[mssql] Parse CROSS/OUTER APPLY

T-SQL (and Oracle) support non-standard syntax, which is similar in
functionality to LATERAL joins in ANSI and PostgreSQL
<https://blog.jooq.org/tag/lateral-derived-table/>: it allows to use
the columns from the tables defined to the left of `APPLY` in the
"derived tables" (subqueries) to the right of `APPLY`. Unlike ANSI
LATERAL (but like Postgres' implementation), APPLY is also used with
table-valued function calls.

Despite them being similar, we represent "APPLY" joins with
`JoinOperator`s of its own (`CrossApply` and `OuterApply`). Doing
otherwise seemed like it would cause unnecessary confusion, as those
interested in dialect-specific parsing would probably not expect APPLY
being parsed as LATERAL, and those wanting to forbid non-standard SQL
would not be helped by this either.

This also renames existing JoinOperator::Cross -> CrossJoin to avoid
confusion with CrossApply.

src/dialect/keywords.rs

@@ -58,6 +58,7 @@ define_keywords!(
     ALTER,
     AND,
     ANY,
+    APPLY,
     ARE,
     ARRAY,
     ARRAY_AGG,
@@ -423,10 +424,7 @@ pub const RESERVED_FOR_TABLE_ALIAS: &[&str] = &[
     WITH, SELECT, WHERE, GROUP, HAVING, ORDER, LIMIT, OFFSET, FETCH, UNION, EXCEPT, INTERSECT,
     // Reserved only as a table alias in the `FROM`/`JOIN` clauses:
     ON, JOIN, INNER, CROSS, FULL, LEFT, RIGHT, NATURAL, USING,
-    // Reserved not because of ambiguity, but so that parsing `SELECT * FROM a
-    // OUTER JOIN b` causes a syntax error, rather than silently parsing to an
-    // inner join where table `a` is aliased as `OUTER`, which is certainly not
-    // what the user intended and also not valid according to the SQL standard.
+    // for MSSQL-specific OUTER APPLY (seems reserved in most dialects)
     OUTER,
 ];
 

src/sqlast/query.rs

@@ -327,7 +327,6 @@ impl ToString for Join {
                 self.relation.to_string(),
                 suffix(constraint)
             ),
-            JoinOperator::Cross => format!(" CROSS JOIN {}", self.relation.to_string()),
             JoinOperator::LeftOuter(constraint) => format!(
                 " {}LEFT JOIN {}{}",
                 prefix(constraint),
@@ -346,6 +345,9 @@ impl ToString for Join {
                 self.relation.to_string(),
                 suffix(constraint)
             ),
+            JoinOperator::CrossJoin => format!(" CROSS JOIN {}", self.relation.to_string()),
+            JoinOperator::CrossApply => format!(" CROSS APPLY {}", self.relation.to_string()),
+            JoinOperator::OuterApply => format!(" OUTER APPLY {}", self.relation.to_string()),
         }
     }
 }
@@ -356,7 +358,11 @@ pub enum JoinOperator {
     LeftOuter(JoinConstraint),
     RightOuter(JoinConstraint),
     FullOuter(JoinConstraint),
-    Cross,
+    CrossJoin,
+    /// CROSS APPLY (non-standard)
+    CrossApply,
+    /// OUTER APPLY (non-standard)
+    OuterApply,
 }
 
 #[derive(Debug, Clone, PartialEq, Hash)]

src/sqlparser.rs

@@ -1618,10 +1618,24 @@ impl Parser {
         let mut joins = vec![];
         loop {
             let join = if self.parse_keyword("CROSS") {
-                self.expect_keyword("JOIN")?;
+                let join_operator = if self.parse_keyword("JOIN") {
+                    JoinOperator::CrossJoin
+                } else if self.parse_keyword("APPLY") {
+                    // MSSQL extension, similar to CROSS JOIN LATERAL
+                    JoinOperator::CrossApply
+                } else {
+                    return self.expected("JOIN or APPLY after CROSS", self.peek_token());
+                };
                 Join {
                     relation: self.parse_table_factor()?,
-                    join_operator: JoinOperator::Cross,
+                    join_operator,
+                }
+            } else if self.parse_keyword("OUTER") {
+                // MSSQL extension, similar to LEFT JOIN LATERAL .. ON 1=1
+                self.expect_keyword("APPLY")?;
+                Join {
+                    relation: self.parse_table_factor()?,
+                    join_operator: JoinOperator::OuterApply,
                 }
             } else {
                 let natural = self.parse_keyword("NATURAL");

tests/sqlparser_common.rs

@@ -1560,7 +1560,7 @@ fn parse_cross_join() {
                 args: vec![],
                 with_hints: vec![],
             },
-            join_operator: JoinOperator::Cross
+            join_operator: JoinOperator::CrossJoin
         },
         only(only(select.from).joins),
     );
@@ -1804,7 +1804,7 @@ fn parse_join_syntax_variants() {
 
     let res = parse_sql_statements("SELECT * FROM a OUTER JOIN b ON 1");
     assert_eq!(
-        ParserError::ParserError("Expected LEFT, RIGHT, or FULL, found: OUTER".to_string()),
+        ParserError::ParserError("Expected APPLY, found: JOIN".to_string()),
         res.unwrap_err()
     );
 }

tests/sqlparser_mssql.rs

@@ -52,6 +52,22 @@ fn parse_mssql_delimited_identifiers() {
     );
 }
 
+#[test]
+fn parse_mssql_apply_join() {
+    let _ = ms_and_generic().verified_only_select(
+        "SELECT * FROM sys.dm_exec_query_stats AS deqs \
+         CROSS APPLY sys.dm_exec_query_plan(deqs.plan_handle)",
+    );
+    let _ = ms_and_generic().verified_only_select(
+        "SELECT * FROM sys.dm_exec_query_stats AS deqs \
+         OUTER APPLY sys.dm_exec_query_plan(deqs.plan_handle)",
+    );
+    let _ = ms_and_generic().verified_only_select(
+        "SELECT * FROM foo \
+         OUTER APPLY (SELECT foo.x + 1) AS bar",
+    );
+}
+
 fn ms() -> TestedDialects {
     TestedDialects {
         dialects: vec![Box::new(MsSqlDialect {})],