date format in oracle/mysql - mysql

Question

Help me to populate below columns as date without using to_date or to_char functions.
day
month
year
1
2
1995
2
3
1998
5
6
2020
output
date
01-02-1995
02-03-1998
05-06-2020

Answer 1

If Oracle, then concatenation of zero left-padded values might do the job (see line #7):
SQL> with test (day, month, year) as
2 (select 1, 2, 1995 from dual union all
3 select 2, 3, 1998 from dual
4 )
5 select day, month, year,
6 --
7 lpad(day, 2, '0') ||'-'|| lpad(month, 2, '0') || '-'|| year as result
8 from test;
DAY MONTH YEAR RESULT
---------- ---------- ---------- ----------------------------------------------
1 2 1995 01-02-1995
2 3 1998 02-03-1998
SQL>

Answer 2

If you want query without to_date and to_char. Let try below query:
SELECT
LPAD(EXTRACT(DAY FROM DATE (year || '-' || month || '-' || day)),2,'0')
|| '-'
|| LPAD(EXTRACT(MONTH FROM DATE (year || '-' || month || '-' || day)),2,'0')
|| '-'
|| EXTRACT(YEAR FROM DATE (year || '-' || month || '-' || day))
FROM YOUR_TABLE;

Answer 3

You should use TO_DATE as that is what it is designed for.
However, as an academic exercise, if you start with DATE '0001-01-01' and then use, for Oracle, ADD_MONTHS (or for MySQL, TIMESTAMPADD) for the years and months and addition for the days:
In Oracle:
SELECT t.*,
TO_DATE(year || '-' || month || '-' || day, 'YYYY-MM-DD') AS dt,
ADD_MONTHS(DATE '0001-01-01', 12 * (year - 1) + (month - 1)) + (day - 1) AS dt2
FROM table_name t
Which, for the sample data:
CREATE TABLE table_name(day, month, year) AS
SELECT 1, 2, 1995 FROM DUAL UNION ALL
SELECT 2, 3, 1998 FROM DUAL UNION ALL
SELECT 5, 6, 2020 FROM DUAL;
Outputs:
DAY
MONTH
YEAR
DT
DT2
1
2
1995
1995-02-01 00:00:00
1995-02-01 00:00:00
2
3
1998
1998-03-02 00:00:00
1998-03-02 00:00:00
5
6
2020
2020-06-05 00:00:00
2020-06-05 00:00:00
fiddle
Or MySQL:
SELECT t.*,
TIMESTAMPADD(MONTH, 12 * (year - 1) + (month - 1), DATE '0001-01-01')
+ (day - 1) AS dt
FROM table_name t
Which outputs:
day
month
year
dt
1
2
1995
19950201
2
3
1998
19980302
5
6
2020
20200605
fiddle