[英]How to update a NULL foreign key and create a new row within another table referencing the same key?
[英]Count rows from another table referencing current row by foreign key
我有如下表格:
inverter [ id, address, ... ]
string [ id, inverter_id (foreign key), ... ]
我想选择所有“逆变器”,以及附加到它们的“字符串”的数量。
我在这里尝试了此查询,但是它给了我空的结果,那么我该怎么做呢?
SELECT inverter.*, COUNT(string.*) as string_count
FROM inverter
LEFT JOIN string ON string.inverter_id = inverter.id
ORDER BY address
我正在使用SQLite3。
这是我现在拥有的测试表的转储:
CREATE TABLE `inverter` (`id` INTEGER NULL PRIMARY KEY AUTOINCREMENT, `address` VARCHAR(3) NULL, `comment` VARCHAR(250) NULL);
INSERT INTO "inverter" ("id","address","comment") VALUES ('2','A1','North side');
INSERT INTO "inverter" ("id","address","comment") VALUES ('3','A2','Broken inverter');
INSERT INTO "inverter" ("id","address","comment") VALUES ('4','A3','');
INSERT INTO "inverter" ("id","address","comment") VALUES ('5','A4','South-west corner');
CREATE TABLE `string` (`id` INTEGER NULL PRIMARY KEY AUTOINCREMENT, `address` VARCHAR(3) NULL, `inverter_id` INTEGER NULL, `comment` VARCHAR(250) NULL, FOREIGN KEY (`inverter_id`) REFERENCES `inverters` (`id`) ON DELETE SET NULL);
INSERT INTO "string" ("id","address","inverter_id","comment") VALUES ('1','XX','3','');
INSERT INTO "string" ("id","address","inverter_id","comment") VALUES ('2','XY','3','Ahoj jak se máš');
INSERT INTO "string" ("id","address","inverter_id","comment") VALUES ('3','XZ','4','Moo');
似乎SQLite3在count(s.*)
上令人窒息,因此请尝试以下操作:
select i.*, count(s.id)
from inverter i
left join string s on i.id = s.inverter_id group by i.address;
这给出:
2|A1|North side|0
3|A2|Broken inverter|2
4|A3||1
5|A4|South-west corner|0
使用聚合始终意味着您正在分组,如果不指定分组,它将是包含所有记录的单个组。 对从inverter
表中获得的字段进行分组。 还可以在count中使用单个字段,而不是string.*
:
select
inverter.id, inverter.address, count(string.id) as string_count
from
inverter
left join string on string.inverter_id = inverter.id
group by
inverter.id, inverter.address
order by
inverter.address
更改表名称“字符串”。 那是保留字。
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