Oracle 21c

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create table SALES (CUSTOMER_NAME varchar2(16)
, LAST_SALE_DATE DATE);
insert into sales
select 'JANE01', date '2022-01-01' from dual union all
select 'JANE01', date '2022-01-02' from dual union all
select 'JANE01', date '2022-01-03' from dual union all
select 'JANE02', date '2022-02-01' from dual union all
select 'JANE02', date '2022-02-02' from dual union all
select 'JANE02', date '2022-02-03' from dual union all
select 'JANE03', date '2022-03-01' from dual union all
select 'JANE03', date '2022-03-02' from dual union all
select 'JANE03', date '2022-03-03' from dual
/
9 rows affected

SELECT DISTINCT CUSTOMER_NAME
FROM SALES
;


CUSTOMER_NAME


JANE01

JANE02

JANE03


SELECT DISTINCT CUSTOMER_NAME
FROM SALES
ORDER BY LAST_SALE_DATE DESC;
ORA-01791: not a SELECTed expression

SELECT DISTINCT CUSTOMER_NAME
FROM SALES
ORDER BY LAST_SALE_DATE desc
FETCH FIRST 6 ROWS ONLY ;


CUSTOMER_NAME


JANE03

JANE03

JANE03

JANE02

JANE02

JANE02


SELECT DISTINCT CUSTOMER_NAME from
(SELECT CUSTOMER_NAME
FROM SALES
ORDER BY LAST_SALE_DATE desc
FETCH FIRST 6 ROWS ONLY );


CUSTOMER_NAME


JANE03

JANE02


insert into sales
select 'JANE04', date '2022-04-01' from dual union all
select 'JANE04', date '2022-04-02' from dual union all
select 'JANE04', date '2022-04-03' from dual union all
select 'JANE05', date '2022-05-01' from dual union all
select 'JANE05', date '2022-05-02' from dual union all
select 'JANE05', date '2022-05-03' from dual union all
select 'JANE06', date '2022-06-01' from dual union all
select 'JANE06', date '2022-06-02' from dual union all
select 'JANE06', date '2022-06-03' from dual union all
select 'JANE07', date '2022-07-01' from dual union all
select 'JANE07', date '2022-07-02' from dual union all
select 'JANE07', date '2022-07-03' from dual
/
12 rows affected

SELECT DISTINCT CUSTOMER_NAME
FROM SALES
ORDER BY LAST_SALE_DATE desc
FETCH FIRST 6 ROWS ONLY ;


CUSTOMER_NAME


JANE07

JANE07

JANE07

JANE06

JANE06

JANE06