Custom Ordering of SELECT Results - sql

Question

I'm working with a Pro*C query, but this question should be general SQL. My research has been a dead end, but I think I'm missing something.
Suppose my server has an array of students' names, {"Alice","Charlie","Bob"}. I query the Student_Ids table for the students' ID numbers:
SELECT id_no FROM student_ids
WHERE student_name IN ('Alice','Charlie','Bob');
To simplify server-side processing, I want to sort the result set in the same order as the students' names. In other words, the result set would be {alice_id_no, charlie_id_no, bob_id_no} regardless of the actual ordering of the table or the behavior of any particular vendor's implementation.
The only solution I can think of is:
. . .
ORDER BY
CASE WHEN student_name='Alice' THEN 0
WHEN student_name='Charlie' THEN 1
WHEN student_name='Bob' THEN 2 END;
but that seems extremely messy and cumbersome when trying to dynamically generate/run this query.
Is there a better way?
UPDATE I gave a terrible example by pre-sorting the students' names. I changed the names to be deliberately unsorted. In other words, I want to sort the names in a non-ASC or DESC-friendly way.
UPDATE II Oracle, but for knowledge's sake, I am looking for more general solutions as well.

Answer 1

The ORDER BY expression you've given for your sample data is equivalent to ORDER BY student_name. Is that what you intended?
If you want a custom ordering that is not alphabetical, I think you might have meant something like this:
ORDER BY
CASE
WHEN student_name = 'Alice' THEN 0
WHEN student_name = 'Charlie' THEN 1
WHEN student_name = 'Bob' THEN 2
END;
You can use a derived table as well, that holds the names as well as the ordering you want. This way you only have to put the names in a single time:
SELECT S.id_no
FROM
student_ids AS S
INNER JOIN (
SELECT Name = 'Alice', Seq = 0 FROM DUAL
UNION ALL SELECT 'Bob', 2 FROM DUAL
UNION ALL SELECT 'Charlie', 1 FROM DUAL
) AS N
ON S.student_name = N.Name
ORDER BY
N.Seq;
You could also put them into a temp table, but in Oracle that could be somewhat of a pain.

Answer 2

Can you do this?
order by student_name
To do a custom sort, you only need one case:
ORDER BY (CASE WHEN student_name = 'Alice' THEN 1
WHEN student_name = 'Bob' THEN 2
WHEN student_name = 'Charlie' THEN 3
ELSE 4
END)

Answer 3

why not this :
SELECT id_no FROM student_ids
WHERE student_name IN ('Alice','Bob','Charlie')
ORDER BY student_name

Answer 4

You can ORDER BY any columns, not necessary those in SELECT list or WHERE clause
SELECT id_no
FROM student_ids
WHERE student_name IN ('Alice','Bob','Charlie)
ORDER BY id_no;

Answer 5

Add a table to hold the sort priorities then you can use the sort_priorities in whatever query you want (and easily update the priorities):
CREATE TABLE student_name_sort_priorities (
student_name VARCHAR2(30) CONSTRAINT student_name_sort_priority__pk PRIMARY KEY,
sort_priority NUMBER(10) CONSTRAINT student_name_sort_priority__nn NOT NULL
CONSTRAINT student_name_sort_priority__u UNIQUE
);
(If you want two values to be equivalently sorted then don't include the UNIQUE constraint.)
INSERT INTO student_name_sort_priorities VALUES ( 'Alice', 0 );
INSERT INTO student_name_sort_priorities VALUES ( 'Charlie', 2 );
INSERT INTO student_name_sort_priorities VALUES ( 'Bob', 1 );
Then you can join the sort priority table with the student_ids table and use the extra column to perform ordering:
SELECT id_no
FROM student_ids s
LEFT OUTER JOIN
student_name_sort_priorities p
ON (s.student_name = p.student_name)
ORDER BY
sort_priority NULLS LAST;
I've used a LEFT OUTER JOIN so that if a name is not contained on the student_name_sort_priorities table then it does not restrict the rows returned from the query; NULLS LAST is used in the ordering for a similar reason - any student names that aren't in the sort priorities table will return a NULL sort priority and be placed at the end of the ordering. If you don't want this then just use INNER JOIN and remove the NULLS LAST.

Answer 6

How about using a 'table of varchar' type like the build-in below:
TYPE dbms_debug_vc2coll is table of varchar2(1000);
test:
SQL> select customer_id, cust_first_name, cust_last_name from customers where cust_first_name in
2 (select column_value from table(sys.dbms_debug_vc2coll('Frederic','Markus','Dieter')));
CUSTOMER_ID CUST_FIRST_NAME CUST_LAST_NAME
----------- -------------------- --------------------
154 Frederic Grodin
149 Markus Rampling
152 Dieter Matthau
That seems to force the order, but that might just be bad luck. I'm not really a sql expert.
The execution plan for this uses 'collection iterator' instead of a big 'or' in the typical:
select customer_id, cust_first_name, cust_last_name from customers where cust_first_name in ('Frederic','Markus','Dieter');
hth, Hein.