嵌套 SQL 从多个表和 COUNT 内联查询

Question

博主们好。

我想创建一个查询，它将 select car name ， model ， price从表tblCars 。 在同一个查询结果中，我需要从表tblLikes中检查某个用户是否喜欢这辆车。 tblLikes有id 、 userId和carId （外键）。 因此，对于特定用户，我需要以下内容作为回报。

1  Corolla Toyota  R100,000  1    (user liked)
2  A3      Audi    R700,000  0    (user did not like)

我尝试过的是以下没有成功。

SELECT
     CarName
   , CarModel
   , CarPrice
   , (
        SELECT COUNT(*)
        FROM tblLikes
        WHERE
                userId = @userId
            AND CarId = @CarID
    ) as Likes
FROM tblCars

SELECT COUNT(*)
FROM tblLikes
WHERE
        userId = @userId
    AND CarId = @CarID 

如果记录存在则返回1 ，如果不存在则返回0 。

Answer 1

declare @tblCars table (
      id int IDENTITY(1,1) NOT NULL
    , CarName varchar(20)
    , CarModel varchar(20)
    , CarPrice money
)

declare @tblLikes table (
      id int IDENTITY(1,1) NOT NULL
    , userId int
    , carId int
)

insert into @tblCars (
      CarName
    , CarModel
    , CarPrice
)
          select 'BMW'          , 'qqq', '1000'
union all select 'Mercedes'     , 'www', '2000'
union all select 'Audi'         , 'eee', '3000'
union all select 'Scania'       , 'rrr', '4000'
union all select 'Opel'         , 'ttt', '5000'
union all select 'Mazda'        , 'yyy', '6000'
union all select 'Budatti'      , 'uuu', '7000'
union all select 'Lamborgini'   , 'iii', '8000'
union all select 'Tesla'        , 'ooo', '9000'
union all select 'Space'        , 'ppp', '10000'

insert into @tblLikes (
      userId
    , carId
)
          select '1', '1'
union all select '1', '3'
union all select '1', '5'
union all select '1', '7'
union all select '1', '9'


select
    c.*
    , (
        case when l.id is not null
            then '1'
            else '0'
        end
    ) as likes
from @tblCars as c
left outer join @tblLikes as l
    on l.carId = c.id