PLSQL : need help to understand a CASE instructions in a ORDER BY - oracle

Question

I have a piece of code that had a ORDER BY with a CASE in it:
ORDER BY
(
CASE
WHEN r.id BETWEEN 900 AND 999 THEN '1AAAAA'
ELSE '2'
|| upper(id.name)
END) ASC,
r.date DESC ;
Could someone explain:
what is the meaning of the '1AAAAA' and '2' ?
what is the meaning of
|| upper(id.name)

Answer 1

In PL/SQL, || is the concatenation operator.
Exactly how the ordering is happening depends on the rest of the query, but it looks like it's putting records with r.id BETWEEN 900 AND 999 before other records, which are sorted by id.name.

Answer 2

The case expression evaluates to a value
CASE
WHEN r.id BETWEEN 900 AND 999 THEN '1AAAAA'
ELSE '2'
|| upper(id.name)
END
The whole above block of code with either evaluate to '1AAAAA', or '2[value-of-r.id]' depending on the value of r.id.
As this is in the order by clause this value will be used to sort the results as follows:
first list all records where r.id is between 900 and 999
then list all other records in ascending order of r.id (the || is the string concatenation operator).

Answer 3

Here is some data. As you can see the name sorts in ASCII order, which is not exactly the same as alphabetical order:
SQL> select id, name, somedate
2 from t42
3 order by name, somedate
4 /
ID NAME SOMEDATE
---------- ---------- ---------
8 Billington 24-MAR-11
13 Cave 19-MAR-11
4 Clarke 28-MAR-11
919 Feuerstein 13-MAR-11
16 Gasparotto 16-MAR-11
1014 KULASH 18-MAR-11
1 Kestelyn 31-MAR-11
917 Kishore 15-MAR-11
2 Lira 30-MAR-11
6 PADFIELD 26-MAR-11
11 Rigby 21-MAR-11
1007 Robertson 25-MAR-11
12 SCHNEIDER 20-MAR-11
9 SPENCER 23-MAR-11
3 TRICHLER 29-MAR-11
918 VERREYNNE 14-MAR-11
10 boehmer 22-MAR-11
15 hall 17-MAR-11
920 poder 12-MAR-11
5 van wijk 27-MAR-11
1021 11-MAR-11
21 rows selected.
SQL>
Sorting by upper(name) makes it case-insensitive:
SQL> select id, name, somedate
2 from t42
3 order by upper(name), somedate
4 /
ID NAME SOMEDATE
---------- ---------- ---------
8 Billington 24-MAR-11
10 boehmer 22-MAR-11
13 Cave 19-MAR-11
4 Clarke 28-MAR-11
919 Feuerstein 13-MAR-11
16 Gasparotto 16-MAR-11
15 hall 17-MAR-11
1 Kestelyn 31-MAR-11
917 Kishore 15-MAR-11
1014 KULASH 18-MAR-11
2 Lira 30-MAR-11
6 PADFIELD 26-MAR-11
920 poder 12-MAR-11
11 Rigby 21-MAR-11
1007 Robertson 25-MAR-11
12 SCHNEIDER 20-MAR-11
9 SPENCER 23-MAR-11
3 TRICHLER 29-MAR-11
5 van wijk 27-MAR-11
918 VERREYNNE 14-MAR-11
1021 11-MAR-11
21 rows selected.
SQL>
The CASE() changes this further by grouping all the records within the specfied ID range first, then all the other records. The records in the selected range are just sorted by the DATE whereas the other records are still sorted by name then date:
SQL> select id, name, somedate
2 from t42
3 ORDER BY
4 (
5 CASE
6 WHEN id BETWEEN 900 AND 999 THEN '1AAAAA'
7 ELSE '2'
8 || upper(name)
9 END) ASC,
10 somedate DESC
11 /
ID NAME SOMEDATE
---------- ---------- ---------
917 Kishore 15-MAR-11
918 VERREYNNE 14-MAR-11
919 Feuerstein 13-MAR-11
920 poder 12-MAR-11
1021 11-MAR-11
8 Billington 24-MAR-11
10 boehmer 22-MAR-11
13 Cave 19-MAR-11
4 Clarke 28-MAR-11
16 Gasparotto 16-MAR-11
15 hall 17-MAR-11
1 Kestelyn 31-MAR-11
1014 KULASH 18-MAR-11
2 Lira 30-MAR-11
6 PADFIELD 26-MAR-11
11 Rigby 21-MAR-11
1007 Robertson 25-MAR-11
12 SCHNEIDER 20-MAR-11
9 SPENCER 23-MAR-11
3 TRICHLER 29-MAR-11
5 van wijk 27-MAR-11
21 rows selected.
SQL>

Answer 4

1. what is the meaning of the '1AAAAA' and '2' ?
That are literal constants.
2. what is the meaning of || upper(id.name)
|| is the SQL standard concatenation operator. 'A' || 'B' produces 'AB'.
IMHO, your question is what the entire order by case means, so, go step by step:
ORDER BY
(
CASE
WHEN r.id BETWEEN 900 AND 999 THEN '1AAAAA'
ELSE '2'
|| upper(id.name)
END) ASC,
r.date DESC ;
This will order your result set by the result of the case expression evaluation (ascendant), then by r.date (descendant).
The case will just return '1AAAAA' for any ID between 900 and 999 (this will then be ordered by r.date, remember?'
For any other value, it will concatenate 2 before the id.name.
This ensures any record with id between 900 and 999 to appear in the first "group", which is ordered just by date, descending. Then a second group will contain all the other records, ordered by the upper of name, then by the date.
You may want to see this data to understand how this works... just add the case expression to your select statement as a new column.
For example if your query starts like this:
SELECT r.id, id.name
FROM
add the case like this:
SELECT r.id, id.name
,
CASE
WHEN r.id BETWEEN 900 AND 999 THEN '1AAAAA'
ELSE '2'|| upper(id.name)
END ORDER_CRITERIA
FROM
This will help you understand what's going on with that expression, as you will see the produced data as the last column of your query.