rvesse commented on code in PR #132:
URL: https://github.com/apache/jena-site/pull/132#discussion_r1030195581
##########
source/documentation/query/lateral-join.md:
##########
@@ -0,0 +1,174 @@
+---
+title: ARQ - Lateral Join
+---
+
+Lateral joins using the keyword `LATERAL` were introduced in Apache jena 4.7.0.
+
+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 input LHS in-scope during each RHS evaluation.
+
+A regular join only executes the RHS once, and the variables from the LHS are
+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`.
+
+Examples:
+```
+## Get exactly one label for each subject with type `:T`
+SELECT * {
+ ?s rdf:type :T
+ 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 takes the graph pattern so far in the group, from
+the `{` starting of the current block, and a `{ }` block afterwards.
+
+#### 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`
+
+#### Variable assignment
+
+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.
+
+Such a variable assignment would conflict with the variable being set in
+variables of the row being joined.
+
+```
+## ** 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](assignment.html) would work.
+`LET` for a variable that is bound acts like a filter.
+
+#### Variable Scopes
+
+In looping on the input, a lateral join makes the bindings of variables in the
current row
+available to the right hand side pattern, settign their value from the top
down.
+
+In SPARQL, it is possible to have variables of the same name which are not
+exposed within a sub-select. These are not lateral-joined to a variable of the
+same name from the LHS.
+
+This is not speciifc to lateral joins. In
+```
+SELECT * {
+ ?s rdf:type :T
+ { SELECT ?label { ?s rdfs:label ?label } }
+```
+the inner `?s` can be replaced by `?z` without changing the results because the
+inner `?s` is not joined to the outer `?s` but instead is hidden by the
`SELECT ?label`.
+```
+SELECT * {
+ ?s rdf:type :T
+ { SELECT ?label { ?z rdfs:label ?label } }
+```
+
+The same rule applies to lateral joins.
+```
+SELECT * {
+ ?s rdf:type :T
+ 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.
+
+### 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).
Review Comment:
```suggestion
There is a similarity to filter `NOT EXISTS`/`EXISTS` expressed as the
non-legal `FILTER ( ASK { pattern } )` where the variables of the row being
filtered are available to "pattern". This is similar to a SQL [correlated
subquery](https://en.wikipedia.org/wiki/Correlated_subquery).
```
##########
source/documentation/query/lateral-join.md:
##########
@@ -0,0 +1,174 @@
+---
+title: ARQ - Lateral Join
+---
+
+Lateral joins using the keyword `LATERAL` were introduced in Apache jena 4.7.0.
+
+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 input LHS in-scope during each RHS evaluation.
+
+A regular join only executes the RHS once, and the variables from the LHS are
+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`.
+
+Examples:
+```
+## Get exactly one label for each subject with type `:T`
+SELECT * {
+ ?s rdf:type :T
+ 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 takes the graph pattern so far in the group, from
+the `{` starting of the current block, and a `{ }` block afterwards.
+
+#### 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`
+
+#### Variable assignment
+
+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.
+
+Such a variable assignment would conflict with the variable being set in
+variables of the row being joined.
+
+```
+## ** 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](assignment.html) would work.
+`LET` for a variable that is bound acts like a filter.
+
+#### Variable Scopes
+
+In looping on the input, a lateral join makes the bindings of variables in the
current row
+available to the right hand side pattern, settign their value from the top
down.
Review Comment:
```suggestion
available to the right hand side pattern, setting their value from the top
down.
```
##########
source/documentation/query/lateral-join.md:
##########
@@ -0,0 +1,174 @@
+---
+title: ARQ - Lateral Join
+---
+
+Lateral joins using the keyword `LATERAL` were introduced in Apache jena 4.7.0.
+
+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 input LHS in-scope during each RHS evaluation.
+
+A regular join only executes the RHS once, and the variables from the LHS are
+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`.
+
+Examples:
+```
+## Get exactly one label for each subject with type `:T`
+SELECT * {
+ ?s rdf:type :T
+ 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 takes the graph pattern so far in the group, from
+the `{` starting of the current block, and a `{ }` block afterwards.
+
+#### 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`
+
+#### Variable assignment
+
+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.
+
+Such a variable assignment would conflict with the variable being set in
+variables of the row being joined.
+
+```
+## ** 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](assignment.html) would work.
+`LET` for a variable that is bound acts like a filter.
+
+#### Variable Scopes
+
+In looping on the input, a lateral join makes the bindings of variables in the
current row
+available to the right hand side pattern, settign their value from the top
down.
+
+In SPARQL, it is possible to have variables of the same name which are not
+exposed within a sub-select. These are not lateral-joined to a variable of the
+same name from the LHS.
+
+This is not speciifc to lateral joins. In
+```
+SELECT * {
+ ?s rdf:type :T
+ { SELECT ?label { ?s rdfs:label ?label } }
+```
+the inner `?s` can be replaced by `?z` without changing the results because the
+inner `?s` is not joined to the outer `?s` but instead is hidden by the
`SELECT ?label`.
+```
+SELECT * {
+ ?s rdf:type :T
+ { SELECT ?label { ?z rdfs:label ?label } }
+```
+
+The same rule applies to lateral joins.
+```
+SELECT * {
+ ?s rdf:type :T
+ 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.
+
+### 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).
+
+## SPARQL Specification Additional Material
+
+### Syntax
+
+`LATERAL` is added to the SPARQL grammar at rule `[[56]
GraphPatternNotTriples](https://www.w3.org/TR/sparql11-query/#rGraphPatternNotTriples)`.
As a syntax form, it is similar to `OPTIONAL`.
+
+```
+[56] GraphPatternNotTriples ::= GroupOrUnionGraphPattern |
OptionalGraphPattern | LateralGraphPattern | ...
+[57] OptionalGraphPattern ::= 'OPTIONAL' GroupGraphPattern
+[ ] LateralGraphPattern ::= 'LATERAL' GroupGraphPattern
+```
+
+### Algebra
+
+The new algebra is operator is `lateral` which takes two expressions
Review Comment:
```suggestion
The new algebra operator is `lateral` which takes two expressions
```
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