Factor (string) to Numeric in PostgreSQL - sql

Question

Similar to this, is it possible to convert a String field to Numeric in PostgreSQL. For instance,
create table test (name text);
insert into test (name) values ('amy');
insert into test (name) values ('bob');
insert into test (name) values ('bob');
insert into test (name) values ('celia');
and add a field that is
name | num
-------+-----
amy | 1
bob | 2
bob | 2
celia | 3

Answer 1

The most effective "hash"-function of all is a serial primary key - giving you a unique number like you wished for in the question.
I also deal with duplicates in this demo:
CREATE TEMP TABLE string (
string_id serial PRIMARY KEY
,string text NOT NULL UNIQUE -- no dupes
,ct int NOT NULL DEFAULT 1 -- count instead of dupe rows
);
Then you would enter new strings like this:
(Data-modifying CTE requires PostgreSQL 9.1 or later.)
WITH x AS (SELECT 'abc'::text AS nu)
, y AS (
UPDATE string s
SET ct = ct + 1
FROM x
WHERE s.string = x.nu
RETURNING TRUE
)
INSERT INTO string (string)
SELECT nu
FROM x
WHERE NOT EXISTS (SELECT 1 FROM y);
If the string nu already exists, the count (ct) is increased by 1. If not, a new row is inserted, starting with a count of 1.
The UNIQUE also adds an index on the column string.string automatically, which leads to optimal performance for this query.
Add additional logic (triggers ?) for UPDATE / DELETE to make this bullet-proof - if needed.
Note, there is a super-tiny race condition here, if two concurrent transactions try to add the same string at the same moment in time. To be absolutely sure, you could use SERIALIZABLE transactions. More info and links under this this related question.
Live demo at sqlfiddle.

Answer 2

How 'bout a hash, such as md5, of name?
create table test (name text, hash text);
-- later
update test set hash = md5(name);
If you need to convert that md5 text to a number: Hashing a String to a Numeric Value in PostgresSQL

Answer 3

If they are all single characters, you could do this:
ALTER TABLE test ADD COLUMN num int;
UPDATE test SET num = ascii(name);
Though that would only return the character for the first letter if the string was more than a single character.

Answer 4

The exact case shown in your request can be produced with the dense_rank window function:
regress=# SELECT name, dense_rank() OVER (ORDER BY name) FROM test;
name | dense_rank
-------+------------
amy | 1
bob | 2
bob | 2
celia | 3
(4 rows)
so if you were adding a number for each row, you'd be able to do something like:
ALTER TABLE test ADD COLUMN some_num integer;
WITH gen(gen_name, gen_num) AS
(SELECT name, dense_rank() OVER (ORDER BY name) FROM test GROUP BY name)
UPDATE test SET some_num = gen_num FROM gen WHERE name = gen_name;
ALTER TABLE test ALTER COLUMN some_num SET NOT NULL;
however I think it's much more sensible to use a hash or to assign generated keys. I'm just showing that your example can be achieved.
The biggest problem with this approach is that inserting new data is a pain. It's a ranking (like your example shows) so if you INSERT INTO test (name) VALUES ('billy'); then the ranking changes.