which allows me to give a higher priority to larger indexes. It shows what columns are indexed and what columns are included, so I can see if there are two indexes that are almost identical with only slight variations in the included columns.
WITH IndexSummary AS
(
SELECT DISTINCT sys.objects.name AS [Table Name],
sys.indexes.name AS [Index Name],
SUBSTRING((SELECT ', ' + sys.columns.Name as [text()]
FROM sys.columns
INNER JOIN sys.index_columns
ON sys.index_columns.column_id = sys.columns.column_id
AND sys.index_columns.object_id = sys.columns.object_id
WHERE sys.index_columns.index_id = sys.indexes.index_id
AND sys.index_columns.object_id = sys.indexes.object_id
AND sys.index_columns.is_included_column = 0
ORDER BY sys.columns.name
FOR XML Path('')), 2, 10000) AS [Indexed Column Names],
ISNULL(SUBSTRING((SELECT ', ' + sys.columns.Name as [text()]
FROM sys.columns
INNER JOIN sys.index_columns
ON sys.index_columns.column_id = sys.columns.column_id
AND sys.index_columns.object_id = sys.columns.object_id
WHERE sys.index_columns.index_id = sys.indexes.index_id
AND sys.index_columns.object_id = sys.indexes.object_id
AND sys.index_columns.is_included_column = 1
ORDER BY sys.columns.name
FOR XML Path('')), 2, 10000), '') AS [Included Column Names],
sys.indexes.index_id, sys.indexes.object_id
FROM sys.indexes
INNER JOIN SYS.index_columns
ON sys.indexes.index_id = SYS.index_columns.index_id
AND sys.indexes.object_id = sys.index_columns.object_id
INNER JOIN sys.objects
ON sys.OBJECTS.object_id = SYS.indexES.object_id
WHERE sys.objects.type = 'U'
)
SELECT IndexSummary.[Table Name],
IndexSummary.[Index Name],
IndexSummary.[Indexed Column Names],
IndexSummary.[Included Column Names],
PhysicalStats.page_count as [Page Count],
CONVERT(decimal(18,2), PhysicalStats.page_count * 8 / 1024.0) AS [Size (MB)],
CONVERT(decimal(18,2), PhysicalStats.avg_fragmentation_in_percent) AS [Fragment %]
FROM IndexSummary
INNER JOIN sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL, NULL, NULL)
AS PhysicalStats
ON PhysicalStats.index_id = IndexSummary.index_id
AND PhysicalStats.object_id = IndexSummary.object_id
WHERE (SELECT COUNT(*) as Computed
FROM IndexSummary Summary2
WHERE Summary2.[Table Name] = IndexSummary.[Table Name]
AND Summary2.[Indexed Column Names] = IndexSummary.[Indexed Column Names]) > 1
ORDER BY [Table Name], [Index Name], [Indexed Column Names], [Included Column Names]
Results of the query look like this:
Table Name Index Indexed Cols Included Cols Pages Size (MB) Frag %
My_Table Indx_1 Col1 Col2, Col3 123 0.96 8.94
My_Table Indx_2 Col1 Col2, Col3 123 0.96 8.94
Complete Description
For the complete explanation see Identifying Duplicate or Redundant Indexes in SQL Server.
Try the script below to show Unused Indexes, hope it helps
/****************************************************************
Description: Script to show Unused Indexes using DMVs
****************************************************************/
SELECT TOP 100
o.name AS ObjectName
, i.name AS IndexName
, i.index_id AS IndexID
, dm_ius.user_seeks AS UserSeek
, dm_ius.user_scans AS UserScans
, dm_ius.user_lookups AS UserLookups
, dm_ius.user_updates AS UserUpdates
, p.TableRows
, 'DROP INDEX ' + QUOTENAME(i.name)
+ ' ON ' + QUOTENAME(s.name) + '.' + QUOTENAME(OBJECT_NAME(dm_ius.object_id)) as 'drop statement'
FROM sys.dm_db_index_usage_stats dm_ius
INNER JOIN sys.indexes i ON i.index_id = dm_ius.index_id AND dm_ius.object_id = i.object_id
INNER JOIN sys.objects o on dm_ius.object_id = o.object_id
INNER JOIN sys.schemas s on o.schema_id = s.schema_id
INNER JOIN (SELECT SUM(p.rows) TableRows, p.index_id, p.object_id
FROM sys.partitions p GROUP BY p.index_id, p.object_id) p
ON p.index_id = dm_ius.index_id AND dm_ius.object_id = p.object_id
WHERE OBJECTPROPERTY(dm_ius.object_id,'IsUserTable') = 1
AND dm_ius.database_id = DB_ID()
AND i.type_desc = 'nonclustered'
AND i.is_primary_key = 0
AND i.is_unique_constraint = 0
ORDER BY (dm_ius.user_seeks + dm_ius.user_scans + dm_ius.user_lookups) ASC
GO
I was just reading some MSDN blogs, noticed a script to do this and remembered this question.
I haven't bothered testing it side by side with Andomar's to see if one has any particular benefit over the other.
One amendment I would likely make to both though would be to take into account the size of both indexes when assessing redundancy.
Edit:
Also see Kimberley Tripp's post on Removing duplicate indexes
I'm trying to show the boroughs and postcodes a particular town in is.
My database is fairly well structured, with a table such as town, postcode and borough. There are also tables for each of the relationships town_postcode & town_borough.
Ideally I want the data returned as:
"Abbey Wood", "SE2", "Bexley, Greenwich"
"Barbican", "EC1, EC2", "City of London"
I've tried a few different approaches and I'm close but not there yet.
Any help would be appreciated... :)
So far I've tried
SELECT DISTINCT t.town,
GROUP_CONCAT( DISTINCT p.postcode SEPARATOR ', ' ) AS 'postcode',
GROUP_CONCAT( DISTINCT b.borough SEPARATOR ', ' ) AS 'borough'
FROM coverage_towns AS t,
coverage_boroughs AS b,
coverage_postcodes AS p,
coverage_towns_boroughs AS tb,
coverage_towns_postcodes AS tp
WHERE t.id = tp.town_id
AND p.id = tp.postcode_id
AND b.id = tb.borough_id
GROUP BY t.town
ORDER BY t.town ASC
Which returns
"Abbey Wood", "SE2", "Southwark, Hammersmith and Fulham, Tower Hamlets, Wandsworth, Enfield, Newham, LOTS MORE HERE"
"Barbican", "EC1, EC2", "Brent, Greenwich, Kensington and Chelsea, Westminster, Camden, LOTS MORE HERE"
I've also tried
SELECT DISTINCT t.town, (
SELECT SQL_CACHE DISTINCT GROUP_CONCAT( p1.postcode
SEPARATOR ', ' )
FROM coverage_postcodes AS p1
WHERE p1.id = tp.postcode_id
) AS 'postcode', (
SELECT SQL_CACHE DISTINCT GROUP_CONCAT( b1.borough
SEPARATOR ', ' )
FROM coverage_boroughs AS b1
WHERE b1.id = tb.borough_id
) AS 'borough'
FROM coverage_towns AS t, coverage_boroughs AS b, coverage_postcodes AS p, coverage_towns_boroughs AS tb, coverage_towns_postcodes AS tp
WHERE t.id = tp.town_id
AND p.id = tp.postcode_id
AND b.id = tb.borough_id
GROUP BY t.town
ORDER BY t.town ASC
Which returns
"Abbey Wood", "SE2", "Greenwich"
"Acton", "W3", "Greenwich"
"Aldersbrook", "E12", "Greenwich"
First query looks good, just add distinct inside the group_concat, like:
SELECT t.town
, GROUP_CONCAT(DISTINCT p.postcode SEPARATOR ', ' ) AS 'postcode'
, GROUP_CONCAT(DISTINCT b.borough SEPARATOR ', ' ) AS 'borough'
<more code here>
GROUP BY
t.town
SOLUTION
I came back to the question after a good coffee and the answer presented itself.
SELECT DISTINCT t.town,
GROUP_CONCAT( DISTINCT p.postcode SEPARATOR ', ' ) AS 'postcode',
GROUP_CONCAT( DISTINCT b.borough SEPARATOR ', ' ) AS 'borough'
FROM towns AS t, boroughs AS b, postcodes AS p, towns_boroughs AS tb, towns_postcodes AS tp
WHERE (t.id = tp.town_id AND t.id = tb.town_id)
AND (p.id = tp.postcode_id AND b.id = tb.borough_id)
GROUP BY t.town
ORDER BY t.town ASC