Oracle takes out the first data in a group ____oracle

OracelAnalysis function: You can remove the first (last) data in the group First_value () over (partition by ...)
Last_value () over (partition by ...) Create TableAtem
(
CO1 VARCHAR2(10),
CO2 INTEGER
) Insert intoAtem (CO1,CO2) Values(' A ', 1);
Insert intoAtem (CO1,CO2) Values(' A ', 2);
Insert intoAtem (CO1,CO2) Values(' A ', 3);
Insert intoAtem (CO1,CO2) Values(' A ', 4);
Insert intoAtem (CO1,CO2) Values(' A ', 1);
Insert intoAtem (CO1,CO2) Values(' B ', 1);
Insert intoAtem (CO1,CO2) Values(' B ', 2);
Insert intoAtem (CO1,CO2) Values(' C ', 1);
Insert intoAtem (CO1,CO2) Values(' d ', 1);
Insert intoAtem (CO1,CO2) Values(' d ', 2); Select distinct(First_value (A.CO2) over ( PARTITION byA.co1 Order byA.CO2)) asCo2,a.co1
fromAtem A//First_value (A.CO2) over ( PARTITION byA.co1 Order by A.CO2)///In the table by A.co1 group, and in the group A.co2 sorted, finally to get the data in the group A.co2