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A laboratory has several chemists who work on one or more projects. While working on
these projects a chemist may use different laboratory equipment.
The laboratory needs to know the following information about its chemists, projects and equipment.
A chemist has an employee ID, name (first and last) , date of birth, age and several phone
numbers. A project has a unique project ID (identifier) , name, and a project start, and finish date. Equipment information which includes a serial number (unique) , name, and
cost . The laboratory wishes to record the date when a given piece of equipment is assigned
to and returned by a particular chemist. The laboratory also wishes to record the hours a chemist works on a project.
The laboratory also has the following business rules. A chemist must be assigned to at least
one project and one equipment item. A given piece of equipment need not be assigned, and a given project need not be assigned either to a chemist.

So, EQUIPMENT to ASSIGNEMENT, if it’s one at a time, a EQUIPMENT can only be assigned to one PROJECT at once N:1,

Or if it’s whole life, a EQUIPMENT can be ASSIGNED to one PROJECT or more in different moments M:N. Picture of the Diagram >> enter image description here

Allen
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    Please use [text, not images/links, for text](https://meta.stackoverflow.com/q/285551/3404097). An image might be nice as an additional alternate or it might add additional information. Although here the image has a pile of irrelevant stuff and the relevant stuff says nothing more than the text you should give. (Although you could give the relevant sub-image as additional alternate.) – philipxy Dec 12 '17 at 01:38

1 Answers1

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A cardinality constraint, like every constraint, is about base tables, which are table-valued variables, and is true (and enforced) in every database state, ie it is also "about the whole life of the data".

philipxy
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