[英]How to use result of an SQL query as a field in another query
我有2張桌子
實體保存有關可出租物品的信息(租賃服務)租金保存實體的特定租金信息,該信息與指向實體ID的外鍵鏈接:
--------------------------
| id | name |
--------------------------
| 1 | Toyota Spacewagon |
| 2 | Volvo SuperCar |
---------------------------------------------------------------
| id | entity_id | rental_date | return_date |
---------------------------------------------------------------
| 1 | 1 | 2016-06-30 12:00:00 | 2016-07-01 12:00:00 |
| 2 | 1 | 2016-06-28 12:00:00 | 2016-07-30 10:00:00 |
| 3 | 1 | 2016-05-30 09:00:00 | 2016-06-01 13:00:00 |
我想做的是獲取所有實體的列表,以及它們的最后租用日期(以確定它們當前的租用狀態),但是我不知道如何在SQL中表達這一點。
-------------------------------------------------
| id | name | rental_date |
-------------------------------------------------
| 1 | Toyota Spacewagon | 2016-06-30 12:00:00 |
| 2 | Volvo SuperCar | (null) |
從未租用的實體應在租車日期返回null,已租用的汽車應最后租車
類似於此,但語法有效...
SELECT id, name rental_date AS (SELECT rental_date FROM rentals WHERE entity_id = THIS_ROW'S_ENTITY) FROM entities;
這樣的事情應該起作用:
SELECT e.Id,e.name,r.rental_date
FROM entities e
LEFT JOIN
(
SELECT entity_id,MAX(rental_date) AS rental_date
FROM rentals
GROUP BY entity_id
) r ON r.entity_id=e.Id
這是一個沒有子查詢的版本:
SELECT a.id, a.name, MAX(b.rental_date)
FROM entities a
LEFT JOIN rentals b on a.id = b.entity_id
GROUP BY a.id, a.name
你可以做:
Select r.rentedtimes,r.lastrentaled , e.id , e.`name`
from entities e
left join (
Select count(entity_id) as rentedtimes,id,entity_id,max(return_date) as lastrentaled
from rentals
group by entity_id)r
on e.id = r.entity_id
group by e.id;
注意:
我花了一些時間進行此查詢,並在Rentals表中添加了每個實體的出現。 看看這個 :)
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