SQL 查询只返回一个 result/row 而不是全部

Question

我有一个复杂的 sql 查询，它意味着 return 所选汽车给出的所有评论，但是，我只看到如下所示的一个，而不是所有的评论在屏幕截图二中。

这是我的 DDL:

CREATE TABLE carsType(
    carsTypeID INTEGER NOT NULL AUTO_INCREMENT,
    carsTypeName VARCHAR(200) NOT NULL,
    PRIMARY KEY(carsTypeID)
);

CREATE TABLE carCategoryType(
    carCategoryTypeID INTEGER NOT NULL AUTO_INCREMENT,
    carCategoryTypeName VARCHAR(200) NOT NULL,
    PRIMARY KEY(carCategoryTypeID)
);

CREATE TABLE cars(
    carID INTEGER NOT NULL AUTO_INCREMENT,
    carsTypeID INTEGER NOT NULL,
    carCategoryTypeID INTEGER NOT NULL,
    carName VARCHAR(200),
    carArea VARCHAR(200),
    FOREIGN KEY(carsTypeID) REFERENCES carsType(carsTypeID),
    FOREIGN KEY(carCategoryTypeID) REFERENCES carCategoryType(carCategoryTypeID),
    PRIMARY KEY(carID)
);

CREATE TABLE carsReview(
    carID INTEGER NOT NULL,
    carMileage INTEGER NOT NULL,
    reviews VARCHAR(200) NOT NULL,
    FOREIGN KEY(carID) REFERENCES cars(carID)
);

INSERT INTO carsType VALUES
(NULL, "Hatchback"),
(NULL, "Saloon"),
(NULL, "Sedan");

INSERT INTO carCategoryType VALUES
(NULL, "A"),
(NULL, "B"),
(NULL, "C");

INSERT INTO cars VALUES
(NULL, 1, 1, "Golf", "Germany"),
(NULL, 2, 2, "Ford", "USA"),
(NULL, 2, 2, "Pontiac", "USA");

INSERT INTO carsReview VALUES
(1, 10000, "Good clean car"),
(2, 3000, "Worth it"),
(3, 1000, "Brand spanking new"),
(1, 45000, "She is good"),
(1, 100000, "Gone son"),
(1, 23000, "Looking fine");

这是我的复杂 SQL 查询：

SELECT tbl_cars.carID, tbl_cars.carName, tbl_cars.carArea, 
        (IFNULL(tbl_carsReview.averageMileage, 0.0)) AS finalaverageMileage , tbl_carsReview.reviews, tbl_carsType.carsTypeName, 
        tbl_carCategoryType.carCategoryTypeName
        FROM cars tbl_cars
        INNER JOIN carsType tbl_carsType ON
        tbl_cars.carsTypeID = tbl_carsType.carsTypeID 
        INNER JOIN carCategoryType tbl_carCategoryType ON
        tbl_cars.carCategoryTypeID = tbl_carCategoryType.carCategoryTypeID
        LEFT JOIN (SELECT carsReview.carID, ROUND(AVG(carsReview.reviews),1) AS averageMileage, carsReview.reviews
        FROM carsReview) tbl_carsReview
        ON tbl_cars.carID = tbl_carsReview.carID 
        WHERE tbl_cars.carID = 1

这是我的sqlfiddle：http://sqlfiddle.com/#!9/c9a92c/8

Answer 1

您在子查询中缺少 group by:

SELECT tbl_cars.carID, tbl_cars.carName, tbl_cars.carArea, 
       (IFNULL(tbl_carsReview.averageMileage, 0.0)) AS finalaverageMileage ,
       tbl_carsReview.reviews, tbl_carsType.carsTypeName, 
       tbl_carCategoryType.carCategoryTypeName
FROM cars tbl_cars INNER JOIN
     carsType tbl_carsType
     ON tbl_cars.carsTypeID = tbl_carsType.carsTypeID INNER JOIN
     carCategoryType tbl_carCategoryType
     ON tbl_cars.carCategoryTypeID = tbl_carCategoryType.carCategoryTypeID LEFT JOIN
     (SELECT carsReview.carID, ROUND(AVG(carsReview.reviews),1) AS averageMileage, carsReview.reviews
      FROM carsReview
      GROUP BY carsReview.carID
------^
     ) tbl_carsReview
     ON tbl_cars.carID = tbl_carsReview.carID 
WHERE tbl_cars.carID = 1;

编辑：

如果你想要评论和的平均值，你需要额外的join:

SELECT tbl_cars.carID, tbl_cars.carName, tbl_cars.carArea, 
       (IFNULL(tbl_carsReviewAvg.averageMileage, 0.0)) AS finalaverageMileage ,
       tbl_carsReview.reviews, tbl_carsType.carsTypeName, 
       tbl_carCategoryType.carCategoryTypeName
FROM cars tbl_cars INNER JOIN
     carsType tbl_carsType
     ON tbl_cars.carsTypeID = tbl_carsType.carsTypeID INNER JOIN
     carCategoryType tbl_carCategoryType
     ON tbl_cars.carCategoryTypeID = tbl_carCategoryType.carCategoryTypeID LEFT JOIN
     (SELECT carsReview.carID, ROUND(AVG(carsReview.reviews),1) AS averageMileage, carsReview.reviews
      FROM carsReview
      GROUP BY carsReview.carID
     ) tbl_carsReviewAvg
     ON tbl_cars.carID = tbl_carsReviewAvg.carID LEFT JOIN
     carsReview tbl_carsReview
     ON tbl_cars.carID = tbl_carsReview.carID
WHERE tbl_cars.carID = 1;

SQL 查询只返回一个 result/row 而不是全部

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