Oracle sql not reading the entire string - sql

Question

I have a oracle sql query to get me the dates from 18/05/2021 to '28/05/2021'.
For some reason the the value after slash is not read, as the values from month 4 is also outputed. I dont know where i am wrong. Please give a hand if you are able, thanks a lot for your time.
NOTE: the dates have been stored in the database with a varchar datatype.
SELECT datadate
FROM mytable
WHERE trailerid= '1' and datattime>'05:00:00' and datattime<'12:00:00'
AND datadate between '18/05/2021' and '28/05/2021'
GROUP BY datadate ORDER BY datadate;
Current output
DATADATE
----------------
18/05/2021
19/04/2021
19/05/2021
20/05/2021
21/04/2021
21/05/2021
22/04/2021
22/05/2021
23/04/2021
23/05/2021
24/04/2021
24/05/2021
25/04/2021
25/05/2021
26/04/2021
26/05/2021
27/04/2021
27/05/2021
28/04/2021
28/05/2021

Answer 1

That's what happens when people store date values as strings.
See if this helps:
AND to_date(datadate, 'dd/mm/yyyy') between to_date('18/05/2021', 'dd/mm/yyyy')
and to_date('28/05/2021', 'dd/mm/yyyy')

Answer 2

You don't have to start over, just move the data to a datetime column
ALTER TABLE t ADD x DATE;
UPDATE t SET x = to_date(concat(datadate,datatime), 'dd/mm/yyyyhh24:mi:ss'))
WHERE datadate in (SELECT to_char(to_date('1999-12-31', 'yyyy-mm-dd') + level, 'dd/mm/yyyy') FROM dual CONNECT BY level <= 10000)
That WH ERE clause should generate a list of valid dates from 2000 to about 2030 - if you have dates outside this range adjust accordingly
Should now be able to find invalid dates (they weren't part of the where clause and x should hence remain null) and fix manually:
SELECT * FROM t WHERE x is null
Then drop your datadate and datatime cols and rename x to datadatetime
Now queries like BETWEEN work properly, and if you need just the date part you can do TRUNC(x). (You can even TRUNC to other date parts like hours, to cut the minutes and seconds off, or week of year to round dates down to the start of the week etc)
If you need just the time you either do x - TRUNC(x) which gives a decimal number like 0.5 for 12 noon or 0.75 for 6pm, or you can TOCHAR depending on what you want to do. It would be better to do x -TRUNC(x) BETWEEN 9.0/24.0 AND 17.0/24.0 than doing a string compare