How to extract Year and Month in SQL Postgres by using Data_Part function - postgresql

Question

I am facing an issue extracting the month and year from the data (in Character varying type) >> InvoiceDate in SQL Postgres. I have seen the solution is relatively easy with MySQL function: DATEFROMPARTS as per the below Code which is not available in SQLpostgres. How can I get the same result DATA_PART function in Postgres SQL, but simultaneously I need to change the data type of the column "InvoiceDate" to the date
Select
CustomerID,
min(InvoiceDate) first_purchase_date,
DATEFROMPARTS(year(min(InvoiceDate)), month(min(InvoiceDate)), 1) Cohort_Date
into #cohort
from #online_retail_main
group by CustomerID
The output:
Customer ID| first_purchase_date |Cohort_Date|
-----------+-------------------------+-----------+
12345 | 2010-12-20 15:47:00:00 | 2010-12-01|
I am trying to make a date consits of Year and Month , while the day to be set as 1 for all

Answer 1

Assuming a valid Postgres timestamp:
select date_trunc('month', '2010-12-20 15:47:00.00'::timestamp)::date;
date_trunc
------------
12/01/2010
--or ISO format
set datestyle = 'ISO,MDY';
select date_trunc('month', '2010-12-20 15:47:00.00'::timestamp)::date;
date_trunc
------------
2010-12-01
Uses date_trunc to truncate the timestamp to a month which means the first of the month. Then cast(::date) to a date type. The DateStyle just deals with how the value is presented to the user. The value is not stored formatted.
To do something similar to what you did in MySQL:
select make_date(extract(year from '2010-12-20 15:47:00.00'::timestamp)::integer, extract(month from '2010-12-20 15:47:00.00'::timestamp)::integer, 1);
This uses make_date from here Date/time functions and extract to build a date.