Find the first n records of each group in SQL

Query the first n articles of a specified category

CREATE TABLE ' person '
(
' Person ' varchar (10),
' Group ' int,
' Age ' int
);

insert INTO ' person ' values
(' Bob ', 1, 32),
(' Jill ', 1, 34),
(' Shawn ', 1, 42),
(' Jake ', 2, 29),
(' Paul ', 2, 36),
(' Laura ', 2, 39);

Query statement
Select A.person, A.group, A.age from Who as a where (SELECT COUNT (*) from person as B where B.group = A.group and B.age >= a.age) <= 2 order by A.group ASC, A.age DESC

SQL: Get the same sortid result set, each ID three data

A table:

SQL code

CREATE TABLE ' book ' (
' BookID ' int (one) unsigned not NULL auto_increment,
' SortID ' int (one) is not NULL,
' BookName ' varchar (255) Not NULL DEFAULT ',
PRIMARY KEY (' BookID ')
) Engine=innodb auto_increment=80729 DEFAULT Charset=utf8;

The data is like this:

xml/html Code
Insert INTO ' book ' (' BookID ', ' sortid ', ' bookname ') VALUES (1, 1, ' 1 '), (2,1, ' 2 '), (3,1, ' 3 '), (4,1, ' 4 '), (5,2, ' 1 '), (6,2, ' 2 ') ), (7,2, ' 3 '), (8,2, ' 4 '), (9,2, ' 5 '), (10,2, ' 6 '), (11,2, ' 7 '), (12,2, ' 8 '), (13,2, ' 9 '), (14,2, ' 10 '), (15,2, ' 11 '), (16,3, ' 1 ') , (17,3, ' 2 '), (18,3, ' 3 '), (19,3, ' 4 '), (20,3, ' 5 '), (21,3, ' 6 '), (22,3, ' 7 '), (23,3, ' 8 '), (24,3, ' 9 '), (25,3, ' 10 '), ("26,3"), 11 '), (27,3, ' 12 '), (28,4, ' 1 '), (29,4, ' 2 '), (30,4, ' 3 '), (31,4, ' 4 '), (32,4, ' 5 '), (33,5, ' 1 '), (34,5, ' 2 '), (35,5, ' 3 '), (() 36,5 , ' 4 ');

In this case, to obtain SortID in (1,2,3) data, the result of each sortid is 3.

This topic is a lot of online, but there is really no special good SQL, there are many kinds of online. At first, I wanted to do it by union, but eventually it was so bad that my colleague clear made the SQL good and tried it, and in the tens of thousands of data, it worked, and the SQL was relatively simple:

SQL code

Select Sortid,bookname from book B where SortID into (1,2) and 3> (select COUNT (1) from the book WHERE B.sortid=sortid and book Id<b.bookid)

Table autocorrelation, indexing on the sortid is a good efficiency

How to select the First/least/max row/group in SQL