Question

我正在尝试在Composer列中找到10个或更多具有NULL值的轨道的播放列表。

以下是表格

CREATE TABLE [Playlist]
(
[PlaylistId] INTEGER  NOT NULL,
[Name] NVARCHAR(120),
CONSTRAINT [PK_Playlist] PRIMARY KEY  ([PlaylistId])
);

CREATE TABLE [PlaylistTrack]
(
[PlaylistId] INTEGER  NOT NULL,
[TrackId] INTEGER  NOT NULL,
CONSTRAINT [PK_PlaylistTrack] PRIMARY KEY  ([PlaylistId], [TrackId]),
FOREIGN KEY ([PlaylistId]) REFERENCES [Playlist] ([PlaylistId]) 
    ON DELETE NO ACTION ON UPDATE NO ACTION,
FOREIGN KEY ([TrackId]) REFERENCES [Track] ([TrackId]) 
    ON DELETE NO ACTION ON UPDATE NO ACTION
);

CREATE TABLE [Track]
(
[TrackId] INTEGER  NOT NULL,
[Name] NVARCHAR(200)  NOT NULL,
[AlbumId] INTEGER,
[MediaTypeId] INTEGER  NOT NULL,
[GenreId] INTEGER,
[Composer] NVARCHAR(220),
[Milliseconds] INTEGER  NOT NULL,
[Bytes] INTEGER,
[UnitPrice] NUMERIC(10,2)  NOT NULL,
CONSTRAINT [PK_Track] PRIMARY KEY  ([TrackId]),
FOREIGN KEY ([AlbumId]) REFERENCES [Album] ([AlbumId]) 
    ON DELETE NO ACTION ON UPDATE NO ACTION,
FOREIGN KEY ([GenreId]) REFERENCES [Genre] ([GenreId]) 
    ON DELETE NO ACTION ON UPDATE NO ACTION,
FOREIGN KEY ([MediaTypeId]) REFERENCES [MediaType] ([MediaTypeId]) 
    ON DELETE NO ACTION ON UPDATE NO ACTION
)

这是我到目前为止所拥有的。我认为查询逻辑很好，但这是计算空值的正确方法吗？

SELECT P.PlaylistId, P.name
FROM Playlist P
    JOIN PlaylistTrack pt ON p.PlaylistId = pt.PlaylistId
    JOIN Track t ON pt.TrackId = t.TrackId
GROUP BY p.PlaylistId
HAVING COUNT(t.Composer IS NULL) >=10;

Answer 1

尝试比较COUNT(T.Compser) >= 10子句中的SELECT是错误的。这是一个比较（总是）返回一个真/假。该子查询也与您的外部查询无关。我真的很惊讶它甚至没有错误地运行。

您应该能够使用HAVING子句，该子句用于比较聚合：

SELECT
    P.PlaylistID,
    P.Name
FROM
    Playlist P
INNER JOIN PlaylistTrack PT ON PT.PlaylistID = P.PlaylistID
INNER JOIN Track T ON
    T.TrackID = PT.TrackID AND
    T.Composer IS NULL
GROUP BY
    P.PlaylistID,
    P.Name
HAVING
    COUNT(*) >= 10

编辑：你已经改变了你的问题。使用新查询时，问题是COUNT()将始终计算一行，无论该值是什么，除非该值为NULL。 T.Composer IS NULL将评估为值 - 无论是真还是假（1/0），这意味着无论T.Composer的值如何，每一行都将计入聚合。

Answer 2

而不是NULL，你只能声明像＆lt;＆gt;这样的东西。 0？或= 0？

或者至少尝试：

T.Composer为NULL或col =＆＃39;＆＃39;

或T.Composer IsNull（col，＆＃39;＆＃39;）=＆＃39;＆＃39;

Answer 3

我会使用left join而不是inner。这样，即使该查询中没有行，它仍将返回一行。有时这有可能破坏你的陈述，因为它不能算作空。

SQL COUNT NULL值

3 个答案: