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My Data base professor uses this for the Conceptual design:
enter image description here

(I had to fix the words cause it wasn't in English, sorry)
Due to a few personal reasons I couldn't watch his classes, and the book I'm using won't tell me why Teacher has this double line connection with Teaches.
It is all over the place on the notes of his, and I can't study properly like this.

Does anyone care to explain it to me?

Tax
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This double line says that a teacher must teach at least 1 subject. Double line shows the full participation and single line shows partial participation and it is also known as minimum cardinality.

Amsal N
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  • I think you mean "1 subject must be taught by one only teacher" instead, right? – Tax Sep 02 '17 at 22:59
  • The 1 and N in the diagram says that 1 teacher can teach N subjects at maximum and 1 subject can be taught by 1 teacher at maximum. The double line say that 1 teacher must teach 1 subject at minimum and there can be a subject that has no teacher at minimum. – Amsal N Sep 02 '17 at 23:02
  • The maximum cardinality is written while minimum cardinality is shown via single or double lines. – Amsal N Sep 02 '17 at 23:03