Does PostgreSQL have an equivalent to SAS' obsnum? - postgresql

Question

When converting scripts, tables, datasets, etc. from a SAS environment to a PostgreSQL environment, is there an equivalent to referencing SAS' obsnum in PostgreSQL? For example, if a query says:
SELECT FROM schema.table
WHERE obsnum = 1
Is there a way to track the observation number or similar in PostgreSQL? Or should a different approach be taken?
Thanks.
Should probably specify that I was told obsnum is a built-in SAS value associated with datasets and tables, and in my SAS scripts there is no declaration for obsnum, only a singular reference to it in a SELECT statement.

Answer 1

OBSNUM isn't an automatic variable in SAS so its a variable value.
You should be able to use a similar query in Postgres to limit where the variable value is 1.
To add on - PROC SQL doesn't have an automatic variable to do numbering, it can use monotonic() for row numbers but it's not supported.
(wrong answer):
Try CTID
See your previous question, the first answer and the comments/questions.
What exactly is the/this data statement in SAS doing? PostgreSQL equivalent?
PostgresSQL
http://www.postgresql.org/docs/8.2/static/ddl-system-columns.html

Answer 2

-- make(fake) a dataset
CREATE TABLE dataset
( val double precision NOT NULL
);
-- populate it with random
INSERT INTO dataset(val)
SELECT random()
FROM generate_series(1,100)
;
-- Use row_number() to enumerate the unordered tuples
-- note the subquery. It is needed because otherwise
-- you cannot refer to row_number
SELECT * FROM (
SELECT val
, row_number() OVER() AS obsnum
FROM dataset
) qq -- subquery MUST have an alias
WHERE qq.obsnum = 1
;
-- you can just as well order over ctid (the result is the same)
SELECT * FROM (
SELECT val
, row_number() OVER(ORDER BY ctid) AS obsnum
FROM dataset
) qq -- subquery MUST have an alias
WHERE qq.obsnum = 1
;
-- In stead of a subquery you could use
-- a CTE to wrap the enumeration part
WITH zzz AS (
SELECT val
, row_number() OVER(ORDER BY ctid) AS obsnum
FROM dataset
)
SELECT * FROM zzz
WHERE obsnum = 1
;
-- Or, if you just want one observation: use LIMIT
-- (order of records is not defined,
-- but the order of ctid is not stable either)
SELECT * FROM dataset
LIMIT 1
;