Retrieving the last record in each group - MySQL
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.
How to get the latest record in each group using GROUP BY?
You should find out last timestamp
values in each group (subquery), and then join this subquery to the table -
SELECT t1.* FROM messages t1
JOIN (SELECT from_id, MAX(timestamp) timestamp FROM messages GROUP BY from_id) t2
ON t1.from_id = t2.from_id AND t1.timestamp = t2.timestamp;
Get the latest records per Group By SQL
The rank
window clause allows you to, well, rank rows according to some partitioning, and then you could just select the top ones:
SELECT oDate, oName, oItem, oQty, oRemarks
FROM (SELECT oDate, oName, oItem, oQty, oRemarks,
RANK() OVER (PARTITION BY oName ORDER BY oDate DESC) AS rk
FROM my_table) t
WHERE rk = 1
SELECT latest record group by one column
You can use variables for this:
SELECT location, parameter, datetime, value
FROM (
SELECT location, parameter, datetime, value,
@seq := IF(@loc = location, @seq + 1,
IF(@loc := location, 1, 1)) AS seq
FROM mytable
CROSS JOIN (SELECT @seq := 0, @loc = '') AS vars
ORDER By location, datetime desc, value desc) AS t
WHERE t.seq = 1
The inner query has an ORDER BY
clause that returns the required latest-per-group record first within its own slice. The variable @seq
is set to 1 for this first record using the logic implemented by the IF
functions. The outer query simply filters the derived table to get the expected record for each location
slice.
Demo here
Select row with most recent date per user
Query:
SQLFIDDLEExample
SELECT t1.*
FROM lms_attendance t1
WHERE t1.time = (SELECT MAX(t2.time)
FROM lms_attendance t2
WHERE t2.user = t1.user)
Result:
| ID | USER | TIME | IO |
--------------------------------
| 2 | 9 | 1370931664 | out |
| 3 | 6 | 1370932128 | out |
| 5 | 12 | 1370933037 | in |
Note that if a user has multiple records with the same "maximum" time, the query above will return more than one record. If you only want 1 record per user, use the query below:
SQLFIDDLEExample
SELECT t1.*
FROM lms_attendance t1
WHERE t1.id = (SELECT t2.id
FROM lms_attendance t2
WHERE t2.user = t1.user
ORDER BY t2.id DESC
LIMIT 1)
Retrieving last record in each group from database - SQL Server 2005/2008
;with cteRowNumber as (
select COMPUTERNAME, SERIALNUMBER, USERNAME, LASTIP, LASTUPDATE, SOURCE,
row_number() over(partition by COMPUTERNAME order by LASTUPDATE desc) as RowNum
from YourTable
)
select COMPUTERNAME, SERIALNUMBER, USERNAME, LASTIP, LASTUPDATE, SOURCE
from cteRowNumber
where RowNum = 1
How to select the last record of each ID
You can use a window function called ROW_NUMBER
.Here is a solution for you given below. I have also made a demo query in db-fiddle for you. Please check link Demo Code in DB-Fiddle
WITH CTE AS
(SELECT product, user_id,
ROW_NUMBER() OVER(PARTITION BY user_id order by product desc)
as RN
FROM Mytable)
SELECT product, user_id FROM CTE WHERE RN=1 ;
Related Topics
Dbms_Metadata.Get_Ddl Not Working
Check Constraint of String to Contain Only Digits. (Oracle SQL)
Problem with Execute Procedure in Pl/SQL Developer
How to Prevent Deletion of the First Row in Table (Postgresql)
SQL to Include Condition in Where If Not Null
How to Select Only the Latest Entry in a Table
Multiple Column Values in a Single Row
Inserting and Transforming Data from SQL Table
Ms Access Displaying Vba Select Query in Datasheet
Problems with Createdb in Postgres
Poor Hibernate Select Performance Comparing to Running Directly - How Debug
Comparing Date with Sysdate in Oracle
Transact-Sql: How to Tokenize a String
MySQL - Search Timestamp by Hour of Day
Generating Xml File from SQL Server 2008
SQL Server:Find Duplicates in a Table Based on Values in a Single Column