If I have a table
CREATE TABLE users (
id int(10) unsigned NOT NULL auto_increment,
name varchar(255) NOT NULL,
profession varchar(255) NOT NULL,
employer varchar(255) NOT NULL,
PRIMARY KEY (id)
)
and I want to get all unique values of profession field, what would be faster (or recommended):
SELECT DISTINCT u.profession FROM users u
or
SELECT u.profession FROM users u GROUP BY u.profession
?
They are essentially equivalent to each other (in fact this is how some databases implement DISTINCT under the hood).
If one of them is faster, it's going to be DISTINCT. This is because, although the two are the same, a query optimizer would have to catch the fact that your GROUP BY is not taking advantage of any group members, just their keys. DISTINCT makes this explicit, so you can get away with a slightly dumber optimizer.
When in doubt, test!
If you have an index on profession, these two are synonyms.
If you don't, then use DISTINCT.
GROUP BY in MySQL sorts results. You can even do:
SELECT u.profession FROM users u GROUP BY u.profession DESC
and get your professions sorted in DESC order.
DISTINCT creates a temporary table and uses it for storing duplicates. GROUP BY does the same, but sortes the distinct results afterwards.
So
SELECT DISTINCT u.profession FROM users u
is faster, if you don't have an index on profession.
All of the answers above are correct, for the case of DISTINCT on a single column vs GROUP BY on a single column.
Every db engine has its own implementation and optimizations, and if you care about the very little difference (in most cases) then you have to test against specific server AND specific version! As implementations may change...
BUT, if you select more than one column in the query, then the DISTINCT is essentially different! Because in this case it will compare ALL columns of all rows, instead of just one column.
So if you have something like:
// This will NOT return unique by [id], but unique by (id,name)
SELECT DISTINCT id, name FROM some_query_with_joins
// This will select unique by [id].
SELECT id, name FROM some_query_with_joins GROUP BY id
It is a common mistake to think that DISTINCT keyword distinguishes rows by the first column you specified, but the DISTINCT is a general keyword in this manner.
So people you have to be careful not to take the answers above as correct for all cases... You might get confused and get the wrong results while all you wanted was to optimize!
Go for the simplest and shortest if you can -- DISTINCT seems to be more what you are looking for only because it will give you EXACTLY the answer you need and only that!
well distinct can be slower than group by on some occasions in postgres (dont know about other dbs).
tested example:
postgres=# select count(*) from (select distinct i from g) a;
count
10001
(1 row)
Time: 1563,109 ms
postgres=# select count(*) from (select i from g group by i) a;
count
10001
(1 row)
Time: 594,481 ms
http://www.pgsql.cz/index.php/PostgreSQL_SQL_Tricks_I
so be careful ... :)
Group by is expensive than Distinct since Group by does a sort on the result while distinct avoids it. But if you want to make group by yield the same result as distinct give order by null ..
SELECT DISTINCT u.profession FROM users u
is equal to
SELECT u.profession FROM users u GROUP BY u.profession order by null
It seems that the queries are not exactly the same. At least for MySQL.
Compare:
describe select distinct productname from northwind.products
describe select productname from northwind.products group by productname
The second query gives additionally "Using filesort" in Extra.
In MySQL, "Group By" uses an extra step: filesort. I realize DISTINCT is faster than GROUP BY, and that was a surprise.
After heavy testing we came to the conclusion that GROUP BY is faster
SELECT sql_no_cache
opnamegroep_intern
FROM telwerken
WHERE opnemergroep IN (7,8,9,10,11,12,13) group by opnamegroep_intern
635 totaal 0.0944 seconds
Weergave van records 0 - 29 ( 635 totaal, query duurde 0.0484 sec)
SELECT sql_no_cache
distinct (opnamegroep_intern)
FROM telwerken
WHERE opnemergroep IN (7,8,9,10,11,12,13)
635 totaal 0.2117 seconds ( almost 100% slower )
Weergave van records 0 - 29 ( 635 totaal, query duurde 0.3468 sec)
(more of a functional note)
There are cases when you have to use GROUP BY, for example if you wanted to get the number of employees per employer:
SELECT u.employer, COUNT(u.id) AS "total employees" FROM users u GROUP BY u.employer
In such a scenario DISTINCT u.employer doesn't work right. Perhaps there is a way, but I just do not know it. (If someone knows how to make such a query with DISTINCT please add a note!)
Here is a simple approach which will print the 2 different elapsed time for each query.
DECLARE #t1 DATETIME;
DECLARE #t2 DATETIME;
SET #t1 = GETDATE();
SELECT DISTINCT u.profession FROM users u; --Query with DISTINCT
SET #t2 = GETDATE();
PRINT 'Elapsed time (ms): ' + CAST(DATEDIFF(millisecond, #t1, #t2) AS varchar);
SET #t1 = GETDATE();
SELECT u.profession FROM users u GROUP BY u.profession; --Query with GROUP BY
SET #t2 = GETDATE();
PRINT 'Elapsed time (ms): ' + CAST(DATEDIFF(millisecond, #t1, #t2) AS varchar);
OR try SET STATISTICS TIME (Transact-SQL)
SET STATISTICS TIME ON;
SELECT DISTINCT u.profession FROM users u; --Query with DISTINCT
SELECT u.profession FROM users u GROUP BY u.profession; --Query with GROUP BY
SET STATISTICS TIME OFF;
It simply displays the number of milliseconds required to parse, compile, and execute each statement as below:
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 2 ms.
SELECT DISTINCT will always be the same, or faster, than a GROUP BY. On some systems (i.e. Oracle), it might be optimized to be the same as DISTINCT for most queries. On others (such as SQL Server), it can be considerably faster.
This is not a rule
For each query .... try separately distinct and then group by ... compare the time to complete each query and use the faster ....
In my project sometime I use group by and others distinct
If you don't have to do any group functions (sum, average etc in case you want to add numeric data to the table), use SELECT DISTINCT. I suspect it's faster, but i have nothing to show for it.
In any case, if you're worried about speed, create an index on the column.
If the problem allows it, try with EXISTS, since it's optimized to end as soon as a result is found (And don't buffer any response), so, if you are just trying to normalize data for a WHERE clause like this
SELECT FROM SOMETHING S WHERE S.ID IN ( SELECT DISTINCT DCR.SOMETHING_ID FROM DIFF_CARDINALITY_RELATIONSHIP DCR ) -- to keep same cardinality
A faster response would be:
SELECT FROM SOMETHING S WHERE EXISTS ( SELECT 1 FROM DIFF_CARDINALITY_RELATIONSHIP DCR WHERE DCR.SOMETHING_ID = S.ID )
This isn't always possible but when available you will see a faster response.
in mySQL i have found that GROUP BY will treat NULL as distinct, while DISTINCT does not.
Took the exact same DISTINCT query, removed the DISTINCT, and added the selected fields as the GROUP BY, and i got many more rows due to one of the fields being NULL.
So.. I tend to believe that there is more to the DISTINCT in mySQL.
Related
I'm making a sample recent screen that will display a list, it displays the list, with id set as primary key.
I have done the correct query as expected but the table with big amount of data can cause slow performance issues.
This is the sample query below:
SELECT distinct H.id -- (Primary Key),
H.partnerid as PartnerId,
H.partnername AS partner, H.accountname AS accountName,
H.accountid as AccountNo,
FROM myschema.mytransactionstable H
INNER JOIN (
SELECT S.accountid, S.partnerid, S.accountname,
max(S.transdate) AS maxDate
from myschema.mytransactionstable S
group by S.accountid, S.partnerid, S.accountname
) ms ON H.accountid = ms.accountid
AND H.partnerid = ms.partnerid
AND H.accountname =ms.accountname
AND H.transdate = maxDate
WHERE H.accountid = ms.accountid
AND H.partnerid = ms.partnerid
AND H.accountname = ms.accountname
AND H.transdate = maxDate
GROUP BY H.partnerid,H.accountid, H.accountname
ORDER BY H.id DESC
LIMIT 5
In my case, there are values which are similar in the selected columns but differ only in their id's
Below is a link to an image without executing the query above. They are all the records that have not yet been filtered.
Sample result query click here
Since I only want to get the 5 most recent by their id but the other columns can contain similar values
accountname,accountid,partnerid.
I already got the correct query but,
I want to improve the performance of the query. Any suggestions for the improvement of query?
You can try using row_number()
select * from
(
select *,row_number() over(order by transdate desc) as rn
from myschema.mytransactionstable
)A where rn<=5
Don't repeat ON and WHERE clauses. Use ON to say how the tables (or subqueries) are "related"; use WHERE for filtering (that is, which rows to keep). Probably in your case, all the WHERE should be removed.
Please provide SHOW CREATE TABLE
This 'composite' index would probably help because of dealing with the subquery and the JOIN:
INDEX(partnerid, accountid, accountname, transdate)
That would also avoid a separate sort for the GROUP BY.
But then the ORDER BY is different, so it cannot avoid a sort.
This might avoid the sort without changing the result set ordering: ORDER BY partnerid, accountid, accountname, transdate DESC
Please provide EXPLAIN SELECT ... and EXPLAIN FORMAT=JSON SELECT ... if you have further questions.
If we cannot get an index to handle the WHERE, GROUP BY, and ORDER BY, the query will generate all the rows before seeing the LIMIT 5. If the index does work, then the outer query will stop after 5 -- potentially a big savings.
I am trying to query a large table something like 70 million records. The hard thing is that the group by query based on three fields does not respond. Is there a clever way of doing that? Is use of VID in group by section better rather than verb field?
This is the query I run :
SELECT
VID, s_name, verb, o_name, count(*) as total
FROM
table_name
group by verb, s_name, o_name;
This is a bit long for a comment. Doing aggregations in MySQL on 70 million rows is an expensive operation.
Can you try the following to see if performance is better?
create index idx_table_name_3 on table_name(s_name, verb, o_name);
select distinct s_name, verb, o_name
from table_name;
If this finishes in a finite amount of time, it might be possible to do the aggregation more efficiently.
Then try this:
select s_name, verb, o_name,
(select count(*)
from table_name t2
where t2.s_name = t.s_name and
t2.verb = t.verb and
t2.o_name = t.o_name
) as cnt
from (select distinct s_name, verb, o_name
from table_name
) t;
The subquery should be materialized using the index. The aggregation should be calculated from the index as well. If so, then this will trick MySQL into not using file sort for the group by -- and the performance may improve.
I have table with user transactions.I need to select users who made total transactions more than 100 000 in a single day.Currently what I'm doing is gather all user ids and execute
SELECT sum ( amt ) as amt from users where date = date("Y-m-d") AND user_id=id;
for each id and checking weather the amt > 100k or not.
Since it's a large table, it's taking lot of time to execute.Can some one suggest an optimised query ?
This will do:
SELECT sum ( amt ) as amt, user_id from users
where date = date("Y-m-d")
GROUP BY user_id
HAVING sum ( amt ) > 1; ' not sure what Lakh is
What about filtering the record 1st and then applying sum like below
select SUM(amt),user_id from (
SELECT amt,user_id from users where user_id=id date = date("Y-m-d")
)tmp
group by user_id having sum(amt)>100000
What datatype is amt? If it's anything but a basic integral type (e.g. int, long, number, etc.) you should consider converting it. Decimal types are faster than they used to be, but integral types are faster still.
Consider adding indexes on the date and user_id field, if you haven't already.
You can combine the aggregation and filtering in a single query...
SELECT SUM(Amt) as amt
FROM users
WHERE date=date(...)
AND user_id=id
GROUP BY user_id
HAVING amt > 1
The only optimization that can be done in your query is by applying primary key on user_id column to speed up filtering.
As far as other answers posted which say to apply GROUP BY on filtered records, it won't have any effect as WHERE CLAUSE is executed first in SQL logical query processing phases.
Check here
You could use MySql sub-queries to let MySql handle all the iterations. For example, you could structure your query like this:
select user_data.user_id, user_data.total_amt from
(
select sum(amt) as total_amt, user_id from users where date = date("Y-m-d") AND user_id=id
) as user_data
where user_data.total_amt > 100000;
We can use * to select all attribute from table ,I am using distinct and my table contain 16 columns, How can I use distinct with it.I cannot do select distinct Id,* from abc;
What would be the best way.
Another way could be select distinct id,col1,col2 etc.
If you want in the results, one row per id, you can use GROUP BY id. But then, it's not advisable to use the other columns in the SELECT list (even if MySQL allows it - that depends on whether you have ANSI setting On or Off). It's advisable to use the other columns with aggregate functions like MIN(), MAX(), COUNT(), etc. In MySQL, there is also a GROUP_CONCAT() aggregate function that will collect all values from a column for a group:
SELECT
id
, COUNT(*) AS number_of_rows_with_same_id
, MIN(col1) AS min_col1
, MAX(col1) AS max_col1
--
, GROUP_CONCAT(col1) AS gc_col1
, GROUP_CONCAT(col2) AS gc_col2
--
, GROUP_CONCAT(col16) AS gc_col16
FROM
abc
GROUP BY
id ;
The query:
SELECT *
FROM abc
GROUP BY id ;
is not valid SQL (up to 92) because you have non-aggregated results in the SELECT list and valid in SQL (2003+). Still, it's invalid here because the other columns are not functionally dependent on the grouping column (id). MySQL unfortunately allows such queries and does no checking of functional dependency.
So, you never know which row (of the many with same id) will be returned or even if - horror! - you get results from different rows (with same id). As #Andriy comments, the consequences are that values for columns other than id will be chosen arbitrarily. If you want predictable results, just don't use such a technique.
An example solution: If you want just one row from every id, and you have a datetime or timestamp (or some other) column that you can use for ordering, you can do this:
SELECT t.*
FROM abc AS t
JOIN
( SELECT id
, MIN(some_column) AS m -- or MAX()
FROM abc
GROUP BY id
) AS g
ON g.id = t.id
AND g.m = t.some_column ;
This will work as long as the (id, some_column) combination is unique.
use group by instead of distinct
group by col1, col2,col3
its doing like distinct
SELECT DISTINCT * FROM `some_table`
Is absolutely valid syntax.
The error is caused by the fact that you call Id, *. Well * includes the Id column too, which usually is unique anyway.
So what you'll need in your case is just:
SELECT DISTINCT * FROM `abc`
SELECT * FROM abc where id in(select distinct id from abc);
You can totally do this.
Hope this helps
Initially I thought it would work for group by is best one. This is same as doing select * froom abc. Sorry guys
This sounds quite simple but I just can't figure it out.
I have a table orders (id, username, telephone_number).
I want to get number of orders from one user by comparing the last 8 numbers in telephone_number.
I tried using SUBSTR(telephone_number, -8), I've searched and experimented a lot, but still I can't get it to work.
Any suggestions?
Untested:
SELECT
COUNT(*) AS cnt,
*
FROM
Orders
GROUP BY
SUBSTR(telephone_number, -8)
ORDER BY
cnt DESC
The idea:
Select COUNT(*) (i.e., number of rows in each GROUPing) and all fields from Orders (*)
GROUP by the last eight digits of telephone_number1
Optionally, ORDER by number of rows in GROUPing descending.
1) If you plan to do this type of query often, some kind of index on the last part of the phone number could be desirable. How this could be best implemented depends on the concrete values stored in the field.
//Memory intensive.
SELECT COUNT(*) FROM `orders` WHERE REGEXP `telephone_number` = '(.*?)12345678'
OR
//The same, but better and quicker.
SELECT COUNT(*) FROM `orders` WHERE `telephone_number` LIKE '%12345678'
You can use the below query to get last 8 characters from a column values.
select right(rtrim(First_Name),8) FROM [ated].[dbo].[Employee]