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The New Relational Database Dictionary by C.J. Date

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Part III

Intervals

Examples in this part of the dictionary are based for the most part on a drastically revised version of the suppliers-and-parts database, involving suppliers and shipments only (“the suppliers-and-shipments database”). Sample values are shown in Fig. 6.

image

Fig. 6: The suppliers-and-shipments database with both since and during relvars—sample values

The relvars illustrated in Fig. 6 are all base relvars. The predicates are as follows:

  • Relvar S_SINCE: Supplier SNO has been under contract ever since day SNO_SINCE (and not the day immediately before day SNO_SINCE) and will continue to be so until further notice, and has had status STATUS ever since day STATUS_SINCE (and not the day immediately before day STATUS_SINCE) and will continue to do so until further notice.

  • Relvar SP_SINCE: Supplier SNO has been able to supply part PNO ever since day SINCE (and not the day immediately before day SINCE) and will continue to be so until further notice.

  • Relvar S_DURING: DURING denotes a maximal interval of days throughout which supplier SNO was under contract.

  • Relvar S_STATUS_DURING: DURING denotes a maximal interval of days throughout which supplier SNO had status STATUS.

  • Relvar SP_DURING: DURING denotes a maximal interval of days throughout which supplier SNO was able to supply part PNO.

    Points arising:

  • First of all, note the subtle change in semantics with respect to shipments ...

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