Sort by given "rank" of column values - sql

Question

I have a table like this (unsorted):
risk
category
Low
A
Medium
B
High
C
Medium
A
Low
B
High
A
Low
C
Low
E
Low
D
High
B
I need to sort rows by category, but first based on the value of risk. The desired result should look like this (sorted):
risk
category
Low
A
Low
B
Low
C
Low
D
Low
E
Medium
A
Medium
B
High
A
High
B
High
C
I've come up with below query but wonder if it is correct:
SELECT
*
FROM
some_table
ORDER BY
CASE
WHEN risk = 'Low' THEN
0
WHEN risk = 'Medium' THEN
1
WHEN risk = 'High' THEN
2
ELSE
3
END,
category;
Just want to understand whether the query is correct or not. The actual data set is huge and there are many other values for risk and categories and hence I can't figure out if the results are correct or not. I've just simplified it here.

Answer 1

Basically correct, but you can simplify:
SELECT *
FROM some_table
ORDER BY CASE risk
WHEN 'Low' THEN 0
WHEN 'Medium' THEN 1
WHEN 'High' THEN 2
-- rest defaults to NULL and sorts last
END
, category;
A "switched" CASE is shorter and slightly cheaper.
In the absence of an ELSE branch, all remaining cases default to NULL, and NULL sorts last in default ascending sort order. So you don't need to do anything extra.
Many other values
... there are many other values for risk
While all other values are lumped together at the bottom of the sort order, this seems ok.
If all of those many values get their individual ranking, I would suggest an additional table to handle ranks of risk values. Like:
CREATE TABLE riskrank (
risk text PRIMARY KEY
, riskrank real
);
INSERT INTO riskrank VALUES
('Low' , 0)
, ('Medium', 1)
, ('High' , 2)
-- many more?
;
Data type real, so it's easy to squeeze in rows with fractional digits in different positions (like enum values do it internally).
Then your query is:
SELECT s.*
FROM some_table s
LEFT JOIN risk_rank rr USING (risk)
ORDER BY rr.riskrank, s.category;
LEFT JOIN, so missing entries in riskrank don't eliminate rows.
enum?
I already mentioned the data type enum. That's a possible alternative as enum values are sorted in the order they are defined (not how they are spelled). They only occupy 4 bytes on disk (real internally), are fast and enforce valid values implicitly. See:
How to change the data type of a table column to enum?
However, I would only even consider an enum if the sort order of your values is immutable. Changing sort order and adding / removing allowed values is cumbersome. The manual:
Although enum types are primarily intended for static sets of values,
there is support for adding new values to an existing enum type, and
for renaming values (see ALTER TYPE). Existing values cannot be
removed from an enum type, nor can the sort ordering of such values be
changed, short of dropping and re-creating the enum type.