SOME RELVARS HAVE FOREIGN KEYS :
{"Image DocumentName": "SOME RELVARS HAVE FOREIGN KEYS :", "Image ImageDescription": "Let R1 and R2 be relvars, not necessarily distinct, and let K be a key for R1. Let FK be a subset of the heading of R2 such that there exists a possibly empty sequence of attribute renamings on R1 that maps K into K\u2019 (say), where K\u2019 and FK contain exactly the same attributes. Let R2 and R1 be subject to the constraint that, at all times, every tuple t2 in R2 has an FK value that\u2019s the K\u2019 value for some (necessarily unique) tuple t1 in R1 at the time in question. Then FK is a foreign key (with the same degree as K); the associated constraint is a referential constraint; and R2 and R1 are the referencing relvar and the corresponding referenced relvar, respectively, for that constraint.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}