MySQL group query before n data

Build table:

CREATE TABLE Hard (
ID INT,
AA varchar (50),
BB INT,
PRIMARY Key (ID)
)
INSERT into hard values (1, ' a ', 9)
INSERT into hard values (2, ' a ', 7)
INSERT into hard values (3, ' a ', 8)
INSERT into hard values (4, ' a ', 6)

INSERT into hard values (5, ' B ', 2)
INSERT into hard values (6, ' B ', 3)
INSERT into hard values (7, ' B ', 4)

INSERT into hard values (8, ' C ', 1)
INSERT into hard values (9, ' C ', 8)
INSERT into hard values (, ' C ', 2)

Query the table with a maximum of two numbers per group:

Select A.* from Hard a Where (select count (*) from hard where aa = A.aa and bb > A.bb) < 2 ORDER BY a.aa,a.bb DESC

Here 2 is n

(Query single table non-grouped top n max data: Select DISTINCT BB from hard ORDER BY bb desc limit 2)

Result diagram:

Principle Analysis:

MySQL group query before n data