Lossless Join Property

Question

What is meant by the lossless join property in a relation schema?

Is it the ability to maintain the semantics of information/data during the decomposition of relations whilst normalising?

score 8 · Accepted Answer · edited Oct 12 '22 at 06:16

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The lossless join property is a feature of decomposition supported by normalisation. It is the ability to ensure that any instance of the original relation can be identified from corresponding instances in the smaller relations.

edited Oct 12 '22 at 06:16

philipxy

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answered Apr 25 '11 at 12:19

user559142

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score 3 · Answer 2 · answered Apr 24 '11 at 16:45

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The word loss in lossless refers to loss of information, not to loss of tuples

This ppt presentation might be helpful.

answered Apr 24 '11 at 16:45

Premraj

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No, that link is about "lossless join with respect to a set of FDs" not "lossless join". – philipxy May 22 '17 at 21:21

score 0 · Answer 3 · edited Oct 11 '22 at 18:47

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Lossless means functioning without a loss. In other words, retain everything.

Important for databases to have this feature.

Formal Definition

Let R be a relation schema.
Let F be a set of functional dependencies on R.
Let R1 and R2 form a decomposition of R.

The decomposition is a lossless-join decomposition of R if at least one of the following functional dependencies are in F+

1) R1 ∩ R2 -> R1
2) R1 ∩ R2 -> R2

In Simpler Terms…

R1 ∩ R2 -> R1
R1 ∩ R2 -> R2

If R is split into R1 and R2, for the decomposition to be lossless then at least one of the two should hold true.

Projecting on R1 and R2, and joining back, results in the relation you started with.

Source: https://en.wikipedia.org/wiki/Lossless-Join_Decomposition

edited Oct 11 '22 at 18:47

Roy Latham

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answered Jul 28 '13 at 06:24

Faisal arzai

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No, this is about "lossless decomposition under a set of FDs", not "lossless decomposition". – philipxy Feb 15 '17 at 07:56
Also this only involves binary decompositions. – philipxy May 20 '19 at 08:23
It's also weirdly exactly the same as the Wikipedia answer I just read. I don't mean to be critical, I'm sure either Faisal was just trying to be helpful or he just wrote both sources but I don't think it's very helpful to duplicate it in both places (tho i see temp to put on wiki so it can be improved and here for karma) but I think you might not want to duicate in a way that doesn't really transform it. – Peter Gerdes Jun 03 '21 at 03:03

score 0 · Answer 4 · answered Feb 15 '17 at 07:58

R1, ... is a lossless decomposition of R when they join back to it.

(R1, ... being a lossless decomposition of R under a set of FDs (functional dependencies) F is a different property. That is when R1, ... is a lossless decomposition of R and satisfies the FDs in F.)

Lossless Join Property

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