How can I get distinct values from multiple fields within one table with just one request.
Option 1
SELECT WM_CONCAT(DISTINCT(FIELD1)) FIELD1S,WM_CONCAT(DISTINCT(FIELD2)) FIELD2S,..FIELD10S
FROM TABLE;
WM_CONCAT is LIMITED
Option 2
select DISTINCT(FIELD1) FIELDVALUE, 'FIELD1' FIELDNAME
FROM TABLE
UNION
select DISTINCT(FIELD2) FIELDVALUE, 'FIELD2' FIELDNAME
FROM TABLE
... FIELD 10
is just too slow
if you were scanning a small range in the data (not full scanning the whole table) you could use WITH to optimise your query
e.g:
WITH a AS
(SELECT field1,field2,field3..... FROM TABLE WHERE condition)
SELECT field1 FROM a
UNION
SELECT field2 FROM a
UNION
SELECT field3 FROM a
.....etc
For my problem, I had
WL1 ... WL2 ... correlation
A B 0.8
B A 0.8
A C 0.9
C A 0.9
how to eliminate the symmetry from this table?
select WL1, WL2,correlation from
table
where least(WL1,WL2)||greatest(WL1,WL2) = WL1||WL2
order by WL1
this gives
WL1 ... WL2 ... correlation
A B 0.8
A C 0.9
:)
The best option in the SQL is the UNION, though you may be able to save some performance by taking out the distinct keywords:
select FIELD1 FROM TABLE
UNION
select FIELD2 FROM TABLE
UNION provides the unique set from two tables, so distinct is redundant in this case. There simply isn't any way to write this query differently to make it perform faster. There's no magic formula that makes searching 200,000+ rows faster. It's got to search every row of the table twice and sort for uniqueness, which is exactly what UNION will do.
The only way you can make it faster is to create separate indexes on the two fields (maybe) or pare down the set of data that you're searching across.
Alternatively, if you're doing this a lot and adding new fields rarely, you could use a materialized view to store the result and only refresh it periodically.
Incidentally, your second query doesn't appear to do what you want it to. Distinct always applies to all of the columns in the select section, so your constants with the field names will cause the query to always return separate rows for the two columns.
I've come up with another method that, experimentally, seems to be a little faster. In affect, this allows us to trade one full-table scan for a Cartesian join. In most cases, I would still opt to use the union as it's much more obvious what the query is doing.
SELECT DISTINCT CASE lvl WHEN 1 THEN field1 ELSE field2 END
FROM table
CROSS JOIN (SELECT LEVEL lvl
FROM DUAL
CONNECT BY LEVEL <= 2);
It's also worthwhile to add that I tested both queries on a table without useful indexes containing 800,000 rows and it took roughly 45 seconds (returning 145,000 rows). However, most of that time was spent actually fetching the records, not running the query (the query took 3-7 seconds). If you're getting a sizable number of rows back, it may simply be the number of rows that is causing the performance issue you're seeing.
When you get distinct values from multiple columns, then it won't return a data table. If you think following data
Column A Column B
10 50
30 50
10 50
when you get the distinct it will be 2 rows from first column and 1 rows from 2nd column. It simply won't work.
And something like this?
SELECT 'FIELD1',FIELD1, 'FIELD2',FIELD2,...
FROM TABLE
GROUP BY FIELD1,FIELD2,...
Related
I know the title is very vague but I didn't know how to explain this question better, if you have any suggestions please do tell me.
I have this table:
CRAccountID DBAccountID Value
And I want to have a row for each CRAccountID and DBAccountID for example:
CRAccountID DBAccountID Value
100 111 10
I want to get these records:
AccountID Value
100 10
111 -10
If you haven't noticed yet I also want to make the DBAccountID value in minus.
I tried it by selecting both of them and doing a Union but it takes way too much time when the table gets bigger (the table can contain more than 4 million records)
Here's my code:
select CRAccountID as AccountID, Value from MyTable
UNION
select DBAccountID as AccountID, -Value as Value from MyTable
Note: my code is much much bigger than this with almost 4 joins in each union therefore it becomes so slow.
I'm using Microsoft SQL.
You want to unpivot the data. I would recommend apply:
select v.account, v.value
from t cross apply
(values (t.CRAccountID, t.value),
(t.DBAccountID, - t.value)
) v(account, value) ;
You can express your complex as a CTE or subquery. Or just include it in the query before the cross apply.
I have two tables A and B with same column names. I have to combine them into table C
when I am running following query, the count is not matching -
select * into C
from
(
select * from A
union
select * from B
)X
The record count of C is not matching with A and B. There is difference of 89 rows. So I figured out that there are duplicates.
I used following query to find duplicates -
select * from A
INTERSECT
select * from B
-- 80 rows returned
Can anybody tell me why intersect returns 80 dups whereas count difference on using union is 89 ?
There are probably duplicates inside of A and/or B as well. All set operators perform an implicit DISTINCT on the result (logically, not necessarily physically).
Duplicate rows are usually a data-quality issue or an outright bug. I usually mitigate this risk by adding unique indexes on all columns and column sets that are supposed to be unique. I especially make sure that every table has a primary key if that is at all possible.
I need help on performance of a query I have that is very slow.
The query is doing a case on a column in order to return a different text value
based on the number.
If I had a table with values 1-5 and 8-10 If something has a value of 1 it should display 'Apple' or if it is 2 it must display 'pear' if it is anything other than 1-5 then it is 'other'. Currently a case statement is being used, but I heard that functions on a query makes it slower.
All I want is the 'Wording' to appear instead of the the number but because the table is so big it feels to me like it is iterating each row just to determine what to display.
Is there a faster way of doing this.I considered doing a join, which seems like it would work nice, but I dont know how to write 'other' for anything other than 1-5
A case statement is not a function. User-defined functions do have additional overhead in some versions of SQL. You don't, as a general rule, need to worry about the overhead for built-in functions.
You could do this with a join as:
with lookup as (
select 1 as val, 'Apple' as str union all
select 2 as val, 'Pear' as str union all
select 3 union all select 4 union all select 5
)
select coalesce(l.val, 'other')
from t left outer join
lookup l
on t.col = l.val
I would expect the case statement to be marginally faster though.
I'm using sql-server 2005 and ASP.NET with C#.
I have Users table with
userId(int),
userGender(tinyint),
userAge(tinyint),
userCity(tinyint)
(simplified version of course)
I need to select always two fit to userID I pass to query users of opposite gender, in age range of -5 to +10 years and from the same city.
Important fact is it always must be two, so I created condition if ##rowcount<2 re-select without age and city filters.
Now the problem is that I sometimes have two returned result sets because I use first ##rowcount on a table. If I run the query.
Will it be a problem to use the DataReader object to read from always second result set? Is there any other way to check how many results were selected without performing select with results?
Can you simplify it by using SELECT TOP 2 ?
Update: I would perform both selects all the time, union the results, and then select from them based on an order (using SELECT TOP 2) as the union may have added more than two. Its important that this next select selects the rows in order of importance, ie it prefers rows from your first select.
Alternatively, have the reader logic read the next result-set if there is one and leave the SQL alone.
To avoid getting two separate result sets you can do your first SELECT into a table variable and then do your ##ROWCOUNT check. If >= 2 then just select from the table variable on its own otherwise select the results of the table variable UNION ALLed with the results of the second query.
Edit: There is a slight overhead to using table variables so you'd need to balance whether this was cheaper than Adam's suggestion just to perform the 'UNION' as a matter of routine by looking at the execution stats for both approaches
SET STATISTICS IO ON
Would something along the following lines be of use...
SELECT *
FROM (SELECT 1 AS prio, *
FROM my_table M1 JOIN my_table M2
WHERE M1.userID = supplied_user_id AND
M1.userGender <> M2.userGender AND
M1.userAge - 5 >= M2.userAge AND
M1.userAge + 15 <= M2.userAge AND
M1.userCity = M2.userCity
LIMIT TO 2 ROWS
UNION
SELECT 2 AS prio, *
FROM my_table M1 JOIN my_table M2
WHERE M1.userID = supplied_user_id AND
M1.userGender <> M2.userGender
LIMIT TO 2 ROWS)
ORDER BY prio
LIMIT TO 2 ROWS;
I haven't tried it as I have no SQL Server and there may be dialect issues.
If I have a table with the hypothetical columns foo and bar. bar might have 50-60 distinct values in it. My goal here is to pick say, up to 5 rows for say 6 unique bars. So if the 6 unique bars that get selected out of the 50-60 each happen to has at least 5 rows of data, we'll have 30 rows in total.
What you'd really want to do is:
SELECT *
FROM `sometable`
WHERE `bar` IN (
SELECT DISTINCT `bar`
FROM `sometable`
ORDER BY RAND()
LIMIT 6
)
Unfortunately, you're likely to get this:
ERROR 1235 (42000): This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
Possibly your version will be more cooperative. Otherwise, you'll probably need to do it as two queries.
Its been a while since I've worked with MySQL (I've been working with MSSQL lately), but two things come to mind:
Some sort of self join
A Cursor
Self join might look something like
SELECT DISTINCT bar FROM table AS t1 LIMIT 5
JOIN table AS t2 ON t1.foo = t2.foo
Again, its been a while, so this might not be valid MySQL. Also, you'd get all the foo's back for the 5 bars, so you'd have to figure out how to trim that down.
I think the easiest way is to use a UNION.
SELECT * FROM table WHERE bar = 'a' LIMIT 5 UNION SELECT * FROM table WHERE bar='b' UNION SEL ....... you get the jist, i hope
EDIT: not sure if this is what you need - you don't say whether this query needs also to somehow determine the bars? or if they are passed in?
A simple solution that takes 7 queries:
SELECT distinct bar FROM sometable ORDER BY rand() LIMIT 6
Then, for each of the 6 bar values above, do this, substituting {$bar} for the value, of course:
SELECT foo,bar FROM sometable WHERE bar='{$bar}' ORDER BY rand() LIMIT 5
Be careful about using "ORDER BY rand()" because it might cause MySQL to fetch a LOT of rows from your table, and compute the rand() function for all of them, and then sort them. This can take a long time if you have a big table.
If it does take a long time, then for the first query, you can remove the ORDER BY and the LIMIT clauses, and select 6 random values in your program code after the query is done.
For the second query, you can split it in to two steps:
SELECT count(*) FROM sometable WHERE bar='{$bar}'
Then, in your program code, you know how many items there are so you can randomly choose which of them to look at, and use OFFSET and LIMIT:
SELECT foo,bar FROM sometable WHERE bar='{$bar}' LIMIT 1 OFFSET {$offset}
Is this getting called from some program?
If so perhaps you can just lookup the bars, and randomly send them into a select statement.
This way your select could simply be: select * from table where bar in (?,?), and you can move the randomness problem into code, which is frankly better at dealing with that.