SQL查询仅返回一个结果/行，而不是全部

Question

我有一个复杂的sql查询，该查询将返回选定汽车给出的所有评论，但是，我只看到如下所示的一个，而不是如屏幕快照2所示的所有评论。

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 /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：

如果您需要评论和平均值，则需要另外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;