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 if/when syntax I'd use to help create the following fields (GPA_TXT, and newfield)?

The logic is: Each GPA should be #.#, each new field should be:

name & "-" & GPA_TXT & (
    case where GPA_TXT > 3.3
        set newfield = newfield & 'GradeA',
    case where GPA_TXT >2.7 and GPA_TXT < 3.3
        set newfield = newfield & "GradeB",
    etc...
)

For example:

name         major     GPA(num) GPA_TXT   [newfield]
Bob          sci       2        02.0      Bob-sci-GradeC-02.0
Jane         chem      3.1      03.1      Jane-chem-GradeB-03.1
Charlie      phys      3.7      03.7      Charlie-phys-GradeA-03.7
Garfield     food      0        00.0      Garfield-food-GradeF-00.0

So I guess I have two questions in here:

1. How to create the GPA TXT field.
2. How to write a case statement to calculate a field according to the values in other fields.

If anyone can link me to a resource with examples or explain I would greatly appreciate it! I'm looking through the documentation but not getting anywhere without examples.

Answer 1

Important note: I would create a view based on your current table and avoided adding new columns, as they will denormalize your schema. Read more here.

Also, I will use lowercase names for all the identifiers to avoid qouting.

• to form GPA_TXT field you can use to_char() function: to_char(gpa, 'FM09.0') (the FM will avoid space in front of the resulting string);

• for the second field, I would use GPA and not GPA_TXT for numeric comparison. You can check more on CASE construct in the docs, but the block might be the following one:

  CASE WHEN gpa >= 3.3 THEN 'A'
       WHEN gpa > 2.7 AND gpa < 3.3 THEN 'B'
       WHEN gpa > 0 THEN 'C'
       ELSE 'F' END
  

Sorry, I don't know how grades are assigned per GPA, please, adjust accordingly.

The resulting query for the view might be (also on SQL Fiddle):

SELECT name,major,gpa,
       to_char(gpa, 'FM09.0') AS gpa_txt,
       name||'-'||major||'-Grade'||
  CASE WHEN gpa >= 3.3 THEN 'A'
       WHEN gpa > 2.7 AND gpa < 3.3 THEN 'B'
       WHEN gpa > 0 THEN 'C'
       ELSE 'F' END || '-' || to_char(gpa, 'FM09.0') AS adesc
  FROM atab;

To build a view just prepend CREATE VIEW aview AS before this query.

---

EDIT

If you still go for adding columns, the following should do the trick:

ALTER TABLE atab ADD gpa_txt text, ADD adesc text;
UPDATE atab SET
    gpa_txt = to_char(gpa, 'FM09.0'),
    adesc = name||'-'||major||'-Grade'||
      CASE WHEN gpa >= 3.3 THEN 'A'
           WHEN gpa > 2.7 AND gpa < 3.3 THEN 'B'
           WHEN gpa > 0 THEN 'C'
           ELSE 'F' END || '-' || to_char(gpa, 'FM09.0');