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

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.

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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');