confusion in using select command in postgresql with timestamp column - postgresql

Question

I have table which has structure like this.
CREATE TABLE users (
id serial NOT NULL,
created_at timestamp NOT NULL
)
I have more than 1000 records in this table.
This is my first query.
1 query
select id,created_at from users where id in (1051,1052)
This returns two rows which is correct as as expected. However when I use
2nd Query
select id,created_at from users where created_at = '2020-06-28'
or
select id,created_at from users where created_at = date '2020-06-28'
It returns nothing, this is not expected result as it should return two rows against it.
Similarly if I use this
3rd Query
select id, created_at from users where created_at between date '2020-06-28' and date '2020-06-28'
It returns nothing however I think this should also return two rows.
While this
4th Query
select id, created_at from users where created_at between date '2020-06-28' and date '2020-06-29'
returns two rows.
Show timezone returns correct timezong in which currently i am
I did`t understand this, why the results are different in 2nd, 3rd and 4th query. How can i get same result as of query 1 using 3rd query.

Answer 1

One single reason for all your queries is that you are comparing timestamp with date
in Query 2
You are comparing 2020-06-28 13:02:53 = 2020-06-28 00:00:00 which will not match so returning no records.
in Query 3
You are using between i.e. 2020-06-28 13:02:53 between 2020-06-28 00:00:00 and 2020-06-28 00:00:00 which will not match so returning no records.
in Query 4
You are using between i.e. 2020-06-28 13:02:53 between 2020-06-28 00:00:00 and 2020-06-29 00:00:00. Here both records are falling in those timestamps and you are getting the records
So you have to compare date values. As right operand is a date type value, you have to convert the left operand to date. try this
for 2nd Query
select id,created_at from users where date(created_at) = '2020-06-28'
for 3rd Query
select id, created_at from users where date(created_at) between date '2020-06-28' and date '2020-06-28'
You should opt 3rd method if you want to compare a date range. For one day only you should use 2nd query.

Answer 2

Because what you are doing is:
test(5432)=# select '2020-06-28'::timestamp;
timestamp
---------------------
06/28/2020 00:00:00
You are selecting for created_at that is exactly at midnight and there is none. The same thing when you do:
select id, created_at from users where created_at between date '2020-06-28' and date '2020-06-28'
You already corrected the mistake in your 3rd query in the 4th query:
select id, created_at from users where created_at between date '2020-06-28' and date '2020-06-29'
where you span the time from midnight of 06/28/2020 to midnight 06/29/2020
An alternate solution is:
create table dt_test(id integer, ts_fld timestamp);
insert into dt_test values (1, '07/04/2020 8:00'), (2, '07/05/2020 1:00'), (3, '07/05/2020 8:15');
select * from dt_test ;
id | ts_fld
----+---------------------
1 | 07/04/2020 08:00:00
2 | 07/05/2020 01:00:00
3 | 07/05/2020 08:15:00
select * from dt_test where date_trunc('days', ts_fld) = '07/05/2020'::date;
id | ts_fld
----+---------------------
2 | 07/05/2020 01:00:00
3 | 07/05/2020 08:15:00
In your case:
select id, created_at from users where date_trunc('days', created_at) = '2020-06-28'::date;