Rejoins the relations in a database into a single data frame, if possible.
This is the inverse of calling autodb, except that the rows
might be returned in a different order.
Arguments
- database
A database containing the data to be rejoined, as returned by
decompose.
Value
A data frame, containing all information contained database if
it is lossless and self-consistent.
Details
The rejoining algorithm might not use all of the given relations: it begins with the relation with the largest number of records, then joins it with enough relations to contain all of the present attributes. This is not limited to relations that the starting relation is linked to by foreign keys, and is not limited to them either, since in some cases this constraint would make it impossible to rejoin with all of the present attributes.
Since the algorithm may not use all of the given relations, the algorithm may
ignore some types of database inconsistency, where different relations hold
data inconsistent with each other. In this case, the rejoining will be lossy.
Rejoining the results of reduce can also be lossy.
Due to the above issues, the algorithm will be changed to use all of the relations in the future.
Not all databases can be represented as a single data frame. A simple example is any database where the same attribute name is used for several difference sources of data, since rejoining results in inappropriate merges.
Examples
# simple example
db <- autodb(ChickWeight)
rj <- rejoin(db)
rj <- rj[order(as.integer(rownames(rj))), ]
all(rj == ChickWeight) # TRUE
#> [1] TRUE
# showing rejoin() doesn't check for inconsistency:
# add another Chick table with the diets swapped
db2 <- db[c(1, 2, 1)]
records(db2)[[3]]$Diet <- rev(records(db2)[[3]]$Diet)
rj2 <- rejoin(db2)
rj2 <- rj2[order(as.integer(rownames(rj2))), ]
all(rj2 == ChickWeight) # TRUE
#> [1] TRUE