MySQL 8.0

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create table sales (CUST int,   Date1 date, Value int);

INSERT INTO sales  VALUES (102,"2019-12-03",7);
INSERT INTO sales  VALUES (101,"2019-12-06",8);
INSERT INTO sales  VALUES (102,"2019-12-06",7);
INSERT INTO sales  VALUES (102,"2019-12-13",10);
INSERT INTO sales  VALUES (102,"2019-12-17",5);
INSERT INTO sales  VALUES (102,"2019-12-18",5);
INSERT INTO sales  VALUES (103,"2019-12-22",5);
INSERT INTO sales  VALUES (102,"2019-12-22",7);

select *
from sales;


CUST
Date1
Value


102
2019-12-03
7

101
2019-12-06
8

102
2019-12-06
7

102
2019-12-13
10

102
2019-12-17
5

102
2019-12-18
5

103
2019-12-22
5

102
2019-12-22
7


select *, avg(value) over(partition by cust) as avg_score from sales






CUST
Date1
Value
avg_score


101
2019-12-06
8
8.0000

102
2019-12-03
7
6.8333

102
2019-12-06
7
6.8333

102
2019-12-13
10
6.8333

102
2019-12-17
5
6.8333

102
2019-12-18
5
6.8333

102
2019-12-22
7
6.8333

103
2019-12-22
5
5.0000

CUST	Date1	Value
102	2019-12-03	7
101	2019-12-06	8
102	2019-12-06	7
102	2019-12-13	10
102	2019-12-17	5
102	2019-12-18	5
103	2019-12-22	5
102	2019-12-22	7

CUST	Date1	Value	avg_score
101	2019-12-06	8	8.0000
102	2019-12-03	7	6.8333
102	2019-12-06	7	6.8333
102	2019-12-13	10	6.8333
102	2019-12-17	5	6.8333
102	2019-12-18	5	6.8333
102	2019-12-22	7	6.8333
103	2019-12-22	5	5.0000