Can I index into an array literal in a SELECT statement? - arrays

Question

I am reading from a PostgreSQL database (that I do not control) that includes an integer column that acts like an enum, but the enum values are not in the database.
This is not my actual data, but consider an example students table:
id | name | class
==================
1 | Adam | 1
2 | Bruce | 1
3 | Chris | 3
4 | Dave | 4
When SELECTing from this table, it is very common to convert the class column to something more humane:
SELECT
id,
name,
CASE class
WHEN 1 THEN 'Freshman'
WHEN 2 THEN 'Sophomore'
WHEN 3 THEN 'Junior'
WHEN 4 THEN 'Senior'
ELSE 'Unknown'
END
FROM students
Is there a better way to write this? I tried constructing a literal array and using the column to index into it, but if that is possible, I have not figured out the syntax yet.
These don't work:
SELECT {"fr", "so", "ju", "se"}[class] FROM students
SELECT '{"fr", "so", "ju", "se"}'[class] FROM students

Answer 1

You can do this using the ARRAY keyword to construct the array:
SELECT (ARRAY['fr', 'so', 'ju', 'se'])[class] FROM students
In PostgreSQL, array subscripts begin with 1, not 0, so if the enum-like column uses 0 as one of its values, you may need to shift it to get what you want. For example, if the values for class were 0, 1, 2, and 3, you can add 1 to class:
SELECT (ARRAY['fr', 'so', 'ju', 'se'])[class + 1] FROM students

Answer 2

Make it a CTE and do the usual join:
with class (class, class_name) as ( values
(1, 'Freshman'),(2,'Sophomore'),(3,'Junior'),(4,'Senior')
), student (id, name, class) as (values
(1, 'Adam',1),(2,'Bruce',1),(3,'Chris',3),(4,'Dave',4)
)
select id, name, class_name
from
student
inner join
class using (class)
;
id | name | class_name
----+-------+------------
1 | Adam | Freshman
2 | Bruce | Freshman
3 | Chris | Junior
4 | Dave | Senior
https://www.postgresql.org/docs/current/static/sql-select.html#SQL-WITH
The WITH clause allows you to specify one or more subqueries that can be referenced by name in the primary query. The subqueries effectively act as temporary tables or views for the duration of the primary query.