Re: getting rid of "Adding missing FROM-clause entry...."

On Thu, 17 Apr 2003 21:22:17 +0530, Rajesh Kumar Mallah
<mallah(at)trade-india(dot)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.id AND 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 | ...

Servus
Manfred