How can I create a column in postgres from values and selections based on other columns?

Question

I want to create a new field (or two) in my table that is a concatenation of other fields, which seems relatively straightforward. But what is the case syntax or

Answer 1

I recommend a "generated" column rather than storing the data redundantly. It will take less space on disk, which is likely to actually make it faster than storing the generated value, and will certainly be less prone to accidentally falling out of sync with the base data. Assuming you like the format provided by @vyegorov, You could create a function like this, using the record type of your table (which matches the table name) as input:

CREATE FUNCTION adesc(rec atab)
  RETURNS text
  IMMUTABLE
  LANGUAGE SQL
AS $$
SELECT to_char($1.gpa, 'FM09.0') AS gpa_txt,
       $1.name||'-'||$1.major||'-Grade'||
  CASE WHEN $1.gpa >= 3.3 THEN 'A'
       WHEN $1.gpa > 2.7 AND $1.gpa < 3.3 THEN 'B'
       WHEN $1.gpa > 0 THEN 'C'
       ELSE 'F' END || '-' || to_char($1.gpa, 'FM09.0') AS adesc;
$$;

You would need to reference this using a relation qualifier (the table name or an alias). When such a reference is not resolved by finding an actual column, PostgreSQL will look for a function taking the table's record type as its only parameter. So you would be able to do something like this:

SELECT name, major, gpa, atab.adesc
  FROM atab;

Such a "generated column" can be used in indexes for fast searching, if that's what you're after, with something like adesc(atab).*.