laravel 查询生成器中的嵌套查询

Question

我需要根据请求 id 获得 1 个帖子，结构如下：

• postId;

• postTitle;

• 发布内容；

• postImage;

• 乐队名；

• 流派名称；

• 标签：[tagId, tagName];

• cmets：[commentId、commentBody、commentCreatedAt]。

表格结构：

• 帖子（id、标题、内容、图片、band_id、时间戳）；

• 标签（id、名称）；

• post_tag(post_id, tag_id);

• cmets（id、body、post_id、user_id、时间戳）。

我尝试了不同的查询变体，例如：

$post = DB::table('posts as p')
    ->select('p.id as postId',
        'p.title as postTitle',
        'p.content as postContent',
        'p.image as postImage',
        'b.name as bandName',
        'g.name as genreName',
            DB::raw("(SELECT t.id as tagId, t.name as tagName
                     FROM tags as t
                     JOIN post_tag as pt ON t.id = pt.tag_id
                     WHERE pt.post_id = $request->postId
                     GROUP BY tagId) as tags"))
    ->join('bands as b', 'b.id', 'p.band_id')
    ->join('genres as g', 'g.id', 'b.genre_id')
    ->where('p.id', $request->postId)
    ->groupBy(
        'postId',
        'postTitle',
        'postContent',
        'postImage',
        'bandName',
        'genreName')
    ->get();

但是我在获取标签时遇到了困难（（它返回错误：SQLSTATE[21000]: Cardinality violation: 1241 Operand should contain 1 column(s), or other.

如何获取帖子的标签（对 cme​​ts 的查询将类似）？ 无法处理这样的嵌套查询（（ 我很感激任何帮助。

更新 1。

试过了：

$post = DB::table('posts as p')
        ->select('p.id as postId',
            'p.title as postTitle',
            'p.content as postContent',
            'p.image as postImage',
            'b.name as bandName',
            'g.name as genreName',
            't.id as tagId',
            't.name as tagName')
        ->join('post_tag as pt', 'p.id', 'pt.post_id')
        ->join('tags as t', 't.id', 'pt.tag_id')
        ->join('bands as b', 'b.id', 'p.band_id')
        ->join('genres as g', 'g.id', 'b.genre_id')
        ->where('p.id', $request->postId)
        ->groupBy(
            'postId',
            'postTitle',
            'postContent',
            'postImage',
            'bandName',
            'genreName',
            'tagId')
        ->get();

结果：

[{
    "postId",
    "postTitle",
    "postContent",
    "postImage",
    "bandName",
    "genreName",
    "tagId",
    "tagName"
},{
    "postId",
    "postTitle",
    "postContent",
    "postImage",
    "bandName",
    "genreName",
    "tagId",
    "tagName"
}]

所以，"postId","postTitle","postContent","postImage","bandName","genreName" 是重复的（（

Answer 1

当您在 select 语句中使用子查询时，它必须返回一个列值，这就是您收到此错误的原因，但是如果您也想获取标签名称和 id 为什么不添加另一个带有标签和 post_tag 表的连接，这里是一个例子：

$post = DB::table('posts as p')
->select('p.id as postId',
    'p.title as postTitle',
    'p.content as postContent',
    'p.image as postImage',
    'b.name as bandName',
    'g.name as genreName',
    't.id as tagId',
    't.name as tagName')
->join('post_tag as pt', 'p.id', 'pt.post_id')
->join('tags as t', 't.tag_id', 'pt.tag_id')
->join('bands as b', 'b.id', 'p.band_id')
->join('genres as g', 'g.id', 'b.genre_id')
->where('p.id', $request->postId)
->groupBy(
    'postId',
    'postTitle',
    'postContent',
    'postImage',
    'bandName',
    'genreName')
->get();

希望对你有帮助