，查詢語法最好使用子查詢不止一次

Question

從表

create table seeplai.item(
    itm_id serial primary key, 
    itp_id int  not null references seeplai.item_type, 
    itm_sq int2 not null default 10, 
    ...) 
create index item_itp_sq on seeplai.item(itp_id, itm_sq); 

使用的item記錄爲基礎，我希望能夠找到具有相同itp_id，並且具有itm_sq少最大itp_sq。現在使用兩個子查詢，這在語法上和邏輯上都是正確的，但感覺不對。

select max(itm_sq) 
into v_prev_sq 
from seeplai.item itm 
where itm.itp_id = (select itp_id from seeplai.item where itm_id=p_itm_id) 
and  itm.itm_sq < (select itm_sq from seeplai.item where itm_id=p_itm_id); 

我知道我使用了一個較好的方法，但它的語法已經溶入我的腦海裏的蜘蛛網

Answer 1

得到的只是一個最大這itm_sq比一個你的行中較小：

SELECT i2.itm_sq 
FROM seeplai.item i 
JOIN seeplai.item i2 USING (itp_id) 
WHERE i.itm_id = p_itm_id 
AND  i2.itm_sq < i.itm_sq 
ORDER BY i2.itm_sq DESC 
LIMIT 1; 

打破平局（具有相同的itm_sq多行）你可以ORDER BY附加列。

這樣你就不必再使用任何東西。如果你不得不，子查詢或CTE將是使用的工具。

---

如果您需要以前itm_sq爲每項目共享相同itp_id窗函數lag()（或lead()有相反的順序）將儀器：

SELECT i2.*, lag(i2.itm_sq) OVER (ORDER BY i2.itm_sq) AS previous_itm_sq 
FROM seeplai.item i 
JOIN seeplai.item i2 USING (itp_id) 
WHERE i.itm_id = p_itm_id 
ORDER BY i2.itm_sq DESC; 

Answer 2

剛子查詢的值存儲在一個變量：

declare @retVal varchar(20); 
set @retVal = select itm_tp from seeplai.item where itm_id=p_itm_id 

select max(itm_sq) 
into v_prev_sq 
from seeplai.item itm 
where itm.itm_tp = @retVal 
and  itm.itm_sq < @retVal; 

Answer 3

我不知道，但也許你可以使用PostgreSQL的WITH Queries (Common Table Expressions)。

Answer 4

也許你正在尋找公用表表達式？ http://www.postgresql.org/docs/9.2/static/queries-with.html

雖然我認爲這是比較容易使用實際上加入做...

select max(itm_0.itm_sq) 
into v_prev_sq 
from seeplai.item itm_0, 
join itm_1 
on itm_0.itm_tp = itm_1.itm_tp 
and itm_0.itm_sq < itm_1.itm_sq 
and itm_0.itm_id = itm_1.p_itm_id 
group by itm_1.p_itm_id 

可能使用DISTINCT ON如果需要

Answer 5

你需要加入表本身：

select max(itm_sq) 
into v_prev_sq 
from seeplai.item itm 
join seeplai.item itm2 on itm.itm_tp = itm2.itm_tp and itm2.itm_id=p_itm_id 
join seeplai.item itm3 on itm.itm_sq < itm3.itm_sq and itm3.itm_id=p_itm_id 

這是您的查詢的「直接翻譯」，但我認爲這種簡化也將工作：

select max(itm_sq) 
into v_prev_sq 
from seeplai.item itm 
join seeplai.item itm2 
    on itm.itm_tp = itm2.itm_tp 
    and itm2.itm_id = p_itm_id 
    and itm.itm_sq < itm2.itm_sq