Question

为简单起见，我们假设我有以下数据库：

{
    id: "a_id",   // This is the key, not a field in the document
    type: "a"
}
{
    id: "b_id",
    type: "b",
    a: "a_id"
}
{
     type: "c",
     b: "b_id"
}

正如你所看到的，有一种经典的亲子关系（＆＃34; a＆＃34;有很多＆＃34; b＆＃34;，＆＃34; b＆＃34;有很多＆＃34; C＆＃34）让我们假设不会总是存在类型＆＃34; c＆＃34;文件 - 例如，我有五个＆＃34; b＆＃34;文件，但只有一个有＆＃34; c＆＃34;与之相关的文件。我想写一个JOIN查询，它会给我所有＆＃34; b＆＃34;属于＆＃34; a＆＃34;，如果它们存在 - 所有＆＃34; c＆＃34;属于＆＃34; b＆＃34;但是对于＆＃34; b＆＃34;没有＆＃34; c＆＃34;我没有得到任何结果 - 我想得到所有五个＆＃34; b＆＃34;文件，以及一个＆＃34; c＆＃34;相关的文件。我的查询如下：

SELECT *
FROM default AS a
JOIN default AS b ON KEY b.a FOR a
JOIN default AS c ON KEY c.b FOR b
WHERE a.type = "a"
AND b.type = "b"
AND c.type = "c";

我在这里缺少什么？谢谢！

Answer 1

WHERE子句post join过滤器，如果c为MISSING，则c.type =“c”为false。

INSERT INTO default VALUES("a_id", { "type": "a" });
INSERT INTO default VALUES("b_id", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id1", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id1", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id2", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id3", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id4", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("c_id", { "type": "c", "b":"b_id"});

CREATE INDEX ia ON default(a);
CREATE INDEX ib ON default(b);

SELECT *
FROM default AS d
LEFT JOIN default AS d1 ON KEY d1.a FOR d
LEFT JOIN default AS d2 ON KEY d2.b FOR d1
WHERE d.type = "a"
AND d1.type = "b"
AND (d2 IS MISSING OR d2.type = "c");

同时结帐https://dzone.com/articles/visually-explaining-n1ql-joins

N1QL（Couchbase） - 对于不存在的关系，在couchbase中进行真正的LEFT JOIN

1 个答案: