Getting timestamp of first day of month - mysql

Question

DB-Fiddle
CREATE TABLE PaL (
id SERIAL PRIMARY KEY,
event_date DATE
);
INSERT INTO PaL
(event_date)
VALUES
('2020-01-01'),
('2020-02-05'),
('2020-03-20'),
('2020-04-15'),
('2020-05-11'),
('2020-06-18'),
('2020-07-19'),
('2020-12-31');
Expected Result:
first_date_of_the_month first_timestamp_of_the_month
2020-01-01 2020-01-01 00:00:00
2020-02-01 2020-02-01 00:00:00
2020-03-01 2020-03-01 00:00:00
2020-04-01 2020-04-01 00:00:00
2020-05-01 2020-05-01 00:00:00
2020-06-01 2020-06-01 00:00:00
2020-07-01 2020-07-01 00:00:00
2020-12-01 2020-12-01 00:00:00
I want to extract the first date and first timestamp of each event_date in the table.
I am doing this with the below query:
SELECT
DATE_ADD(DATE_ADD(LAST_DAY(pl.event_date), INTERVAL 1 DAY), INTERVAL -1 MONTH) AS first_date_of_the_month,
DATE_FORMAT(DATE_ADD(DATE_ADD(LAST_DAY(pl.event_date), INTERVAL 1 DAY), INTERVAL -1 MONTH), '%Y-%m-%d %H:%i:%s') AS first_timestamp_of_the_month
FROM PaL pl
However, HeidiSQL is somehow interpeting the timestamp as value and not as a TIMESTAMP format.
How do I need to change the query so it displays the result as TIMESTAMP?

Answer 1

SELECT *,
DATE_FORMAT(event_date, '%Y-%m-01') AS first_date_of_the_month,
DATE_FORMAT(event_date, '%Y-%m-01 00:00:00') AS first_timestamp_of_the_month
FROM PaL
HeidiSQL is somehow interpeting the timestamp as value and not as a TIMESTAMP format.
This is client problem.
MySQL's datatype system is soft, i.e. each value is converted to needed datatype according to current datatype context implicitly. But you may use correct final context or excplicit final CAST.
https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=21a7d3fccaca1329ef8e1797d366c5a6

Answer 2

You don't have to convert to strings to accomplish this. It is simple enough with basic date functions:
SELECT event_date + interval (1 - day(event_date)) day as first_date_of_the_month,
timestamp(event_date + interval (1 - day(event_date)) day) as first_timestamp_of_the_month
FROM PaL pl;
Here is a db<>fiddle.

Answer 3

Gotta chime in here.... The way I prefer in MySQL, the way that uses the inbuilt LAST_DAY() function, is this:
SELECT LAST_DAY(event_date) + INTERVAL 1 DAY - INTERVAL 1 MONTH
For what it's worth the string constants 2021-03-13 00:00:00 and the shorter 2021-03-13 have precisely the same meaning when used as DATE, DATETIME, or TIMESTAMP values. There's little need for a separate first_timestamp_of_the_month value.
It works in Oracle too. In SQL Server, it's EOMONTH(). postgreSQL, not so much.