Insert certain weekday within a week if it doesn't exist - mysql

Question

I have a simple table as follows:
DROP TABLE IF EXISTS table_name;
CREATE TABLE table_name
(symbol CHAR(4)
,time DATE
,bid DECIMAL (7,2)
,ask DECIMAL (7,2)
,PRIMARY KEY(symbol,time)
);
insert into table_name
(symbol,`time`,bid,ask)
values
('CN50','2020-09-25',15077.5, 15087.5),
('CN50','2020-09-28',15255 , 15265 ),
('CN50','2020-09-29',15257 , 15267 ),
('CN50','2020-09-30',15258 , 15268 ),
('CN50','2020-10-01',15259 , 15269 );
SELECT *, DATE_FORMAT(time,'%a') dow FROM table_name;
+--------+------------+----------+----------+------+
| symbol | time | bid | ask | dow |
+--------+------------+----------+----------+------+
| CN50 | 2020-09-25 | 15077.50 | 15087.50 | Fri |
| CN50 | 2020-09-28 | 15255.00 | 15265.00 | Mon |
| CN50 | 2020-09-29 | 15257.00 | 15267.00 | Tue |
| CN50 | 2020-09-30 | 15258.00 | 15268.00 | Wed |
| CN50 | 2020-10-01 | 15259.00 | 15269.00 | Thu |
+--------+------------+----------+----------+------+
What I wish to achieve is that if there is no value for Sunday within a week, insert a new row between this Friday and next Monday with Sunday date in time column and copy the bid/ask values from Friday.
In this example, 2020-09-25 is Friday and 2020-09-28 is Monday, I would expect to insert a row with 2020-09-27 as time and keep the bid and ask value from last Friday which is 2020-09-25.
Before:
Symbol
time
bid
ask
CN50
2020-09-25
15077.5
15087.5
CN50
2020-09-28
15255
15265
After:
Symbol
time
bid
ask
CN50
2020-09-25
15077.5
15087.5
CN50
2020-09-27
15077.5
15087.5
CN50
2020-09-28
15255
15265

Answer 1

As mentioned, I would handle this kind of thing in application code, but if you know that Friday's going to be there (and, crucially, that Sunday isn't!) , then something like this would suffice...
SELECT *
FROM table_name
UNION
SELECT symbol
, time + INTERVAL 2 DAY
, bid
, ask
FROM table_name
WHERE DATE_FORMAT(time + INTERVAL 2 DAY,'%a') = 'sun'
ORDER
BY symbol
, time;
+--------+------------+----------+----------+
| symbol | time | bid | ask |
+--------+------------+----------+----------+
| CN50 | 2020-09-25 | 15077.50 | 15087.50 |
| CN50 | 2020-09-27 | 15077.50 | 15087.50 |
| CN50 | 2020-09-28 | 15255.00 | 15265.00 |
| CN50 | 2020-09-29 | 15257.00 | 15267.00 |
| CN50 | 2020-09-30 | 15258.00 | 15268.00 |
| CN50 | 2020-10-01 | 15259.00 | 15269.00 |
+--------+------------+----------+----------+
If there's a chance that Sunday could be there already, then you could left join this result onto the original data set, and use COALESCE to filter the correct value - but by this point you'd probably want switch back to application code or employ the CTE/Windowing tools available in MySQL 8.0+