Repository: tobymao/sqlglot
Issue category: Feature Request

feat: support for mysql group_concat with multi columns
**Is your feature request related to a problem? Please describe.**

```python 
import sqlglot
sql = "select a, group_concat(b, ' ', c SEPARATOR ',') from table group by a;"
sqlglot.parse_one(sql, read="mysql")
# or 
sqlglot.transpile(sql, read="mysql", write="postgres", pretty=True)

```

```
sqlglot/parser.py:1255, in Parser.raise_error(self, message, token)
   1243 error = ParseError.new(
   1244     f"{message}. Line {token.line}, Col: {token.col}.\n"
   1245     f"  {start_context}\033[4m{highlight}\033[0m{end_context}",
   (...)
   1251     end_context=end_context,
   1252 )
   1254 if self.error_level == ErrorLevel.IMMEDIATE:
-> 1255     raise error
   1257 self.errors.append(error)

ParseError: Expecting ). Line 1, Col: 38.
```


**Describe the solution you'd like**

transpile the sql to postgresql
output:
select a STRING_AGG(b || ' ' || c, ',')  from table group  a;

**Additional context**
Add any other context or screenshots about the feature request here.



Functions to edit:
sqlglot/dialects/mysql.py:MySQL

Patch:
diff --git a/sqlglot/dialects/mysql.py b/sqlglot/dialects/mysql.py
index 750d4a36f3..b3c45b1de2 100644
--- a/sqlglot/dialects/mysql.py
+++ b/sqlglot/dialects/mysql.py
@@ -319,11 +319,7 @@ class Parser(parser.Parser):
         FUNCTION_PARSERS = {
             **parser.Parser.FUNCTION_PARSERS,
             "CHAR": lambda self: self._parse_chr(),
-            "GROUP_CONCAT": lambda self: self.expression(
-                exp.GroupConcat,
-                this=self._parse_lambda(),
-                separator=self._match(TokenType.SEPARATOR) and self._parse_field(),
-            ),
+            "GROUP_CONCAT": lambda self: self._parse_group_concat(),
             # https://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html#function_values
             "VALUES": lambda self: self.expression(
                 exp.Anonymous, this="VALUES", expressions=[self._parse_id_var()]
@@ -617,6 +613,39 @@ def _parse_chr(self) -> t.Optional[exp.Expression]:
 
             return self.expression(exp.Chr, **kwargs)
 
+        def _parse_group_concat(self) -> t.Optional[exp.Expression]:
+            def concat_exprs(
+                node: t.Optional[exp.Expression], exprs: t.List[exp.Expression]
+            ) -> exp.Expression:
+                if isinstance(node, exp.Distinct) and len(node.expressions) > 1:
+                    concat_exprs = [
+                        self.expression(exp.Concat, expressions=node.expressions, safe=True)
+                    ]
+                    node.set("expressions", concat_exprs)
+                    return node
+                if len(exprs) == 1:
+                    return exprs[0]
+                return self.expression(exp.Concat, expressions=args, safe=True)
+
+            args = self._parse_csv(self._parse_lambda)
+
+            if args:
+                order = args[-1] if isinstance(args[-1], exp.Order) else None
+
+                if order:
+                    # Order By is the last (or only) expression in the list and has consumed the 'expr' before it,
+                    # remove 'expr' from exp.Order and add it back to args
+                    args[-1] = order.this
+                    order.set("this", concat_exprs(order.this, args))
+
+                this = order or concat_exprs(args[0], args)
+            else:
+                this = None
+
+            separator = self._parse_field() if self._match(TokenType.SEPARATOR) else None
+
+            return self.expression(exp.GroupConcat, this=this, separator=separator)
+
     class Generator(generator.Generator):
         LOCKING_READS_SUPPORTED = True
         NULL_ORDERING_SUPPORTED = None

Test patch: diff --git a/tests/dialects/test_mysql.py b/tests/dialects/test_mysql.py
index 5f23c4401a..e15876012d 100644
--- a/tests/dialects/test_mysql.py
+++ b/tests/dialects/test_mysql.py
@@ -723,6 +723,52 @@ def test_mysql(self):
                 "postgres": "STRING_AGG(DISTINCT x, '' ORDER BY y DESC NULLS LAST)",
             },
         )
+        self.validate_all(
+            "GROUP_CONCAT(a, b, c SEPARATOR ',')",
+            write={
+                "mysql": "GROUP_CONCAT(CONCAT(a, b, c) SEPARATOR ',')",
+                "sqlite": "GROUP_CONCAT(a || b || c, ',')",
+                "tsql": "STRING_AGG(CONCAT(a, b, c), ',')",
+                "postgres": "STRING_AGG(CONCAT(a, b, c), ',')",
+                "presto": "ARRAY_JOIN(ARRAY_AGG(CONCAT(CAST(a AS VARCHAR), CAST(b AS VARCHAR), CAST(c AS VARCHAR))), ',')",
+            },
+        )
+        self.validate_all(
+            "GROUP_CONCAT(a, b, c SEPARATOR '')",
+            write={
+                "mysql": "GROUP_CONCAT(CONCAT(a, b, c) SEPARATOR '')",
+                "sqlite": "GROUP_CONCAT(a || b || c, '')",
+                "tsql": "STRING_AGG(CONCAT(a, b, c), '')",
+                "postgres": "STRING_AGG(CONCAT(a, b, c), '')",
+            },
+        )
+        self.validate_all(
+            "GROUP_CONCAT(DISTINCT a, b, c SEPARATOR '')",
+            write={
+                "mysql": "GROUP_CONCAT(DISTINCT CONCAT(a, b, c) SEPARATOR '')",
+                "sqlite": "GROUP_CONCAT(DISTINCT a || b || c, '')",
+                "tsql": "STRING_AGG(CONCAT(a, b, c), '')",
+                "postgres": "STRING_AGG(DISTINCT CONCAT(a, b, c), '')",
+            },
+        )
+        self.validate_all(
+            "GROUP_CONCAT(a, b, c ORDER BY d SEPARATOR '')",
+            write={
+                "mysql": "GROUP_CONCAT(CONCAT(a, b, c) ORDER BY d SEPARATOR '')",
+                "sqlite": "GROUP_CONCAT(a || b || c, '')",
+                "tsql": "STRING_AGG(CONCAT(a, b, c), '') WITHIN GROUP (ORDER BY d)",
+                "postgres": "STRING_AGG(CONCAT(a, b, c), '' ORDER BY d NULLS FIRST)",
+            },
+        )
+        self.validate_all(
+            "GROUP_CONCAT(DISTINCT a, b, c ORDER BY d SEPARATOR '')",
+            write={
+                "mysql": "GROUP_CONCAT(DISTINCT CONCAT(a, b, c) ORDER BY d SEPARATOR '')",
+                "sqlite": "GROUP_CONCAT(DISTINCT a || b || c, '')",
+                "tsql": "STRING_AGG(CONCAT(a, b, c), '') WITHIN GROUP (ORDER BY d)",
+                "postgres": "STRING_AGG(DISTINCT CONCAT(a, b, c), '' ORDER BY d NULLS FIRST)",
+            },
+        )
         self.validate_identity(
             "CREATE TABLE z (a INT) ENGINE=InnoDB AUTO_INCREMENT=1 CHARACTER SET=utf8 COLLATE=utf8_bin COMMENT='x'"
         )