How to find "holes" in a table - sql

I recently inherited a database on which one of the tables has the primary key composed of encoded values (Part1*1000 + Part2).
I normalized that column, but I cannot change the old values.
So now I have
select ID from table order by ID
ID
100001
100002
101001
...
I want to find the "holes" in the table (more precisely, the first "hole" after 100000) for new rows.
I'm using the following select, but is there a better way to do that?
select /* top 1 */ ID+1 as newID from table
where ID > 100000 and
ID + 1 not in (select ID from table)
order by ID
newID
100003
101029
...
The database is Microsoft SQL Server 2000. I'm ok with using SQL extensions.

select ID +1 From Table t1
where not exists (select * from Table t2 where t1.id +1 = t2.id);
not sure if this version would be faster than the one you mentioned originally.

SELECT (ID+1) FROM table AS t1
LEFT JOIN table as t2
ON t1.ID+1 = t2.ID
WHERE t2.ID IS NULL

This solution should give you the first and last ID values of the "holes" you are seeking. I use this in Firebird 1.5 on a table of 500K records, and although it does take a little while, it gives me what I want.
SELECT l.id + 1 start_id, MIN(fr.id) - 1 stop_id
FROM (table l
LEFT JOIN table r
ON l.id = r.id - 1)
LEFT JOIN table fr
ON l.id < fr.id
WHERE r.id IS NULL AND fr.id IS NOT NULL
GROUP BY l.id, r.id
For example, if your data looks like this:
ID
1001
1002
1005
1006
1007
1009
1011
You would receive this:
start_id stop_id
1003 1004
1008 1008
1010 1010
I wish I could take full credit for this solution, but I found it at Xaprb.

from How do I find a "gap" in running counter with SQL?
select
MIN(ID)
from (
select
100001 ID
union all
select
[YourIdColumn]+1
from
[YourTable]
where
--Filter the rest of your key--
) foo
left join
[YourTable]
on [YourIdColumn]=ID
and --Filter the rest of your key--
where
[YourIdColumn] is null

The best way is building a temp table with all IDs
Than make a left join.
declare #maxId int
select #maxId = max(YOUR_COLUMN_ID) from YOUR_TABLE_HERE
declare #t table (id int)
declare #i int
set #i = 1
while #i <= #maxId
begin
insert into #t values (#i)
set #i = #i +1
end
select t.id
from #t t
left join YOUR_TABLE_HERE x on x.YOUR_COLUMN_ID = t.id
where x.YOUR_COLUMN_ID is null

Have thought about this question recently, and looks like this is the most elegant way to do that:
SELECT TOP(#MaxNumber) ROW_NUMBER() OVER (ORDER BY t1.number)
FROM master..spt_values t1 CROSS JOIN master..spt_values t2
EXCEPT
SELECT Id FROM <your_table>

This solution doesn't give all holes in table, only next free ones + first available max number on table - works if you want to fill in gaps in id-es, + get free id number if you don't have a gap..
select numb + 1 from temp
minus
select numb from temp;

This will give you the complete picture, where 'Bottom' stands for gap start and 'Top' stands for gap end:
select *
from
(
(select <COL>+1 as id, 'Bottom' AS 'Pos' from <TABLENAME> /*where <CONDITION*/>
except
select <COL>, 'Bottom' AS 'Pos' from <TABLENAME> /*where <CONDITION>*/)
union
(select <COL>-1 as id, 'Top' AS 'Pos' from <TABLENAME> /*where <CONDITION>*/
except
select <COL>, 'Top' AS 'Pos' from <TABLENAME> /*where <CONDITION>*/)
) t
order by t.id, t.Pos
Note: First and Last results are WRONG and should not be regarded, but taking them out would make this query a lot more complicated, so this will do for now.

Many of the previous answer are quite good. However they all miss to return the first value of the sequence and/or miss to consider the lower limit 100000. They all returns intermediate holes but not the very first one (100001 if missing).
A full solution to the question is the following one:
select id + 1 as newid from
(select 100000 as id union select id from tbl) t
where (id + 1 not in (select id from tbl)) and
(id >= 100000)
order by id
limit 1;
The number 100000 is to be used if the first number of the sequence is 100001 (as in the original question); otherwise it is to be modified accordingly
"limit 1" is used in order to have just the first available number instead of the full sequence

For people using Oracle, the following can be used:
select a, b from (
select ID + 1 a, max(ID) over (order by ID rows between current row and 1 following) - 1 b from MY_TABLE
) where a <= b order by a desc;

The following SQL code works well with SqLite, but should be used without issues also on MySQL, MS SQL and so on.
On SqLite this takes only 2 seconds on a table with 1 million rows (and about 100 spared missing rows)
WITH holes AS (
SELECT
IIF(c2.id IS NULL,c1.id+1,null) as start,
IIF(c3.id IS NULL,c1.id-1,null) AS stop,
ROW_NUMBER () OVER (
ORDER BY c1.id ASC
) AS rowNum
FROM |mytable| AS c1
LEFT JOIN |mytable| AS c2 ON c1.id+1 = c2.id
LEFT JOIN |mytable| AS c3 ON c1.id-1 = c3.id
WHERE c2.id IS NULL OR c3.id IS NULL
)
SELECT h1.start AS start, h2.stop AS stop FROM holes AS h1
LEFT JOIN holes AS h2 ON h1.rowNum+1 = h2.rowNum
WHERE h1.start IS NOT NULL AND h2.stop IS NOT NULL
UNION ALL
SELECT 1 AS start, h1.stop AS stop FROM holes AS h1
WHERE h1.rowNum = 1 AND h1.stop > 0
ORDER BY h1.start ASC

Related

Filter rows with same column value but IDs are not the biggest [duplicate]

There is a table messages that contains data as shown below:
Id Name Other_Columns
-------------------------
1 A A_data_1
2 A A_data_2
3 A A_data_3
4 B B_data_1
5 B B_data_2
6 C C_data_1
If I run a query select * from messages group by name, I will get the result as:
1 A A_data_1
4 B B_data_1
6 C C_data_1
What query will return the following result?
3 A A_data_3
5 B B_data_2
6 C C_data_1
That is, the last record in each group should be returned.
At present, this is the query that I use:
SELECT
*
FROM (SELECT
*
FROM messages
ORDER BY id DESC) AS x
GROUP BY name
But this looks highly inefficient. Any other ways to achieve the same result?
MySQL 8.0 now supports windowing functions, like almost all popular SQL implementations. With this standard syntax, we can write greatest-n-per-group queries:
WITH ranked_messages AS (
SELECT m.*, ROW_NUMBER() OVER (PARTITION BY name ORDER BY id DESC) AS rn
FROM messages AS m
)
SELECT * FROM ranked_messages WHERE rn = 1;
This and other approaches to finding groupwise maximal rows are illustrated in the MySQL manual.
Below is the original answer I wrote for this question in 2009:
I write the solution this way:
SELECT m1.*
FROM messages m1 LEFT JOIN messages m2
ON (m1.name = m2.name AND m1.id < m2.id)
WHERE m2.id IS NULL;
Regarding performance, one solution or the other can be better, depending on the nature of your data. So you should test both queries and use the one that is better at performance given your database.
For example, I have a copy of the StackOverflow August data dump. I'll use that for benchmarking. There are 1,114,357 rows in the Posts table. This is running on MySQL 5.0.75 on my Macbook Pro 2.40GHz.
I'll write a query to find the most recent post for a given user ID (mine).
First using the technique shown by #Eric with the GROUP BY in a subquery:
SELECT p1.postid
FROM Posts p1
INNER JOIN (SELECT pi.owneruserid, MAX(pi.postid) AS maxpostid
FROM Posts pi GROUP BY pi.owneruserid) p2
ON (p1.postid = p2.maxpostid)
WHERE p1.owneruserid = 20860;
1 row in set (1 min 17.89 sec)
Even the EXPLAIN analysis takes over 16 seconds:
+----+-------------+------------+--------+----------------------------+-------------+---------+--------------+---------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+----------------------------+-------------+---------+--------------+---------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 76756 | |
| 1 | PRIMARY | p1 | eq_ref | PRIMARY,PostId,OwnerUserId | PRIMARY | 8 | p2.maxpostid | 1 | Using where |
| 2 | DERIVED | pi | index | NULL | OwnerUserId | 8 | NULL | 1151268 | Using index |
+----+-------------+------------+--------+----------------------------+-------------+---------+--------------+---------+-------------+
3 rows in set (16.09 sec)
Now produce the same query result using my technique with LEFT JOIN:
SELECT p1.postid
FROM Posts p1 LEFT JOIN posts p2
ON (p1.owneruserid = p2.owneruserid AND p1.postid < p2.postid)
WHERE p2.postid IS NULL AND p1.owneruserid = 20860;
1 row in set (0.28 sec)
The EXPLAIN analysis shows that both tables are able to use their indexes:
+----+-------------+-------+------+----------------------------+-------------+---------+-------+------+--------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+----------------------------+-------------+---------+-------+------+--------------------------------------+
| 1 | SIMPLE | p1 | ref | OwnerUserId | OwnerUserId | 8 | const | 1384 | Using index |
| 1 | SIMPLE | p2 | ref | PRIMARY,PostId,OwnerUserId | OwnerUserId | 8 | const | 1384 | Using where; Using index; Not exists |
+----+-------------+-------+------+----------------------------+-------------+---------+-------+------+--------------------------------------+
2 rows in set (0.00 sec)
Here's the DDL for my Posts table:
CREATE TABLE `posts` (
`PostId` bigint(20) unsigned NOT NULL auto_increment,
`PostTypeId` bigint(20) unsigned NOT NULL,
`AcceptedAnswerId` bigint(20) unsigned default NULL,
`ParentId` bigint(20) unsigned default NULL,
`CreationDate` datetime NOT NULL,
`Score` int(11) NOT NULL default '0',
`ViewCount` int(11) NOT NULL default '0',
`Body` text NOT NULL,
`OwnerUserId` bigint(20) unsigned NOT NULL,
`OwnerDisplayName` varchar(40) default NULL,
`LastEditorUserId` bigint(20) unsigned default NULL,
`LastEditDate` datetime default NULL,
`LastActivityDate` datetime default NULL,
`Title` varchar(250) NOT NULL default '',
`Tags` varchar(150) NOT NULL default '',
`AnswerCount` int(11) NOT NULL default '0',
`CommentCount` int(11) NOT NULL default '0',
`FavoriteCount` int(11) NOT NULL default '0',
`ClosedDate` datetime default NULL,
PRIMARY KEY (`PostId`),
UNIQUE KEY `PostId` (`PostId`),
KEY `PostTypeId` (`PostTypeId`),
KEY `AcceptedAnswerId` (`AcceptedAnswerId`),
KEY `OwnerUserId` (`OwnerUserId`),
KEY `LastEditorUserId` (`LastEditorUserId`),
KEY `ParentId` (`ParentId`),
CONSTRAINT `posts_ibfk_1` FOREIGN KEY (`PostTypeId`) REFERENCES `posttypes` (`PostTypeId`)
) ENGINE=InnoDB;
Note to commenters: If you want another benchmark with a different version of MySQL, a different dataset, or different table design, feel free to do it yourself. I have shown the technique above. Stack Overflow is here to show you how to do software development work, not to do all the work for you.
UPD: 2017-03-31, the version 5.7.5 of MySQL made the ONLY_FULL_GROUP_BY switch enabled by default (hence, non-deterministic GROUP BY queries became disabled). Moreover, they updated the GROUP BY implementation and the solution might not work as expected anymore even with the disabled switch. One needs to check.
Bill Karwin's solution above works fine when item count within groups is rather small, but the performance of the query becomes bad when the groups are rather large, since the solution requires about n*n/2 + n/2 of only IS NULL comparisons.
I made my tests on a InnoDB table of 18684446 rows with 1182 groups. The table contains testresults for functional tests and has the (test_id, request_id) as the primary key. Thus, test_id is a group and I was searching for the last request_id for each test_id.
Bill's solution has already been running for several hours on my dell e4310 and I do not know when it is going to finish even though it operates on a coverage index (hence using index in EXPLAIN).
I have a couple of other solutions that are based on the same ideas:
if the underlying index is BTREE index (which is usually the case), the largest (group_id, item_value) pair is the last value within each group_id, that is the first for each group_id if we walk through the index in descending order;
if we read the values which are covered by an index, the values are read in the order of the index;
each index implicitly contains primary key columns appended to that (that is the primary key is in the coverage index). In solutions below I operate directly on the primary key, in you case, you will just need to add primary key columns in the result.
in many cases it is much cheaper to collect the required row ids in the required order in a subquery and join the result of the subquery on the id. Since for each row in the subquery result MySQL will need a single fetch based on primary key, the subquery will be put first in the join and the rows will be output in the order of the ids in the subquery (if we omit explicit ORDER BY for the join)
3 ways MySQL uses indexes is a great article to understand some details.
Solution 1
This one is incredibly fast, it takes about 0,8 secs on my 18M+ rows:
SELECT test_id, MAX(request_id) AS request_id
FROM testresults
GROUP BY test_id DESC;
If you want to change the order to ASC, put it in a subquery, return the ids only and use that as the subquery to join to the rest of the columns:
SELECT test_id, request_id
FROM (
SELECT test_id, MAX(request_id) AS request_id
FROM testresults
GROUP BY test_id DESC) as ids
ORDER BY test_id;
This one takes about 1,2 secs on my data.
Solution 2
Here is another solution that takes about 19 seconds for my table:
SELECT test_id, request_id
FROM testresults, (SELECT #group:=NULL) as init
WHERE IF(IFNULL(#group, -1)=#group:=test_id, 0, 1)
ORDER BY test_id DESC, request_id DESC
It returns tests in descending order as well. It is much slower since it does a full index scan but it is here to give you an idea how to output N max rows for each group.
The disadvantage of the query is that its result cannot be cached by the query cache.
Use your subquery to return the correct grouping, because you're halfway there.
Try this:
select
a.*
from
messages a
inner join
(select name, max(id) as maxid from messages group by name) as b on
a.id = b.maxid
If it's not id you want the max of:
select
a.*
from
messages a
inner join
(select name, max(other_col) as other_col
from messages group by name) as b on
a.name = b.name
and a.other_col = b.other_col
This way, you avoid correlated subqueries and/or ordering in your subqueries, which tend to be very slow/inefficient.
I arrived at a different solution, which is to get the IDs for the last post within each group, then select from the messages table using the result from the first query as the argument for a WHERE x IN construct:
SELECT id, name, other_columns
FROM messages
WHERE id IN (
SELECT MAX(id)
FROM messages
GROUP BY name
);
I don't know how this performs compared to some of the other solutions, but it worked spectacularly for my table with 3+ million rows. (4 second execution with 1200+ results)
This should work both on MySQL and SQL Server.
Solution by sub query fiddle Link
select * from messages where id in
(select max(id) from messages group by Name)
Solution By join condition fiddle link
select m1.* from messages m1
left outer join messages m2
on ( m1.id<m2.id and m1.name=m2.name )
where m2.id is null
Reason for this post is to give fiddle link only.
Same SQL is already provided in other answers.
An approach with considerable speed is as follows.
SELECT *
FROM messages a
WHERE Id = (SELECT MAX(Id) FROM messages WHERE a.Name = Name)
Result
Id Name Other_Columns
3 A A_data_3
5 B B_data_2
6 C C_data_1
We will look at how you can use MySQL at getting the last record in a Group By of records. For example if you have this result set of posts.
id
category_id
post_title
1
1
Title 1
2
1
Title 2
3
1
Title 3
4
2
Title 4
5
2
Title 5
6
3
Title 6
I want to be able to get the last post in each category which are Title 3, Title 5 and Title 6. To get the posts by the category you will use the MySQL Group By keyboard.
select * from posts group by category_id
But the results we get back from this query is.
id
category_id
post_title
1
1
Title 1
4
2
Title 4
6
3
Title 6
The group by will always return the first record in the group on the result set.
SELECT id, category_id, post_title
FROM posts
WHERE id IN (
SELECT MAX(id)
FROM posts
GROUP BY category_id );
This will return the posts with the highest IDs in each group.
id
category_id
post_title
3
1
Title 3
5
2
Title 5
6
3
Title 6
Reference Click Here
Here are two suggestions. First, if mysql supports ROW_NUMBER(), it's very simple:
WITH Ranked AS (
SELECT Id, Name, OtherColumns,
ROW_NUMBER() OVER (
PARTITION BY Name
ORDER BY Id DESC
) AS rk
FROM messages
)
SELECT Id, Name, OtherColumns
FROM messages
WHERE rk = 1;
I'm assuming by "last" you mean last in Id order. If not, change the ORDER BY clause of the ROW_NUMBER() window accordingly. If ROW_NUMBER() isn't available, this is another solution:
Second, if it doesn't, this is often a good way to proceed:
SELECT
Id, Name, OtherColumns
FROM messages
WHERE NOT EXISTS (
SELECT * FROM messages as M2
WHERE M2.Name = messages.Name
AND M2.Id > messages.Id
)
In other words, select messages where there is no later-Id message with the same Name.
Clearly there are lots of different ways of getting the same results, your question seems to be what is an efficient way of getting the last results in each group in MySQL. If you are working with huge amounts of data and assuming you are using InnoDB with even the latest versions of MySQL (such as 5.7.21 and 8.0.4-rc) then there might not be an efficient way of doing this.
We sometimes need to do this with tables with even more than 60 million rows.
For these examples I will use data with only about 1.5 million rows where the queries would need to find results for all groups in the data. In our actual cases we would often need to return back data from about 2,000 groups (which hypothetically would not require examining very much of the data).
I will use the following tables:
CREATE TABLE temperature(
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
groupID INT UNSIGNED NOT NULL,
recordedTimestamp TIMESTAMP NOT NULL,
recordedValue INT NOT NULL,
INDEX groupIndex(groupID, recordedTimestamp),
PRIMARY KEY (id)
);
CREATE TEMPORARY TABLE selected_group(id INT UNSIGNED NOT NULL, PRIMARY KEY(id));
The temperature table is populated with about 1.5 million random records, and with 100 different groups.
The selected_group is populated with those 100 groups (in our cases this would normally be less than 20% for all of the groups).
As this data is random it means that multiple rows can have the same recordedTimestamps. What we want is to get a list of all of the selected groups in order of groupID with the last recordedTimestamp for each group, and if the same group has more than one matching row like that then the last matching id of those rows.
If hypothetically MySQL had a last() function which returned values from the last row in a special ORDER BY clause then we could simply do:
SELECT
last(t1.id) AS id,
t1.groupID,
last(t1.recordedTimestamp) AS recordedTimestamp,
last(t1.recordedValue) AS recordedValue
FROM selected_group g
INNER JOIN temperature t1 ON t1.groupID = g.id
ORDER BY t1.recordedTimestamp, t1.id
GROUP BY t1.groupID;
which would only need to examine a few 100 rows in this case as it doesn't use any of the normal GROUP BY functions. This would execute in 0 seconds and hence be highly efficient.
Note that normally in MySQL we would see an ORDER BY clause following the GROUP BY clause however this ORDER BY clause is used to determine the ORDER for the last() function, if it was after the GROUP BY then it would be ordering the GROUPS. If no GROUP BY clause is present then the last values will be the same in all of the returned rows.
However MySQL does not have this so let's look at different ideas of what it does have and prove that none of these are efficient.
Example 1
SELECT t1.id, t1.groupID, t1.recordedTimestamp, t1.recordedValue
FROM selected_group g
INNER JOIN temperature t1 ON t1.id = (
SELECT t2.id
FROM temperature t2
WHERE t2.groupID = g.id
ORDER BY t2.recordedTimestamp DESC, t2.id DESC
LIMIT 1
);
This examined 3,009,254 rows and took ~0.859 seconds on 5.7.21 and slightly longer on 8.0.4-rc
Example 2
SELECT t1.id, t1.groupID, t1.recordedTimestamp, t1.recordedValue
FROM temperature t1
INNER JOIN (
SELECT max(t2.id) AS id
FROM temperature t2
INNER JOIN (
SELECT t3.groupID, max(t3.recordedTimestamp) AS recordedTimestamp
FROM selected_group g
INNER JOIN temperature t3 ON t3.groupID = g.id
GROUP BY t3.groupID
) t4 ON t4.groupID = t2.groupID AND t4.recordedTimestamp = t2.recordedTimestamp
GROUP BY t2.groupID
) t5 ON t5.id = t1.id;
This examined 1,505,331 rows and took ~1.25 seconds on 5.7.21 and slightly longer on 8.0.4-rc
Example 3
SELECT t1.id, t1.groupID, t1.recordedTimestamp, t1.recordedValue
FROM temperature t1
WHERE t1.id IN (
SELECT max(t2.id) AS id
FROM temperature t2
INNER JOIN (
SELECT t3.groupID, max(t3.recordedTimestamp) AS recordedTimestamp
FROM selected_group g
INNER JOIN temperature t3 ON t3.groupID = g.id
GROUP BY t3.groupID
) t4 ON t4.groupID = t2.groupID AND t4.recordedTimestamp = t2.recordedTimestamp
GROUP BY t2.groupID
)
ORDER BY t1.groupID;
This examined 3,009,685 rows and took ~1.95 seconds on 5.7.21 and slightly longer on 8.0.4-rc
Example 4
SELECT t1.id, t1.groupID, t1.recordedTimestamp, t1.recordedValue
FROM selected_group g
INNER JOIN temperature t1 ON t1.id = (
SELECT max(t2.id)
FROM temperature t2
WHERE t2.groupID = g.id AND t2.recordedTimestamp = (
SELECT max(t3.recordedTimestamp)
FROM temperature t3
WHERE t3.groupID = g.id
)
);
This examined 6,137,810 rows and took ~2.2 seconds on 5.7.21 and slightly longer on 8.0.4-rc
Example 5
SELECT t1.id, t1.groupID, t1.recordedTimestamp, t1.recordedValue
FROM (
SELECT
t2.id,
t2.groupID,
t2.recordedTimestamp,
t2.recordedValue,
row_number() OVER (
PARTITION BY t2.groupID ORDER BY t2.recordedTimestamp DESC, t2.id DESC
) AS rowNumber
FROM selected_group g
INNER JOIN temperature t2 ON t2.groupID = g.id
) t1 WHERE t1.rowNumber = 1;
This examined 6,017,808 rows and took ~4.2 seconds on 8.0.4-rc
Example 6
SELECT t1.id, t1.groupID, t1.recordedTimestamp, t1.recordedValue
FROM (
SELECT
last_value(t2.id) OVER w AS id,
t2.groupID,
last_value(t2.recordedTimestamp) OVER w AS recordedTimestamp,
last_value(t2.recordedValue) OVER w AS recordedValue
FROM selected_group g
INNER JOIN temperature t2 ON t2.groupID = g.id
WINDOW w AS (
PARTITION BY t2.groupID
ORDER BY t2.recordedTimestamp, t2.id
RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
)
) t1
GROUP BY t1.groupID;
This examined 6,017,908 rows and took ~17.5 seconds on 8.0.4-rc
Example 7
SELECT t1.id, t1.groupID, t1.recordedTimestamp, t1.recordedValue
FROM selected_group g
INNER JOIN temperature t1 ON t1.groupID = g.id
LEFT JOIN temperature t2
ON t2.groupID = g.id
AND (
t2.recordedTimestamp > t1.recordedTimestamp
OR (t2.recordedTimestamp = t1.recordedTimestamp AND t2.id > t1.id)
)
WHERE t2.id IS NULL
ORDER BY t1.groupID;
This one was taking forever so I had to kill it.
Here is another way to get the last related record using GROUP_CONCAT with order by and SUBSTRING_INDEX to pick one of the record from the list
SELECT
`Id`,
`Name`,
SUBSTRING_INDEX(
GROUP_CONCAT(
`Other_Columns`
ORDER BY `Id` DESC
SEPARATOR '||'
),
'||',
1
) Other_Columns
FROM
messages
GROUP BY `Name`
Above query will group the all the Other_Columns that are in same Name group and using ORDER BY id DESC will join all the Other_Columns in a specific group in descending order with the provided separator in my case i have used || ,using SUBSTRING_INDEX over this list will pick the first one
Fiddle Demo
Hi #Vijay Dev if your table messages contains Id which is auto increment primary key then to fetch the latest record basis on the primary key your query should read as below:
SELECT m1.* FROM messages m1 INNER JOIN (SELECT max(Id) as lastmsgId FROM messages GROUP BY Name) m2 ON m1.Id=m2.lastmsgId
I've not yet tested with large DB but I think this could be faster than joining tables:
SELECT *, Max(Id) FROM messages GROUP BY Name
SELECT
column1,
column2
FROM
table_name
WHERE id IN
(SELECT
MAX(id)
FROM
table_name
GROUP BY column1)
ORDER BY column1 ;
You can take view from here as well.
http://sqlfiddle.com/#!9/ef42b/9
FIRST SOLUTION
SELECT d1.ID,Name,City FROM Demo_User d1
INNER JOIN
(SELECT MAX(ID) AS ID FROM Demo_User GROUP By NAME) AS P ON (d1.ID=P.ID);
SECOND SOLUTION
SELECT * FROM (SELECT * FROM Demo_User ORDER BY ID DESC) AS T GROUP BY NAME ;
If you need the most recent or oldest record of a text column in a grouped query, and you would rather not use a subquery, you can do this...
Ex. You have a list of movies and need to get the count in the series and the latest movie
id
series
name
1
Star Wars
A New hope
2
Star Wars
The Empire Strikes Back
3
Star Wars
Return of The Jedi
SELECT COUNT(id), series, SUBSTRING(MAX(CONCAT(id, name)), LENGTH(id) + 1),
FROM Movies
GROUP BY series
This returns...
id
series
name
3
Star Wars
Return of The Jedi
MAX will return the row with the highest value, so by concatenating the id to the name, you now will get the newest record, then just strip off the id for your final result.
More efficient than using a subquery.
So for the given example:
SELECT MAX(Id), Name, SUBSTRING(MAX(CONCAT(Id, Other_Columns)), LENGTH(Id) + 1),
FROM messages
GROUP BY Name
Happy coding, and "May The Force Be With You" :)
Try this:
SELECT jos_categories.title AS name,
joined .catid,
joined .title,
joined .introtext
FROM jos_categories
INNER JOIN (SELECT *
FROM (SELECT `title`,
catid,
`created`,
introtext
FROM `jos_content`
WHERE `sectionid` = 6
ORDER BY `id` DESC) AS yes
GROUP BY `yes`.`catid` DESC
ORDER BY `yes`.`created` DESC) AS joined
ON( joined.catid = jos_categories.id )
Here is my solution:
SELECT
DISTINCT NAME,
MAX(MESSAGES) OVER(PARTITION BY NAME) MESSAGES
FROM MESSAGE;
SELECT * FROM table_name WHERE primary_key IN (SELECT MAX(primary_key) FROM table_name GROUP BY column_name )
**
Hi, this query might help :
**
SELECT
*
FROM
message
WHERE
`Id` IN (
SELECT
MAX(`Id`)
FROM
message
GROUP BY
`Name`
)
ORDER BY
`Id` DESC
i find best solution in https://dzone.com/articles/get-last-record-in-each-mysql-group
select * from `data` where `id` in (select max(`id`) from `data` group by `name_id`)
The below query will work fine as per your question.
SELECT M1.*
FROM MESSAGES M1,
(
SELECT SUBSTR(Others_data,1,2),MAX(Others_data) AS Max_Others_data
FROM MESSAGES
GROUP BY 1
) M2
WHERE M1.Others_data = M2.Max_Others_data
ORDER BY Others_data;
If you want the last row for each Name, then you can give a row number to each row group by the Name and order by Id in descending order.
QUERY
SELECT t1.Id,
t1.Name,
t1.Other_Columns
FROM
(
SELECT Id,
Name,
Other_Columns,
(
CASE Name WHEN #curA
THEN #curRow := #curRow + 1
ELSE #curRow := 1 AND #curA := Name END
) + 1 AS rn
FROM messages t,
(SELECT #curRow := 0, #curA := '') r
ORDER BY Name,Id DESC
)t1
WHERE t1.rn = 1
ORDER BY t1.Id;
SQL Fiddle
If performance is really your concern you can introduce a new column on the table called IsLastInGroup of type BIT.
Set it to true on the columns which are last and maintain it with every row insert/update/delete. Writes will be slower, but you'll benefit on reads. It depends on your use case and I recommend it only if you're read-focused.
So your query will look like:
SELECT * FROM Messages WHERE IsLastInGroup = 1
MariaDB 10.3 and newer using GROUP_CONCAT.
The idea is to use ORDER BY + LIMIT:
SELECT GROUP_CONCAT(id ORDER BY id DESC LIMIT 1) AS id,
name,
GROUP_CONCAT(Other_columns ORDER BY id DESC LIMIT 1) AS Other_columns
FROM t
GROUP BY name;
db<>fiddle demo
How about this:
SELECT DISTINCT ON (name) *
FROM messages
ORDER BY name, id DESC;
I had similar issue (on postgresql tough) and on a 1M records table. This solution takes 1.7s vs 44s produced by the one with LEFT JOIN.
In my case I had to filter the corrispondant of your name field against NULL values, resulting in even better performances by 0.2 secs
Yet another option without subqueries.
This solution uses MySQL LAST_VALUE window function, exploiting Window Function Frame available MySQL tool from .
SELECT DISTINCT
LAST_VALUE(Id)
OVER(PARTITION BY Name
ORDER BY Id
ROWS BETWEEN 0 PRECEDING
AND UNBOUNDED FOLLOWING),
Name,
LAST_VALUE(Other_Columns)
OVER(PARTITION BY Name
ORDER BY Id
ROWS BETWEEN 0 PRECEDING
AND UNBOUNDED FOLLOWING)
FROM
tab
Try it here.
Hope below Oracle query can help:
WITH Temp_table AS
(
Select id, name, othercolumns, ROW_NUMBER() over (PARTITION BY name ORDER BY ID
desc)as rank from messages
)
Select id, name,othercolumns from Temp_table where rank=1
Another approach :
Find the propertie with the max m2_price withing each program (n properties in 1 program) :
select * from properties p
join (
select max(m2_price) as max_price
from properties
group by program_id
) p2 on (p.program_id = p2.program_id)
having p.m2_price = max_price
What about:
select *, max(id) from messages group by name
I have tested it on sqlite and it returns all columns and max id value for all names.
As of MySQL 8.0.14, this can also be achieved using Lateral Derived Tables:
SELECT t.*
FROM messages t
JOIN LATERAL (
SELECT name, MAX(id) AS id
FROM messages t1
WHERE t.name = t1.name
GROUP BY name
) trn ON t.name = trn.name AND t.id = trn.id
db<>fiddle

SQL - for each entry in a table - check for associated row

I have a log table which logs a start row, and a finish row for a particular event.
Each event should have a start row, and if everything goes ok it should have an end row.
But if something goes wrong then the end row may not be created.
I want to SELECT everything in the table that has a start row but not an associated end row.
For example, consider the table like this:
id event_id event_status
1 123 1
2 123 2
3 234 1
4 234 2
5 456 1
6 678 1
7 678 2
Notice that the id column 5 has a start row but no end row. Start is an event_status of 1, end is an event_status of 2.
How can i pull back all the event_ids which have a start row but not an end row>?
This is for mssql.
You could use a not exists subquery to demand that no other row exists that ends the event:
select *
from YourTable t1
where status = 1
and not exists
(
select *
from YourTable t2
where t2.event_id = t1.event_id
and t2.status = 2
)
You can try with left self join as below:
select y1.event_id from #yourevents y1 left join #yourevents y2
on y1.event_id = y2.event_id
and y1.event_status = 1
and y2.event_status = 2
where y2.event_id is null
and y1.event_status = 1
In this particular case you could use one of 3 solutions:
Solution 1. The classic
Check if there is no end status
SELECT *
FROM myTable t1
WHERE NOT EXISTS (
SELECT *
FROM myTable t2
WHERE t1.event_id = t2.event_id AND t2.status=2
)
Solution 2. Make it pretty. Don't do subqueries with so many parentheses
The same check, but in a more concise and pretty manner
SELECT t1.*
FROM myTable t1
LEFT JOIN myTable t2 ON t1.event_id = t2.event_id AND t2.status=2
-- Doesn't exist
WHERE t2.event_id IS NULL
Solution 3. Look for the last status for each event
More flexibility in case the status logic becomes more complicated
WITH last_status AS (
SELECT
id,
event_id,
status,
-- The ROWS BETWEEN ..yadda yadda ... FOLLOWING might be unnecessary. Try, check.
last_value(status) OVER (PARTITION BY event_id ORDER BY status ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS last_status
FROM myTable
)
SELECT
id,
event_id,
status
FROM last_events
WHERE last_status<>2
There are more, with min/max queries and others. Pick what best suits your need for cleanliness, readability and versatility.

Generate SQL rows

Given a number of types and a number of occurrences per type, I would like to generate something like this in T-SQL:
Occurrence | Type
-----------------
0 | A
1 | A
0 | B
1 | B
2 | B
Both the number of types and the number of occurrences per type are presented as values in different tables.
While I can do this with WHILE loops, I'm looking for a better solution.
Thanks!
This works with a number-table which i would use.
SELECT Occurrence = ROW_NUMBER() OVER (PARTITION BY Type ORDER BY Type) - 1
, Type
FROM Numbers num
INNER JOIN #temp1 t
ON num.n BETWEEN 1 AND t.Occurrence
Tested with this sample data:
create table #temp1(Type varchar(10),Occurrence int)
insert into #temp1 VALUES('A',2)
insert into #temp1 VALUES('B',3)
How to create a number-table? http://sqlperformance.com/2013/01/t-sql-queries/generate-a-set-1
If you have a table with the columns type and num, you have two approaches. One way is to use recursive CTEs:
with CTE as (
select type, 0 as occurrence, num
from table t
union all
select type, 1 + occurrence, num
from cte
where occurrence + 1 < num
)
select cte.*
from cte;
You may have to set the MAXRECURSION option, if the number exceeds 100.
The other way is to join in a numbers table. SQL Server uses spt_values for this purpose:
select s.number - 1 as occurrence, t.type
from table t join
spt_values s
on s.number <= t.num ;

How to SELECT top N rows that sum to a certain amount?

Suppose:
MyTable
--
Amount
1
2
3
4
5
MyTable only has one column, Amount, with 5 rows. They are not necessarily in increasing order.
How can I create a function, which takes a #SUM INT, and returns the TOP N rows that sum to this amount?
So for input 6, I want
Amount
1
2
3
Since 1 + 2 + 3 = 6. 2 + 4 / 1 + 5 won't work since I want TOP N ROWS
For 7/8/9/10, I want
Amount
1
2
3
4
I'm using MS SQL Server 2008 R2, if this matters.
Saying "top N rows" is indeed ambiguous when it comes to relational databases.
I assume that you want to order by "amount" ascending.
I would add a second column (to a table or view) like "sum_up_to_here", and create something like that:
create view mytable_view as
select
mt1.amount,
sum(mt2.amount) as sum_up_to_here
from
mytable mt1
left join mytable mt2 on (mt2.amount < mt1.amount)
group by mt1.amount
or:
create view mytable_view as
select
mt1.amount,
(select sum(amount) from mytable where amount < mt1.amount)
from mytable mt1
and then I would select the final rows:
select amount from mytable_view where sum_up_to_here < (some value)
If you don't bother about performance you may of course run it in one query:
select amount from
(
select
mt1.amount,
sum(mt2.amount) as sum_up_to_here
from
mytable mt1
left join mytable mt2 on (mt2.amount < mt1.amount)
group by mt1.amount
) t where sum_up_to_here < 20
One approach:
select t1.amount
from MyTable t1
left join MyTable t2 on t1.amount > t2.amount
group by t1.amount
having coalesce(sum(t2.amount),0) < 7
SQLFiddle here.
In Sql Server you can use CDEs to make it pretty simple to read.
Here is a CDE I did to sum up totals used in sequence. The CDE is similar to the joins above, and holds the total up to any given index. Outside of the CDE I join it back to the original table so I can select it along with other fields.
;with summrp as (
select m1.idx, sum(m2.QtyReq) as sumUsed
from #mrpe m1
join #mrpe m2 on m2.idx <= m1.idx
group by m1.idx
)
select RefNum, RefLineSuf, QtyReq, ProjectedDate, sumUsed from #mrpe m
join summrp on summrp.idx=m.idx
In SQL Server 2012 you can use this shortcut to get a result like Grzegorz's.
SELECT amount
FROM (
SELECT * ,
SUM(amount) OVER (ORDER BY amount ASC) AS total
from demo
) T
WHERE total <= 6
A fiddle in the hand... http://sqlfiddle.com/#!6/b8506/6

How to find missing numbers between 2 columns?

I am looking for a way to find missing numbers within a range. I have a beginning number column and a ending number column in the same table.
I am trying to get the skipped numbers. I can get the next skipped number, but don't know how to get a list of the numbers that were not in the range. I have a numbers table if that would be useful.
Here is my example:
doc_num_begin doc_num_end
------------- -----------
20000007 20000008
20000011 20000015
20000016 20000017
I'd like to get 20000009,20000010. I have searched but not able to find out how to do this using beginning and ending columns.
Thanks
If you have a numbers table, then this is pretty easy:
select n.num
from Numbers n left outer join
RangeTable rt
on n.number between rt.doc_num_begin and doc_num_end
where rt.doc_num_begin is null
This is doing a left outer join from the numbers to the range table, and then keeping the ones that don't match.
Although pretty easy to express, the performance will probably be rather poor due to the non-equijoin. You may also want to put in conditions on the numbers table, so you don't start at 0, 1, . . ., when the ranges start at 20000007. You would do this as:
select n.num
from Numbers n join
(select MIN(doc_num_begin) as MinVal, MAX(doc_num_end) as MaxVal from RangeTable) const
on n.number between const.MinVal and const.MaxVal left outer join
RangeTable rt
on n.number between rt.doc_num_begin and doc_num_end
where rt.doc_num_begin is null
If you just have to find the missing ranges, you could use this query:
SELECT
t1.doc_num_end + 1 as start_missing_range,
MIN(t2.doc_num_begin) - 1 as end_missing_range
FROM
your_table t1 INNER JOIN your_table t2
ON t1.doc_num_end < t2.doc_num_begin
GROUP BY
t1.doc_num_end
HAVING
MIN(t2.doc_num_begin) - t1.doc_num_end > 1
EDIT: And this query could be used to expand a range:
SELECT num+start_missing_range
FROM
(select 0 as num
union all select 1 as num
union all select 2 as num
union all select 3 as num
union all select 4 as num
union all select 5 as num
union all select 6 as num
union all select 7 as num
union all select 8 as num
union all select 9 as num) numbers inner join
(SELECT
t1.doc_num_end + 1 as start_missing_range,
MIN(t2.doc_num_begin) - 1 as end_missing_range
FROM
your_table t1 INNER JOIN your_table t2
ON t1.doc_num_end < t2.doc_num_begin
GROUP BY
t1.doc_num_end
HAVING
MIN(t2.doc_num_begin) - t1.doc_num_end > 1) rg
on end_missing_range-start_missing_range>=numbers.num
(it will work only if a range contains at maximum 10 numbers, it could be easily expanded to some more... of course, there will always be a limit, but at least you don't need a table with all of the numbers)
You can use known sequence number in any table or sample database for this purpose to filter with this id.
Cross Joining this id, would extend the limit you seek.
SELECT i from (select (w2.WorkOrderID-1)+(w1.WorkOrderID-1)*10000 as i
from AdventureWorks.Production.WorkOrder w1
cross join AdventureWorks.Production.WorkOrder w2
where w1.WorkOrderID<10000 and w2.WorkOrderID<10000) as MyNumbers
WHERE i BETWEEN #StartRange and #EndRange
and not exists (SELECT 1 FROM MyTable
WHERE i BETWEEN doc_num_begin doc_num_end)