MySQL: Allow optional constraint name after CONSTRAINT keyword

MySQL allows `CONSTRAINT CHECK (expr)` without a name - the database will
auto-generate a constraint name. This is a MySQL extension; the SQL standard
requires a name after the CONSTRAINT keyword. See [docs].

Previously, we required an identifier after CONSTRAINT, causing us to
interpret CONSTRAINT as a column name in cases like:

```sql
CREATE TABLE t (x INT, CONSTRAINT CHECK (x > 1))
```

Now we check if the token after CONSTRAINT is a constraint type keyword
(CHECK, PRIMARY, UNIQUE, FOREIGN) and treat the name as optional for
dialects that support this. This is just MySQL as far as I know (and
Generic), but we introduce a new `Dialect` flag even though it's a minor
feature. We could remove the flag and allow the more permissive optional
name syntax across dialects if desired.

[docs]: https://dev.mysql.com/doc/refman/8.4/en/create-table.html

Author: mvzink

src/dialect/generic.rs

@@ -275,4 +275,8 @@ impl Dialect for GenericDialect {
     fn supports_comment_optimizer_hint(&self) -> bool {
         true
     }
+
+    fn supports_constraint_keyword_without_name(&self) -> bool {
+        true
+    }
 }

src/dialect/mod.rs

@@ -1156,6 +1156,23 @@ pub trait Dialect: Debug + Any {
         false
     }
 
+    /// Returns true if the dialect supports the `CONSTRAINT` keyword without a name
+    /// in table constraint definitions.
+    ///
+    /// Example:
+    /// ```sql
+    /// CREATE TABLE t (a INT, CONSTRAINT CHECK (a > 0))
+    /// ```
+    ///
+    /// This is a MySQL extension; the SQL standard requires a name after `CONSTRAINT`.
+    /// When the name is omitted, the output normalizes to just the constraint type
+    /// without the `CONSTRAINT` keyword (e.g., `CHECK (a > 0)`).
+    ///
+    /// <https://dev.mysql.com/doc/refman/8.4/en/create-table.html>
+    fn supports_constraint_keyword_without_name(&self) -> bool {
+        false
+    }
+
     /// Returns true if the specified keyword is reserved and cannot be
     /// used as an identifier without special handling like quoting.
     fn is_reserved_for_identifier(&self, kw: Keyword) -> bool {

src/dialect/mysql.rs

@@ -186,6 +186,11 @@ impl Dialect for MySqlDialect {
     fn supports_comment_optimizer_hint(&self) -> bool {
         true
     }
+
+    /// See: <https://dev.mysql.com/doc/refman/8.4/en/create-table.html>
+    fn supports_constraint_keyword_without_name(&self) -> bool {
+        true
+    }
 }
 
 /// `LOCK TABLES`

src/parser/mod.rs

@@ -9243,7 +9243,20 @@ impl<'a> Parser<'a> {
         &mut self,
     ) -> Result<Option<TableConstraint>, ParserError> {
         let name = if self.parse_keyword(Keyword::CONSTRAINT) {
-            Some(self.parse_identifier()?)
+            if self.dialect.supports_constraint_keyword_without_name()
+                && self
+                    .peek_one_of_keywords(&[
+                        Keyword::CHECK,
+                        Keyword::PRIMARY,
+                        Keyword::UNIQUE,
+                        Keyword::FOREIGN,
+                    ])
+                    .is_some()
+            {
+                None
+            } else {
+                Some(self.parse_identifier()?)
+            }
         } else {
             None
         };

tests/sqlparser_mysql.rs

@@ -3462,6 +3462,27 @@ fn parse_create_table_unallow_constraint_then_index() {
     assert!(mysql_and_generic().parse_sql_statements(sql).is_ok());
 }
 
+#[test]
+fn parse_create_table_constraint_check_without_name() {
+    let dialects = all_dialects_where(|d| d.supports_constraint_keyword_without_name());
+    dialects.one_statement_parses_to(
+        "CREATE TABLE t (x INT, CONSTRAINT PRIMARY KEY (x))",
+        "CREATE TABLE t (x INT, PRIMARY KEY (x))",
+    );
+    dialects.one_statement_parses_to(
+        "CREATE TABLE t (x INT, CONSTRAINT UNIQUE (x))",
+        "CREATE TABLE t (x INT, UNIQUE (x))",
+    );
+    dialects.one_statement_parses_to(
+        "CREATE TABLE t (x INT, CONSTRAINT FOREIGN KEY (x) REFERENCES t2(id))",
+        "CREATE TABLE t (x INT, FOREIGN KEY (x) REFERENCES t2(id))",
+    );
+    dialects.one_statement_parses_to(
+        "CREATE TABLE t (x INT, CONSTRAINT CHECK (x > 1))",
+        "CREATE TABLE t (x INT, CHECK (x > 1))",
+    );
+}
+
 #[test]
 fn parse_create_table_with_fulltext_definition() {
     mysql_and_generic().verified_stmt("CREATE TABLE tb (id INT, FULLTEXT (id))");