I have a few tables that have similar fields but not exactly the same.
The same fields they have are description (text field) and modified (unixtime)
I would like to select the last modified items from these tables based on unixtime. I cant use UNION since the tables aren't the same and the multiple table select times out.
I've been trying to look for this but no luck, either people are using JOINS or SELECT A., B. FROM table A, table B
How different they are?
Maybe you can get the common fields out:
select t1.name1 as name from table1
union
select t2.name2 as name from table2
Try this:
SELECT
IF (A.modified > B.modified, A.modified, B.modified) AS modified,
IF (A.modified > B.modified, A.description, B.description) AS description,
FROM
(SELECT description, modified FROM A ORDER BY modified DESC LIMIT 1) AS A,
(SELECT description, modified FROM B ORDER BY modified DESC LIMIT 1) AS B
LIMIT 1
However, it's pretty much the same as just doing two queries (only more complicated) so I wouldn't recommend it.
Try adding desc index on 'modified' if your select timesou, and use limit on select to return just one (last) row.
Then you can:
SELECT
A,B,C,D, desc, modified
FROM
TABLEA
UNION ALL
SELECT
CAST(E as <A type>), CAST(F AS <B type>) ..., desc, modified
FROM
TABLE B
Related
I am looking to run this query on a list of tables.
SELECT Description,Code,count(*) as count
FROM table1
group by Description,code
having count(*) > 1
I will have to run this query on 30+ different tables, I was wondering If I could change the from statement and just list off the table names.
In addition, is there some functionality that will add the name of the table that it came from in a seperate column to distinguish where the results came from?
Thanks in advance
You might use UNION ALL to put it together. Unless you need some dynamic table selection.
SELECT Description,Code,count(*) as count, 'table1' as tableNane
FROM table1
group by Description,code
having count(*) > 1
UNION ALL
SELECT Description,Code,count(*) as count, 'table2' as tableNane
FROM table2
group by Description,code
having count(*) > 1
...
Actualy I like #Shubhradeep Majumdar version. It will generate more concise code.
SELECT Description,Code, Count(Code), tableName FROM (
SELECT Description,Code, 'table1' as tableName
FROM table1
UNION ALL
SELECT Description,Code, 'table2' as tableName
FROM table2
) tables
GROUP BY tableName, Description, Code
HAVING COUNT(Code) > 1
But there might be a little catch to it. It is more elegant code, but it might actually be slower than first version. The problem is that tableName is appended at every record before grouping while in my first version you do that on already processed data.
Carrying over from #Marek's answer, You could first append all the tables to a table with union all.
select *, 'tab1' as tabnm from tab1
union all
select *, 'tab2' as tabnm from tab2
union all
select *, 'tab3' as tabnm from tab3
-- and so on...
And then use your code to process that final table.
will save you a great deal of time.
EDITED with a column specifying the table name
Exist a better way to do what the following SQL query does? I have the feeling that table1 will be searched twice and may be that can be avoided with some trick and increase the efficient of the query, but I just can't figure out how ;( Here is the query (in MySQL):
SELECT a, SUM(count)
FROM table1
GROUP BY a
HAVING SUM(count) = (SELECT SUM(count) as total FROM table1 GROUP BY a ORDER BY total DESC LIMIT 1)
The goal is return the number(s) with the major accumulate, with its accumulate.
being table1 a two field table like:
a,count
1,10
1,30
1,0
2,1
2,100
2,4
3,10
4,50
4,55
The result with that data sample is:
2,105
4,105
Thanks in advance.
SELECT a, total FROM
(SELECT a AS a, SUM(COUNT) AS total
FROM table1
GROUP BY a) AS xyz
HAVING total = MAX(total)
Hope this will work for you
This sub-query is executed only once, and you don't have to bother with creating any pre-query as other answers may suggest (although doing so this is still correct, just not needed). Database engine will realise, that the sub-query is not using any variable dependent on the other part of the query. You can use EXPLAIN to see how the query is executed.
More on the topic in this answer:
https://stackoverflow.com/a/658954/1821029
I think you could probably do it by moving your HAVING sub-select query into its on prequery. Since it will always include a single row, you won't require any "JOIN", and it does not have to keep recomputing the COUNT(*) every time the HAVING is applied. Do it once, then the rest
SELECT
a,
SUM(count)
FROM
table1,
( SELECT SUM(count) as total
FROM table1
GROUP BY a
ORDER BY total DESC
LIMIT 1 ) PreQuery
GROUP BY
a
HAVING
SUM(count) = PreQuery.Total
This query return one row with two columns:
1- a list of comma separated values of "a" column, which have the biggest "Total"
2- and the biggest Total value
select group_concat(a), Total
from
(select a, sum(count) as Total
from table1
group by a) OnTableQuery
group by Total
order by Total desc
limit 1
Note that it queries table1 just one time. The query was already tested.
I had 3 tables which are not identical to each other. According to one of my requirement I had to copy all these tables records to another table.
That part is okay.What my problem is that the records I inserted is in a order now.
Like
first 100 records from table1
second 100 records from table2
third 100 records from table3
what I wanted to do is change/mix the record positions.Like if i selected first 100 records there should be records from all three table.
selecting data from ORDER BY Rand() is not I want.I just need to select data and display those data.
Is there any way that i can solve this out?Thanks
A great post handling several cases, from simple, to gaps, to non-uniform with gaps.
http://jan.kneschke.de/projects/mysql/order-by-rand/
For most general case, here's how you do it:
SELECT name
FROM random AS r1 JOIN
(SELECT (RAND() *
(SELECT MAX(id)
FROM random)) AS id)
AS r2
WHERE r1.id >= r2.id
ORDER BY r1.id ASC
LIMIT 1
This supposes that the distribution of ids is equal, and that there can be gaps in the id list. See the article for more advanced examples
If you don't want to query later on with rand() you could create the table by inserting from a union select ordered by rand() in the first place:
INSERT INTO merged (a, b)
SELECT a, b FROM (
SELECT a, b, rand() AS r FROM t1
UNION ALL
SELECT a, b, rand() AS r FROM t2
) ORDER BY r
However, also consider this post I just came across: INSERT INTO SELECT strange order using UNION, perhaps someone can comment.
I'am trying to understand what causes the following, maybe you could help me:
I have a query like:
select field1,fieldDate from table1
union all
select field1,fieldDate from table2
order by fieldDate desc
and the another one like this:
select field1,field2,fieldDate from table1
union all
select field1,field2,fieldDate from table2
order by fieldDate desc
So basically they are the same with the exception that in the second I retrieve an extra field.
Now, both results come with a diferent ordering, but just for the cases that the dates are exacly the same. For example there are 2 rows (row1,row2) with date 2009-11-25 09:41:55. For query 1 row1 comes before row2 and for query 2 row2 comes before row1.
Does somebody knows why this happens?
Thanks,
Regards
The ordering based on any fields that you don't explicitly order by is undefined, and the optimizer can change the ordering if it thinks that results in a better execution plan. Given two rows with the exact same value in the order by field you can not depend on them being in any particularly order in relation to each other unless you explicitly order by another field with different values.
Can you do this
select * from ( select
field1,field2,fieldDate, 0 as ordercol from table1
union all select
field1,field2,fieldDate, 1 as ordercol from table2) t1
order by fieldDate desc, ordercol asc
Straight from the MySQl manual, to user order by on a union you have to parenthesis the individual tables.
(select field1,fieldDate from table1)
union all
(select field1,fieldDate from table2)
order by fieldDate desc
This is not SQL standards compliant! The code you entered should order the union of both tables but to my surprise MySQL has the above syntax.
The order in which rows with the same fieldDate are returned can differ for each query execution. Usually this order will be the same but you should not count on it. If you want any extra ordering state more order by fields.
EDIT: This answer is wrong: the order by works on the entire union. I'll leave it here to save others the trouble :)
Your order by only works on the second part of the union. You can use a subquery to make the order by work on the entire union:
select field1,field2,fieldDate
from (
select field1,field2,fieldDate
from table1
union all
select field1,field2,fieldDate
from table2
) SubQueryName
order by fieldDate desc
I'm trying out the following query:
SELECT A,B,C FROM table WHERE field LIKE 'query%'
UNION
SELECT A,B,C FROM table WHERE field LIKE '%query'
UNION
SELECT A,B,C FROM table WHERE field LIKE '%query%'
GROUP BY B ORDER BY B ASC LIMIT 5
That's three queries stuck together, kinda sorta. However, the result set that comes back reflects results from query #3 before the results from query #1 which is undesired.
Is there any way to prioritize these so that results come as all for query #1, then all for query #2 then all for query #3? I don't want to do this in PHP just yet (not to mention having to control for results that showed up in the first query not to show in the second and so forth).
Maybe you should try including a fourth column, stating the table it came from, and then order and group by it:
SELECT A,B,C, "query 1" as origin FROM table WHERE field LIKE 'query%'
UNION
SELECT A,B,C, "query 2" as origin FROM table WHERE field LIKE '%query'
UNION
SELECT A,B,C, "query 3" as origin FROM table WHERE field LIKE '%query%'
GROUP BY origin, B ORDER BY origin, B ASC LIMIT 5
Add an additional column with hard-coded values that you will use to sort the overall resultset, like so:
SELECT A,B,C,1 as [order] FROM table WHERE field LIKE 'query%'
UNION
SELECT A,B,C,2 as [order] FROM table WHERE field LIKE '%query'
UNION
SELECT A,B,C,3 as [order] FROM table WHERE field LIKE '%query%'
GROUP BY B ORDER BY [order] ASC, B ASC LIMIT 5
Can you do it as a subselect, something like
SELECT * FROM (
SELECT A,B,C FROM table WHERE field LIKE 'query%'
UNION
SELECT A,B,C FROM table WHERE field LIKE '%query'
UNION
SELECT A,B,C FROM table WHERE field LIKE '%query%'
) ORDER BY B ASC LIMIT 5
SELECT distinct a,b,c FROM (
SELECT A,B,C,1 as o FROM table WHERE field LIKE 'query%'
UNION
SELECT A,B,C,2 as o FROM table WHERE field LIKE '%query'
UNION
SELECT A,B,C,3 as o FROM table WHERE field LIKE '%query%'
)
ORDER BY o ASC LIMIT 5
Would be my way of doing it. I dont know how that scales.
I don't understand the
GROUP BY B ORDER BY B ASC LIMIT 5
Does it apply only to the last SELECT in the union?
Does mysql actually allow you to group by a column and still not do aggregates on the other columns?
EDIT: aaahh. I see that mysql actually does. Its a special version of DISTINCT(b) or something. I wouldnt want to try to be an expert on that area :)
If there isn't a sort that makes sense to order them you desire, don't union the results together - just return 3 separate recordsets, and deal with them accordingly in your data tier.
I eventually (looking at all suggestions) came to this solution, its a bit of a compromise between what I need and time.
SELECT * FROM
(SELECT A, B, C, "1" FROM table WHERE B LIKE 'query%' LIMIT 3
UNION
SELECT A, B, C, "2" FROM table WHERE B LIKE '%query%' LIMIT 5)
AS RS
GROUP BY B
ORDER BY 1 DESC
it delivers 5 results total, sorts from the fourth "column" and gives me what I need; a natural result set (its coming over AJAX), and a wildcard result set following right after.
:)
/mp
There are two varients of UNION.
'UNION' and 'UNION ALL'
In most cases what you really want to say is UNION ALL as it does not do duplicate elimination (Think SELECT DISTINCT) between sets which can result in quite a bit of savings in terms of execution time.
Others have suggested multiple result sets which is a workable solution however I would caution against this in time sensitive applications or applications connected over WANs as doing so can result in significantly more round trips on the wire between server and client.
I don't understand why the need of union for taking the data from single table
SELECT A, B, C
FROM table
WHERE field LIKE 'query%'
OR field LIKE '%query'
OR field LIKE '%query%'
GROUP BY B
ORDER BY B ASC LIMIT 5