afs opened a new issue, #1615:
URL: https://github.com/apache/jena/issues/1615
### Version
4.6.1
### Feature
# LATERAL join
_Proposed experimental feature_
_This issue description may be edited in-place._
A `LATERAL` join is like a foreach loop, looping on the results from the
left-hand side (LHS), the pattern before the `LATERAL` keyword, and executing
the right-hand side (RHS) query pattern once for each row, with the variables
from the RHS in-scope during each RHS evaluation.
A regular join only executes the RHS once, and the variables from the LHS
are only used for the join condition after evaluation of the left and right
sub-patterns.
Another way to think of a lateral join is as a `flatmap`.
Example:
```
## Get exactly one label for each subject in a row.
SELECT * {
?s ?p ?o
LATERAL {
SELECT * { ?s rdfs:label ?label } LIMIT 1
}
}
```
```
## Get zero or one labels for each subject.
SELECT * {
?s ?p ?o
LATERAL { OPTIONAL { SELECT * ?s rdfs:label ?label } LIMIT 1}
}
}
```
### Syntax
The `LATERAL` keyword which has the graph pattern so far (from the `{`
starting the current block) and a `{ }` block afterwards.
Possible addition: `LATERAL ( ?var1 ?var2 ...)` to specify certain variables
to expose to the RHS. Other variables would be (inner)joined as usual. This may
be an unnecessary feature.
### Scope
A sub-select may have variables of the same name that are not lateral-joined
to a variable of the same name from the RHS.
```
SELECT * {
?s ?p ?o
LATERAL {
SELECT ?label { ?s rdfs:label ?label } LIMIT 1
}
}
```
The inner `?s` in the `SELECT ?label` is not the outer `?s` because the
`SELECT ?label` does not pass out `?s`. As a sub-query the `?s` could be any
name except `?label` for the same results.
This is the same situation as a sub-query in other situations.
There needs to be a new syntax restriction: there can no variable introduced
by `AS` (`BIND`, or sub-query) or `VALUES` in-scope at the top level of the
`LATERAL` RHS, that is the same name as any
[in-scope](https://www.w3.org/TR/sparql11-query/#variableScope) variable from
the LHS.
```
## ** Illegal **
SELECT * {
?s ?p ?o
LATERAL { BIND( 123 AS ?o) }
}
```
See [SPARQL Grammar note
12](https://www.w3.org/TR/sparql11-query/#sparqlGrammar).
In ARQ, [LET](https://www.w3.org/TR/sparql11-query/#variableScope) would
work.
`LET` for a variable that is bound acts like a filter.
### Evaluation
[Substituting variables](https://afs.github.io/substitute.html) from the LHS
into the RHS (with the same restrictions), then executing the pattern, gives
the evaluation of `LATERAL`
### Notes
There is a similarity to filter `NOT EXISTS`/`EXISTS` expressed as the
not-legal `FILTER ( ASK { pattern } )` where the variables of the row being
filtered are available to "pattern". This is similar to ab SQL [correlated
subquery](https://en.wikipedia.org/wiki/Correlated_subquery).
### Elsewhere
* https://github.com/w3c/sparql-12/issues/100
* Jena's [SERVICE
<loop:>](https://jena.apache.org/documentation/query/service_enhancer.html)
* Oxigraph:
[oxigraph/issues/267](https://github.com/oxigraph/oxigraph/issues/267),
[oxigraph/pull/274](https://github.com/oxigraph/oxigraph/pull/274)
*
https://docs.stardog.com/query-stardog/stored-query-service#correlated-subqueries
*
https://www.postgresql.org/docs/current/queries-table-expressions.html#QUERIES-LATERAL
* https://dev.mysql.com/doc/refman/8.0/en/lateral-derived-tables.html
* https://en.wikipedia.org/wiki/Correlated_subquery
### Are you interested in contributing a solution yourself?
Yes
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