如何將一個select語句的結果用作另一個變量的結果？

Question

這就是我想要做的

WITH 
loc_id AS (// take the result of the following select statement 
SELECT l.location_id 
    FROM mopdb.lulocation l 
    WHERE (l.location_short = 'FOO')) 

SELECT d.device 
    FROM mopdb.ludevice d 
    LEFT JOIN lulocation l ON (d.location_id = l.location_id) 
    WHERE (d.location_id = loc_id) //use it here 

Answer 1

如果你正在嘗試使用公用表表達式，然後loc_id將是可以像一個子查詢中使用的表，所以你可以這樣做：

with loc_id as (
    select l.location_id 
    from mopdb.lulocation l 
    where (l.location_short = 'FOO') 
) select 
    d.device 
from mopdb.ludevice d 
left join lulocation l on d.location_id = l.location_id 
where d.location_id = (select location_id from loc_id); 

然而，變量在SQL的每個方言中都會有不同的語法，因此它將在很大程度上取決於您使用的產品。

Answer 2

醜陋，但是：

SELECT d.device 
     FROM mopdb.ludevice d 
     LEFT JOIN lulocation l ON d.location_id = l.location_id 
     WHERE d.location_id = 
     (
     SELECT x.location_id 
     FROM mopdb.lulocation x 
     WHERE (x.location_short = 'FOO') 
    )