从PostgreSQL查询中按条件获取行

Question

I have the following query select * from func1(ID)

If I do: select * from func1(10)

I get:

rownum  date    qty
1       1.1.10  -5
2       1.10.10  6
3       2.10.10  6
4       5.10.10  -2
5       6.10.10  -8

If I do select * from func1(7) I get

rownum  date qty
1       1.1.10  -7
2       1.10.10  -6

If I do select * from func1(6) I get

rownum  date qty

rownum is the order of the column its being calculated in func1() based on my needs (it's not random numbering) you assume that the numbering is correct.

I want to write a query which returns me that find the first appears of qty>=0 in qty (search from bottom up! from highest rownum to lowest) and gives me the date in the rownum+1 row. If no rows match for qty>=0 it will return the date of the 1st row. If no rows at all return NULL .

meaning that for:

select * from func1(10) the output should be 5.10.10 (as rownum=3 is the first qty>=0 from bottom up and 5.10.10 is the date in the rownum+1)

select * from func1(7) the output should be 1.1.10 (as there are no qty>=0 so it gives the date of the rownum=1)

select * from func1(7) the output should be NULL (as there are no rows)

How can i do that?

Answer 1

first not negative from the bottom

t=# with a(i,q) as (values(1,-5),(2,3),(3,-9),(4,-1))
select * from a where i=1 or q>=0 order by case when i=1 then null else i end desc nulls last, q limit 1;
 i | q
---+---
 2 | 3
(1 row)

all negatives - first row:

t=# with a(i,q) as (values(1,-5),(2,-3),(3,-9),(4,-1))
select * from a where i=1 or q>=0 order by case when i=1 then null else i end desc nulls last, q limit 1;
 i | q
---+----
 1 | -5
(1 row)

empty set:

t=# with a(i,q) as (select 1,1 where false)
select * from a where i=1 or q>=0 order by case when i=1 then null else i end desc nulls last, q limit 1;
 i | q
---+---
(0 rows)