What will be the main key of the "Pictures" table in this example
I would say that PictureID is the primary key because it (or should, anyway) be unique for each image. In the end, the definition of a primary key, according to Wikipedia :
In a relational database structure, a unique key or primary key is a candidate key for uniquely identifying each row in a table. A unique key or primary key contains one column or a set of columns. There are no two different rows in a table that can have the same value (or combination of values) in those columns. Depending on its design, a table can have arbitrarily many unique keys, but no more than one primary key.
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If you want each image to belong to one and only one album, then only the PictureID is ok.
If you want each image to appear in multiple albums, you can make PictureID and AlbumID the combined primary key. That is, the picture is uniquely identified by the combination of the PictureID and the album to which it belongs. Then keep the PictureData in another table (the image would be better described as a PictureAlbum or something)
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In the first view, pictureId must be the primary key. BUT if I assume there will be a table that will store the Maps information (say PictureMaster) that has PitctureId as the primary key THEN
In the Pictures table, pictureId and AlbumId will create a PrimaryKey (Composite Primary Key) and AlbumId, PictureId will be a ForeignKey.
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