I'm keep getting lost in this one and struggling to find the right method. Hopefully someone out there might know of a good way of doing what I want to do.
I have two tables and I want to update one table using concatenated data from the other where the ids are the same. As an example....
Table1
ItemID CategoryID
1 20
1 30
1 40
2 10
3 40
3 20
4 10
4 20
Table2
ItemID CatIDs
1
2
3
4
I want to update Table2.CatIDs with all the Category IDs from Table1 where the ItemIDs match. It seems straightforward when I write it down like that but after trying Inner Joins, Sub Queries and so on as I've found online, I keep getting "You have errors in your SQL Syntax..."
I want Table2 to look something like
ItemID CatIDs
1 20,30,40
2 10
3 40,20
4 10,20
I've tried Inner Joins and also sub queries and the closest I've got without an error was this....
UPDATE Table2
SET Table2.CatIDs = Table2.CatIDs + ", " +
(SELECT CategoryID FROM Table1 WHERE Table2.ItemID = Table1.ItemID)
But it doesn't seem finished and all it done was update four rows with the same CatIDs and then give me the message
#1242 - Subquery returns more than 1 row
I'm sure someone out there will be able to see where I'm going wrong and point me in the right direction.
Thanks in advance
You should realize that table1 is the right way to store this information. It is called a junction or association table. Sometimes you need to do the concatenation for presentation purposes, but you need to keep the junction table for full flexibility.
You can do what you want using an update with join and group by:
UPDATE Table2 t2 JOIN
(SELECT t1.ItemId, GROUP_CONCAT(t1.CategoryId SEPARATOR ', ') as cats
FROM table1 t1
GROUP BY t1.ItemId
) tc
ON t2.ItemId = tc.ItemId
SET t2.CatIDs = tc.cats;
Try like below by using group_concat() you can get the , separated list and then join between the tables. But storing comma separated values is never a good idea.
update table2 t2
join
(
select ItemID,group_concat(CategoryID) as newcat
from table1 group by ItemID
) tab on t2.ItemID = tab.ItemID
set t2.CatIDs = tab.newcat
Related
I need to join / loop over some data but not sure how to do it using only Mysql nested / WHERE IN type queries. I have one table that looks like this
id, code1, code2, code3, code4........... code20
1 12 41 1 55
So a lot of columns, they don't all have values, but of the ones that do, I need to take each of those codes and return a row for the code from a table that looks like this:
codeid, description
1 item1
12 item12
13 item13
41 item41
...
I was hoping I could use a wildcard in my select for the columns like
SELECT description FROM table2 WHERE IN(SELECT code* FROM table1)
It doesn't look like the wildcard is possible from my googling, but someone here may know a trick. How can I take all the values from the first table as a list, run the query on it to return the definitions of each code as separate rows?
No, you cannot use a wildcard. This is a problem with your data structure. Whenever you have columns that are only distinguished by numbers, you probably have an issue with the data model.
Instead, you should have a table with one row per id and per code.
You can do something like:
select t2.description
from table2 t2
where exists (select 1
from table1 t1
where t2.code in (t1.code1, t1.code2, . . . )
);
Performance will not be very good. For that, see the first suggestion for fixing the data model.
I have got a MySQL database, managed with phpMyAdmin.
I am not very clever at MySQL requests.
In table1 are (among others) the 2 following columns :
- id_product
- active
In table2 are (among others) the 2 following columns :
- id_product
- description
I would like to write a request that display a table as follows :
having, at least, the id_product column and the description column
and having only product for which the active field is equal to 1 (the active field can only have a value of 0 or 1)
Thank you in advance for any help in this matter.
Patrick
You want to make an Inner Join between the tables. This matches rows from one table to rows in another, excluding rows that don't have a match. Then you want to add a restriction to only return those with active = 1.
This looks like:
Select t2.id_product, t2.description from table2 t2
inner join table1 t1
on t1.id_product = t2.id_product
where t1.active = 1;
What I have is 2 tables, the first table I want it to display all results, no "where" or anything to limit it.
The second table I want to match an id to the first table, it can have multiple rows referencing it so I want to count the number.
So lets say the first table is like this:
ID - name
1 - one
2 - two
3 - three
4 - four
And the second table is like this
ID - REF
1 - 1
2 - 1
3 - 2
4 - 2
5 - 3
6 - 3
7 - 4
8 - 4
I want to combine them like so:
ID - name - count
1 - one - 2
2 - two - 2
3 - three- 2
4 - four - 2
I have tried using subqueries, left joins, right joins, inner joins, sub query joins, grouping and 9 times out of ten I get 20 results of the first ID out of 1300 results I should get. The rest I only get an incorrect count and no name.
I feel this is MySQL 101 but after attempting multiple variations and coming up with nothing I feel there must be something I am missing.
I would be happy to be directed to a question that is in the exact same situation (2 hours of looking and nothing that works exactly like this) Or a simple query to point out the logic of this method, Thanks in an advance to anyone that answers, you will have made my day.
If any additional information is needed let me know, I have left out the query deliberately because I have adapted it so many times that it will not have much relevance (I would have to list every query I tried and that would be far to much scrolling)
Ok I have tested the first and answer and it seemed to work in this context so I will expand my answer, the question is "answered" so this is just an expansion if there are no replies I will close this with the answer as follows:
SELECT t.id, t.name, count(*) AS suppliers
FROM #__tiresku AS t
LEFT JOIN #__mrsp AS m ON t.name = m.tiresku_id
GROUP BY t.id, t.name
The expansions is an inner join, I have another table that is more of a list, it has an id and a name and that's it, I reference that table with an id to get the "name" instead.
This might have a better option then joins (like foreign keys or something).
I had this added to the select b.name AS brand_name
And a join INNER JOIN #__brands AS b ON t.brand = b.id
Worked with a sub query rather then join
This is a basic join with aggregation:
select t1.id, t1.name, count(*) as `count`
from table1 t1 join
table2 t2
on t1.id = t2.ref
group by t1.id, t1.name;
As asked, the example does not include records in the first table that are not in the second table, but this may be possible and is implied.
I am inclined to create a nested table of the counts in the second table without regard to "exists in the first table" either, unless the counts are huge and then the probe becomes cheaper.
I would do the count of the values in the second table first as the first table is a defacto decode of a description.
select ID, name, coalesce('count',0)
from (select ref, count(*) as 'count'
from table2
group by ref) as T2
right join table1
on ref = ID;
I have got 5 tables of which the structures are the same. Only the PAGEVISITS field is unique
ie. table 1:
ITEM | PAGEVISITS | Commodity
1813 50 Griddle
1851 10 Griddle
11875 100 Refrigerator
2255 25 Refrigerator
ie. table 2:
ITEM | PAGEVISITS | Commodity
1813 0 Griddle
1851 10 Griddle
11875 25 Refrigerator
2255 10 Refrigerator
I want it to add up the Commodity to spit out:
table1 | table2 | Commodity
60 10 Griddle
125 35 Refrigerator
Some of the data is actually correct but some are WAY off given the below query:
SELECT
SUM(MT.PAGEVISITS) as table1,
SUM(CT1.PAGEVISITS) as table2,
SUM(CT2.PAGEVISITS) as table3,
SUM(CT3.PAGEVISITS) as table4,
SUM(CT4.PAGEVISITS) as table5,
(COUNT(DISTINCT MT.ITEM)) + (COUNT(DISTINCT CT1.ITEM)) + (COUNT(DISTINCT CT2.ITEM)) + (COUNT(DISTINCT CT3.ITEM)) + (COUNT(DISTINCT CT4.ITEM)) as Total,
MT.Commodity
FROM table1 as MT
LEFT JOIN table2 CT1
on MT.ITEM = CT1.ITEM
LEFT JOIN table3 CT2
on MT.ITEM = CT2.ITEM
LEFT JOIN table4 CT3
on MT.ITEM = CT3.ITEM
LEFT JOIN table5 CT4
on MT.ITEM = CT4.ITEM
GROUP BY Commodity
I believe this may be cause by using the LEFT JOIN incorrectly. I have also tried the INNER JOIN with the same inconsistent results.
I would do a UNION on all five of those tables to get them as one rowset (inline view), and then run a query on that, start with something like this...
SELECT SUM(IF(t.source='MT',t.pagevisits,0)) AS table1
, SUM(IF(t.source='CT1',t.pagevisits,0)) AS table2
, t.commodity
FROM ( SELECT 'MT' as source, table1.* FROM table1
UNION ALL
SELECT 'CT1', table2.* FROM table2
UNION ALL
SELECT 'CT2', table3.* FROM table3
UNION ALL
SELECT 'CT3', table4.* FROM table4
UNION ALL
SELECT 'CT4', table5.* FROM table5
) t
GROUP BY t.commodity
(But I would specify the column list for each of those tables, rather than using the '.*' and having my query dependent on no one adding/dropping/renaming/reordering columns in any of those tables.)
I include an "extra" literal value (aliased as "source") to identify which table the row came from. I can use a conditional test in an expression in the SELECT list, to figure out whether the row came from a particular table.
This approach is particularly flexible, and can be used to get more complicated resultsets. For example, if I also wanted to get a total number page visits from table3, 4 and 5 added together, along with the individual counts.
SUM(IF(t.source IN ('CT2','CT3','CT4'),t.pagevisits,0) AS total_345
To get the equivalent of your COUNT(DISTINCT item) + COUNT(DISTINCT item) + ... expression...
I would use an expression that makes a single value from both the "source" and "item" columns, being careful to have some sort of guarantee that any particular "source"+"item" will not create a duplicate of some other "source"+"item". (If we just concatenate strings, for example, we don't have any way to distinguish between 'A'+'11' and 'A1'+'1'.) The most common approach I see here is a carefully chosen delimiter which is guaranteed not to appear in either value. We can distinguish between 'A::11' and 'A1::1', so something like this will work:
COUNT(DISINCT CONCAT(t.source,'::',t.item))
In your current query, if item is NULL, then the row doesn't get included in the COUNT. To fully replicate that behavior, you would need something like this:
COUNT(DISINCT IF(t.item IS NOT NULL,CONCAT(t.source,'::',t.item),NULL)) AS Total
Or course, getting a count of distinct item values over the whole set of five tables is much simpler (but then, it does return a different result)
COUNT(DISINCT t.item)
But to answer your question about the use of the LEFT JOIN, the left side table is the "driver" so a matching row has to be in that table for a corresponding row to be retrieved from a table on the right. That is, unmatched rows from the tables on the right side will not be returned.
If what you have is basically five "partitions", and you want to process all of the rows whether or not a matching row appears in any of the other "partitions", I would go with the UNION ALL approach to simply concatenate all of the rows from all of those tables together, and process the rows as if they were from a single table.
NOTE: For very large tables, this may not be a feasible approach, since MySQL is going to have to materialize that inline view. There are other approaches which don't require concatenating all of the rows together.
Specifying a list of only the columns you need, in the SELECT from each table, may help performance, if there are columns in those tables you don't need to reference in your query.
I want to be able to limit the amount of duplicate records in a mySQL database table to 2.
(Excluding the id field which is auto increment)
My table is set up like
id city item
---------------------
1 Miami 4
2 Detroit 5
3 Miami 4
4 Miami 18
5 Miami 4
So in that table, only row 5 would be deleted.
How can I do this?
MySQL has some foibles when reading and writing to the same table. So I don't actually know if this will work, the syntax is fine in many implementations of SQL, but I don't know if it's MySQL friendly...
DELETE
yourTable
WHERE
1 < (SELECT COUNT(*)
FROM yourTable as Lookup
WHERE city = yourTable.city AND item = yourTable.item AND id < yourTable.id)
EDIT
Amazingly convoluted, but worth a try?
DELETE
yourTable
FROM
yourTable
INNER JOIN
(
SELECT
id
FROM
(
SELECT
id
FROM
yourTable
WHERE
1 < (SELECT COUNT(*)
FROM yourTable as Lookup
WHERE city = yourTable.city AND item = yourTable.item AND id < yourTable.id)
)
AS inner_deletes
)
AS deletes
ON deletes.id = yourTable.id
I think your problem here is that both your code and/or table structure allows inserting duplicates and you are asking this question when you should really fix your db and/or code.
i think a better solution is avoid allow more than 5 registers, you have to implement a validation where if select count(*) > 3 you will not accept the new insert.
because if you want to do this into the data tier, you have to use a stored procedure , because first you need to identify all the register with more than 3 registers and delete only the last .
Saludos
Due to MySQL being notoriously difficult when it comes to updating queried tables (see for example the answers from Dems), the best I can figure out is sadly more than one statement but on the plus side fairly readable;
CREATE TEMPORARY TABLE Dump AS SELECT id FROM table1 WHERE id NOT IN
(SELECT MIN(id) FROM table1 GROUP BY city,item UNION
SELECT MAX(id) FROM table1 GROUP BY city,item);
DELETE FROM table1 where id in (select * from Dump);
DROP TABLE DUMP;
Not sure if it was important which duplicate was removed, this keeps the first and last.
In your reply to Joachim's answer, you ask about saving 3 or 5 rows, this is one way to accomplish it. Depending on how you are using this database, you could either call this in a loop, or you could turn it into a stored procedure. Either way, you would continue to run this entire block of code until Rows Affected = 0:
drop table if exists TempTable;
create table TempTable
select city, item,
count(*) as record_count,
min(id) as ItemToDrop -- this could be changed to max() if you
-- want to delete new stuff instead
from YourTable
group by city, item
having count(*) > 2; -- This value = number of rows you save
delete from YourTable
where id in (select ItemToDrop from TempTable);