Question

SQLite数据库中有一个经典的父/子表集：

CREATE TABLE ImageGallery (
    ImageGalleryId INTEGER PRIMARY KEY AUTOINCREMENT  NOT NULL , 
    Name VARCHAR(80) UNIQUE NOT NULL
    );

CREATE TABLE Image (
    ImageId INTEGER PRIMARY KEY AUTOINCREMENT  NOT NULL , 
    ImageGalleryId INTEGER NOT NULL, 
    Filename VARCHAR(260) NOT NULL, 
    FullPath VARCHAR(260) NOT NULL 
    );

在删除图像后需要触发一个触发器，如果该图库中没有更多图像，则会删除ImageGallery，基本上是这样的：

if ((select count(*) from Image where ImageGalleryId = old.imageGalleryId) == 0) then
begin
  delete from ImageGallery where ImageGalleryId = old.imageGalleryId
end

在SQLite方言中究竟是什么样的？

Answer 1

SQLite没有IF语句。

但是，触发器有WHEN clause：

CREATE TRIGGER xxx
AFTER DELETE ON Image
WHEN (SELECT COUNT(*) FROM Image WHERE ImageGalleryId = OLD.ImageGalleryId) = 0
BEGIN
    DELETE FROM ImageGallery WHERE ImageGalleryId = OLD.ImageGalleryId;
END;

SQLite触发器expr用于在没有子节点时删除父节点

1 个答案: