On Thu, 17 Apr 2003 21:22:17 +0530, Rajesh Kumar Mallah
<mallah@trade-india.com> wrote:
>i think two of your queries will work for me.
I hope so, but ...
>I wrote:
>> UPDATE t_a
>> SET fname2=foo.fname , mname2=foo.mname
>> FROM (SELECT DISTINCT ON (b2.id) b2.id, b2.fname, b2.mname
>> FROM t_b AS b1, t_b AS b2
>> WHERE b1.id = b2.id AND b1.con_id < b2.con_id
>> ORDER BY b2.id, b2.con_id) AS foo
>> WHERE t_a.id = foo.id;
... this can be slow, if you have large groups of equal id, because
each group blows up to n^2/2 rows. You might be better off with a
subselect like
SELECT DISTINCT ON (b2.id) b2.id, b2.fname, b2.mname FROM t_b AS b2 WHERE EXISTS (SELECT * FROM t_b AS
b1 WHERE b1.id = b2.id AND b1.con_id < b2.con_id) ORDER BY b2.id, b2.con_id
or
SELECT b2.id, b2.fname, b2.mname FROM t_b AS b2 WHERE (SELECT count(*) FROM t_b AS b1 WHERE b1.id =
b2.idAND b1.con_id < b2.con_id) = 1
The latter having the advantage of being standard SQL. You have to
experiment a little to find out what works best for you.
BTW, my proposed UPDATE statements don't handle this case very well:
id | con_id | fname | mname
----+--------+-------+------- 1 | 1 | first | ... 1 | 1 | 2nd | ... 1 | 2 | 3rd | ... 1 | 2 |
4th | ...
ServusManfred
