Question

我只是在学习SQL，而且我真的很难理解为什么我的左联接返回重复的行。这是我正在使用的查询：

SELECT "id", "title"
FROM "posts"
LEFT JOIN "comments" "comment"
ON "comment"."post_id"="id" AND ("comment"."status" = 'hidden')

它返回 4行，但只能返回3。返回的两行包含重复（相同的值）。我可以使用DISTINCT上的"id"前缀来解决此问题。

SELECT DISTINCT "id", "title"
FROM "posts"
LEFT JOIN "comments" "comment"
ON "comment"."post_id"="id" AND ("comment"."status" = 'hidden')

查询返回 3行，我得到了想要的结果。但是我仍然想知道为什么在世界上我会首先从第一个查询中得到重复的行？我正在尝试编写一个聚合查询，这似乎是我遇到的问题。

我正在使用PostgreSQL。

更具体： （由我的ORM创建）

Shift DDL

CREATE TABLE shift (
    id uuid DEFAULT uuid_generate_v4() PRIMARY KEY,
    "gigId" uuid REFERENCES gig(id) ON DELETE CASCADE,
    "categoryId" uuid REFERENCES category(id),
    notes text,
    "createdAt" timestamp without time zone NOT NULL DEFAULT now(),
    "updatedAt" timestamp without time zone NOT NULL DEFAULT now(),
    "salaryFixed" numeric,
    "salaryHourly" numeric,
    "salaryCurrency" character varying(3) DEFAULT 'SEK'::character varying,
    "staffingMethod" character varying(255) NOT NULL DEFAULT 'auto'::character varying,
    "staffingIspublished" boolean NOT NULL DEFAULT false,
    "staffingActivateon" timestamp with time zone,
    "staffingTarget" integer NOT NULL DEFAULT 0
);

ShiftEmployee DDL

CREATE TABLE "shiftEmployee" (
    "employeeId" uuid REFERENCES employee(id) ON DELETE CASCADE,
    "shiftId" uuid REFERENCES shift(id) ON DELETE CASCADE,
    status character varying(255) NOT NULL,
    "updatedAt" timestamp without time zone NOT NULL DEFAULT now(),
    "salaryFixed" numeric,
    "salaryHourly" numeric,
    "salaryCurrency" character varying(3) DEFAULT 'SEK'::character varying,
    CONSTRAINT "PK_6acfd2e8f947cee5a62ebff08a5" PRIMARY KEY ("employeeId", "shiftId")
);

查询

SELECT "id", "staffingTarget" FROM "shift" LEFT JOIN "shiftEmployee" "se" ON "se"."shiftId"="id" AND ("se"."status" = 'confirmed');

结果

id                                      staffingTarget
68bb0892-9bce-4d08-b40e-757cb0889e87    3
12d88ff7-9144-469f-8de5-3e316c4b3bbd    6
73c65656-e028-4f97-b855-43b00f953c7b    5
68bb0892-9bce-4d08-b40e-757cb0889e88    3
e3279b37-2ba5-4f1d-b896-70085f2ba345    4
e3279b37-2ba5-4f1d-b896-70085f2ba346    5
e3279b37-2ba5-4f1d-b896-70085f2ba346    5
789bd2fb-3915-4cda-a3d7-2186cf5bb01a    3

Answer 1

如果一个帖子具有多个隐藏评论，您将多次看到该帖子，因为联接每次匹配都会返回一行-这就是联接的本质。外部联接的行为也没有不同。

如果您打算只列出带有隐藏评论的帖子，则最好使用EXISTS查询：

SELECT p.id, p.title
FROM posts p
where exists (select *
              from comments c 
              where c.post_id = p.id 
                and c.status = 'hidden');

左联接返回重复的行

1 个答案: