Allow optional keywords in create table unique constraints (#2077)

* Add optional keywords to `create table` unique constraint

* More tests

* Move [INDEX | KEY] [index_name] to MySql dialect

* Linter

* Remove files moved

src/sqlfluff/dialects/dialect_mysql.py

@@ -231,6 +231,47 @@ class CreateTableStatementSegment(
     )
 
 
+@mysql_dialect.segment(replace=True)
+class TableConstraintSegment(BaseSegment):
+    """A table constraint, e.g. for CREATE TABLE."""
+
+    type = "table_constraint_segment"
+    # Later add support for CHECK constraint, others?
+    # e.g. CONSTRAINT constraint_1 PRIMARY KEY(column_1)
+    match_grammar = Sequence(
+        Sequence(  # [ CONSTRAINT <Constraint name> ]
+            "CONSTRAINT", Ref("ObjectReferenceSegment"), optional=True
+        ),
+        OneOf(
+            Sequence(  # UNIQUE [INDEX | KEY] [index_name] ( column_name [, ... ] )
+                "UNIQUE",
+                OneOf("INDEX", "KEY", optional=True),
+                Ref("ObjectReferenceSegment", optional=True),
+                Ref("BracketedColumnReferenceListGrammar"),
+                # Later add support for index_parameters?
+            ),
+            Sequence(  # PRIMARY KEY ( column_name [, ... ] ) index_parameters
+                Ref("PrimaryKeyGrammar"),
+                # Columns making up PRIMARY KEY constraint
+                Ref("BracketedColumnReferenceListGrammar"),
+                # Later add support for index_parameters?
+            ),
+            Sequence(  # FOREIGN KEY ( column_name [, ... ] )
+                # REFERENCES reftable [ ( refcolumn [, ... ] ) ]
+                Ref("ForeignKeyGrammar"),
+                # Local columns making up FOREIGN KEY constraint
+                Ref("BracketedColumnReferenceListGrammar"),
+                "REFERENCES",
+                Ref("ColumnReferenceSegment"),
+                # Foreign columns making up FOREIGN KEY constraint
+                Ref("BracketedColumnReferenceListGrammar"),
+                # Later add support for [MATCH FULL/PARTIAL/SIMPLE] ?
+                # Later add support for [ ON DELETE/UPDATE action ] ?
+            ),
+        ),
+    )
+
+
 mysql_dialect.add(
     DoubleForwardSlashSegment=StringParser(
         "//", SymbolSegment, name="doubleforwardslash", type="statement_terminator"

test/fixtures/dialects/mysql/create_table_constraint_unique.sql

@@ -0,0 +1,9 @@
+CREATE TABLE a(
+    a INT NOT NULL,
+    UNIQUE (a),
+    UNIQUE idx_c(a),
+    UNIQUE KEY (a),
+    UNIQUE KEY idx_a(a),
+    UNIQUE INDEX (a),
+    UNIQUE INDEX idx_b(a)
+)

test/fixtures/dialects/mysql/create_table_constraint_unique.yml

@@ -0,0 +1,81 @@
+# YML test files are auto-generated from SQL files and should not be edited by
+# hand. To help enforce this, the "hash" field in the file must match a hash
+# computed by SQLFluff when running the tests. Please run
+# `python test/generate_parse_fixture_yml.py`  to generate them after adding or
+# altering SQL files.
+_hash: 89f5acbd9b7f7b2a7f14cc5faa9666129d97246fbc7eb70c4d34e8c193eec5ab
+file:
+  statement:
+    create_table_statement:
+    - keyword: CREATE
+    - keyword: TABLE
+    - table_reference:
+        identifier: a
+    - bracketed:
+      - start_bracket: (
+      - column_definition:
+          identifier: a
+          data_type:
+            data_type_identifier: INT
+          column_constraint_segment:
+          - keyword: NOT
+          - keyword: 'NULL'
+      - comma: ','
+      - table_constraint_segment:
+          keyword: UNIQUE
+          bracketed:
+            start_bracket: (
+            column_reference:
+              identifier: a
+            end_bracket: )
+      - comma: ','
+      - table_constraint_segment:
+          keyword: UNIQUE
+          object_reference:
+            identifier: idx_c
+          bracketed:
+            start_bracket: (
+            column_reference:
+              identifier: a
+            end_bracket: )
+      - comma: ','
+      - table_constraint_segment:
+        - keyword: UNIQUE
+        - keyword: KEY
+        - bracketed:
+            start_bracket: (
+            column_reference:
+              identifier: a
+            end_bracket: )
+      - comma: ','
+      - table_constraint_segment:
+        - keyword: UNIQUE
+        - keyword: KEY
+        - object_reference:
+            identifier: idx_a
+        - bracketed:
+            start_bracket: (
+            column_reference:
+              identifier: a
+            end_bracket: )
+      - comma: ','
+      - table_constraint_segment:
+        - keyword: UNIQUE
+        - keyword: INDEX
+        - bracketed:
+            start_bracket: (
+            column_reference:
+              identifier: a
+            end_bracket: )
+      - comma: ','
+      - table_constraint_segment:
+        - keyword: UNIQUE
+        - keyword: INDEX
+        - object_reference:
+            identifier: idx_b
+        - bracketed:
+            start_bracket: (
+            column_reference:
+              identifier: a
+            end_bracket: )
+      - end_bracket: )