如何分割複合列像ROW，ARRAY？

Question

我與這個停止的線程有同樣的問題。

http://www.mail-archive.com/pgsql-sql@postgresql.org/msg28070.html

QUERY-1

SELECT 
r.id, 
(
     SELECT 
     rl.reminder_header, 
     rl.reminder_footer 
     FROM reminder_levels AS rl 
     WHERE rl.lookup = 
     (
       SELECT MAX(reminder_level_lookup) 
       FROM reminders 
       WHERE customer_id = r.customer_id 
     ) 
) 
FROM reminders AS r 

PostgreSQL的回答說：
ERROR：子查詢必須返回只有一列

QUERY-2

SELECT 
r.id, 
(
     SELECT 
     rl.reminder_header 
     FROM reminder_levels AS rl 
     WHERE rl.lookup = 
     (
       SELECT MAX(reminder_level_lookup) 
       FROM reminders 
       WHERE customer_id = r.customer_id 
     ) 
) AS reminder_header, 
(
     SELECT 
     rl.reminder_footer 
     FROM reminder_levels AS rl 
     WHERE rl.lookup = 
     (
       SELECT MAX(reminder_level_lookup) 
       FROM reminders 
       WHERE customer_id = r.customer_id 
     ) 
) AS reminder_footer 
FROM reminders AS r 

id | reminder_header | reminder_footer 
----+-------------------+-------------------- 
1 | hogehoge   | fugafuga 

...這工作，但運行在同一子查詢塊的兩倍。
這會導致性能下降。
（但是，這個結果表是我想要的。）

查詢3

SELECT 
r.id, 
(
     SELECT 
     ROW(rl.reminder_header, rl.reminder_header) 
     FROM reminder_levels AS rl 
     WHERE rl.lookup = 
     (
       SELECT MAX(reminder_level_lookup) 
       FROM reminders 
       WHERE customer_id = r.customer_id 
     ) 
) AS rec 
FROM reminders AS r 

id | rec 
----+---------------------- 
1 | (hogehoge, fugafuga) 

...它的工作原理，但列 'REC' 是複合。

如何將這個'rec'分隔爲類似於Query-2的reminder_header和reminder_footer， 。

有一些程序或tequnique？ 或其他解決方案？

謝謝。

Answer 1

要從子查詢中選擇多個列，通常使用連接。現在，我不知道你究竟表如何相關，但像這樣將是一個良好的開端：

SELECT r.id 
,  rl.reminder_header 
,  rl.reminder_footer 
FROM reminders r 
JOIN reminder_levels AS rl 
ON  rl.customer_id = r.customer_id 
WHERE rl.lookup = 
     (
     SELECT MAX(reminder_level_lookup) 
     FROM reminders r2 
     WHERE r2.customer_id = r.customer_id 
     ) 

Answer 2

你可以這樣做：

with query as ( 
SELECT 
r.id, 
(
     SELECT 
     r1::reminder_levels 
     FROM reminder_levels AS rl 
     WHERE rl.lookup = 
     (
       SELECT MAX(reminder_level_lookup) 
       FROM reminders 
       WHERE customer_id = r.customer_id 
     ) 
) AS rec 
FROM reminders AS r) 
select id, 
     (rec::reminder_levels).reminder_header, 
     (rec::reminder_levels).reminder_footer 
from query