Question

我有一个简单的查询，但我被卡住了，我真的无法弄清楚如何做到这一点。进一步来说：表Allarmi有很多条目，我显示所有条目都按Allarmi.IDComponente

分组

类似这样的事情

SELECT * 
FROM ALLARMI AS ALLARMI
WHERE ID_CELLA = 1
GROUP BY ID_Componente

我只需要在此表中添加COUNT列，例如

(SELECT COUNT( * ) 
FROM allarmi
WHERE isAttivo = 1
GROUP BY id_componente)

但即使COUNT或0

，我也想展示NULL

我该怎么办？提前致谢

Answer 1

假设表Componenti定义了组件，并且您希望从allarmi获取每个组件的记录数，那么您真正想要做的是从Componenti中选择组件，在allarmi

上添加LEFT OUTER JOIN

SELECT COMPONENTI.ID_Componente, COUNT(ALLARMI.ID_Componente)
FROM COMPONENTI
LEFT OUTER JOIN ALLARMI ON
   -- insert your join here!
   -- maybe COMPONENTI.ID_Componente= ALLARMI.ID_Componente?
   -- add any query for this table here as well.
   AND ALLARMI.ID_Cella = 1
GROUP BY COMPONENTI.ID_Componente

坚持一个简单的查询 - OUTER JOIN

1 个答案: