Defining a new variable in SELECT clause in SQL developer - sql

Question

I am new to SQL and the extraction of data from databases, so please bear with me. I only have experience with coding in statistical programs, including Stata, SAS, and R.
I currently have a SELECT clause that extracts a table from an Oracle database.
To simplify the question, I make use of a illustrative example:
I am interested in CREATING a new variable, which is not included in the database and must be defined based on the other variables, that contains the weight of their mother. Since I am new to SQL, I do not know if this is possible to do in the SELECT clause or if there exist more efficient options
Note that,
Mother and Mother_number are referring to the "same numbers", meaning that mothers and daughters are represented in the model.
AA (number 1) and CC (number 3) have the same mother (BB) (number 2)
I need to do some conversion of the date, e.g. to_char(a.from_date, 'dd-mm-yyyy') as fromdat since SQL confuses the year with the day-of-the month
The SQL code:
select to_char(a.from_date, 'dd-mm-yyyy') as fromdate, a.Name, a.Weight, a.Number, a.Mother_number
from table1 a, table2 b
where 1=1
and a.family_ref=b.family_ref
and .. (other conditions)
What I currently obtain:
| fromdate | Name | Weight | Number | Mother_number |
|------------|------|--------|--------|---------------|
| 06-07-2021 | AA | 100 | 1 | 2 |
| 06-07-2021 | BB | 200 | 2 | 3 |
| 06-07-2021 | CC | 300 | 3 | 2 |
| 06-07-2021 | DD | 400 | 4 | 5 |
| 06-07-2021 | EE | 500 | 5 | 6 |
| ... | ... | ... | ... | ... |
What I wish to obtain:
| fromdate | Name | Weight | Number | Mother_number | Mother_weight |
|------------|------|--------|--------|---------------|---------------|
| 06-07-2021 | AA | 100 | 1 | 2 | 200 |
| 06-07-2021 | BB | 200 | 2 | 3 | 300 |
| 06-07-2021 | CC | 300 | 3 | 2 | 200 |
| 06-07-2021 | DD | 400 | 4 | 5 | 500 |
| 06-07-2021 | EE | 500 | 5 | 6 | … |
| | … | … | … | … | …

Answer 1

Assuming the MOTHER_NUMBER value is referencing the same value as the NUMBER variable just join the table with itself.
select a.fromdate
, a.name
, a.weight
, a.number
, a.mother_number
, b.weight as mother_weight
from HAVE a
left join HAVE b
on a.mother_number = b.number

Answer 2

Although I'm not sure I'm following the "mother" logic, the way you need to implement the last column in your SELECT statement is to add b.weight as Mother_Weight in the end of the first line, before the for keyword.
Since the b table references "Mothers", you can add the column just by taking the weight of the person in table b.
If instead you wish to add the data of a person's mother's weight, you can do that by adding a column to the relevant table and then updating each row in your table by executing the statements below:
ALTER TABLE table1 ADD Mother_weight FLOAT;
UPDATE table1 SET Mother_weight=(SELECT (Weight) FROM table2 WHERE table1.family_ref=table2.familyref);
Then you add the a.Mother_weight clause in your SELECT statement.

Answer 3

Use a hierarchical query:
SELECT to_char(a.fromdate, 'dd-mm-yyyy') as fromdate,
a.Name,
a.Weight,
a."NUMBER",
a.Mother_number,
PRIOR weight AS mother_weight
FROM table1 a
INNER JOIN table2 b
ON (a.family_ref=b.family_ref)
WHERE LEVEL = 2
OR ( LEVEL = 1
AND NOT EXISTS(
SELECT 1
FROM table1 x
WHERE a.mother_number = x."NUMBER"
)
)
CONNECT BY NOCYCLE
PRIOR "NUMBER" = mother_number
AND PRIOR a.family_ref = a.family_ref
ORDER BY a."NUMBER"
Or, a sub-query factoring clause and a self-join:
WITH data (fromdate, name, weight, "NUMBER", mother_number) AS (
SELECT to_char(a.fromdate, 'dd-mm-yyyy'),
a.Name,
a.Weight,
a."NUMBER",
a.Mother_number
FROM table1 a
INNER JOIN table2 b
ON (a.family_ref=b.family_ref)
)
SELECT d.*,
m.weight AS mother_weight
FROM data d
LEFT OUTER JOIN data m
ON (d.mother_number = m."NUMBER")
ORDER BY d."NUMBER"
Which, for the sample data:
CREATE TABLE table1 (family_ref, fromdate, Name, Weight, "NUMBER", Mother_number) AS
SELECT 1, DATE '2021-07-06', 'AA', 100, 1, 2 FROM DUAL UNION ALL
SELECT 1, DATE '2021-07-06', 'BB', 200, 2, 3 FROM DUAL UNION ALL
SELECT 1, DATE '2021-07-06', 'CC', 300, 3, 2 FROM DUAL UNION ALL
SELECT 1, DATE '2021-07-06', 'DD', 400, 4, 5 FROM DUAL UNION ALL
SELECT 1, DATE '2021-07-06', 'EE', 500, 5, 6 FROM DUAL;
CREATE TABLE table2 (family_ref) AS
SELECT 1 FROM DUAL;
Both output:
FROMDATE
NAME
WEIGHT
NUMBER
MOTHER_NUMBER
MOTHER_WEIGHT
06-07-2021
AA
100
1
2
200
06-07-2021
BB
200
2
3
300
06-07-2021
CC
300
3
2
200
06-07-2021
DD
400
4
5
500
06-07-2021
EE
500
5
6
db<>fiddle here