not so sequential sequences

rhys.stewart@gmail.com ("Rhys Stewart") writes:

Hi all, looking for a method to number a table sequentially, but the
sequence only increments if the value in a certain column is
different. as in

seq | parish
------------------------
1 | Kingston
1 | Kingston
1 | Kingston
1 | Kingston
2 | Lucea
3 | Morant Bay
3 | Morant Bay
3 | Morant Bay
4 | Port Antonio
5 | Savannah-La-Mar
5 | Savannah-La-Mar
5 | Savannah-La-Mar

so i guess i would order by a certain column and then the 'magic
sequence' would be a column that only increments when the column
changes.

The usual way to do this would be to separate out what clearly is a
dependant table; the seq and parish values could be associated with a
smaller table that increments any time you get a new parish.

The process of doing this sort of separation is called "normalization."
<http://en.wikipedia.org/wiki/Database_normalization&gt;
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On Oct 17, 2006, at 23:18 , Rhys Stewart wrote:

Hi all, looking for a method to number a table sequentially, but the
sequence only increments if the value in a certain column is
different. as in

[snip]

Normalization could solve your problem and also improve your schema:

-- The parish table
create table parishes (
id serial primary key,
name varchar
);

-- Your main table
create table ... (
...
parish_id integer references parishes (id)
);

Now you have the parishes table:

id | name
------------------------
1 | Kingston
2 | Lucea
3 | Morant Bay
4 | Port Antonio
5 | Savannah-La-Mar

And your main table:

... | parish_id
------------------------
... | 1
... | 1
... | 1
... | 1
... | 2
... | 3
... | 3
... | 3
... | 4
... | 5
... | 5
... | 5

As you can see, the parish_id field is now your "magic sequence".

so i guess i would order by a certain column and then the 'magic
sequence' would be a column that only increments when the column
changes.

I'm assuming (perhaps wrongly) here that you don't care about a
strictly sequential number; for example, a field indicating the
position of something in a queue or list. I'm also assuming that the
number has no semantic meaning, which might not fit your use case.

Alexander.

Rhys,
You could create a sequence, then make the "seq" attribute to your table
have a default value of:
seq integer default nextval('your_sequence')

Then every time an insert is done into your table, the "seq" will
increment. You alternatively could make your "insert" statement have
for that position, the nextval('your_sequence')

Does that help?

Rhys Stewart wrote:

Hi all, looking for a method to number a table sequentially, but the
sequence only increments if the value in a certain column is
different. as in

seq | parish
------------------------
1 | Kingston
1 | Kingston
1 | Kingston
1 | Kingston
2 | Lucea
3 | Morant Bay
3 | Morant Bay
3 | Morant Bay
4 | Port Antonio
5 | Savannah-La-Mar
5 | Savannah-La-Mar
5 | Savannah-La-Mar

so i guess i would order by a certain column and then the 'magic
sequence' would be a column that only increments when the column
changes.

Rhys

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