feat: dollar-quoted strings support (#772)

* feat: support dollar-quoted strings

* remove comment

* unused code

* removed debugging

* added tests

* fmt

* clippy

* updated tests

tests/sqlparser_postgres.rs

@@ -2507,3 +2507,84 @@ fn parse_drop_function() {
         }
     );
 }
+
+#[test]
+fn parse_dollar_quoted_string() {
+    let sql = "SELECT $$hello$$, $tag_name$world$tag_name$, $$Foo$Bar$$, $$Foo$Bar$$col_name, $$$$, $tag_name$$tag_name$";
+
+    let stmt = pg().parse_sql_statements(sql).unwrap();
+
+    let projection = match stmt.get(0).unwrap() {
+        Statement::Query(query) => match &*query.body {
+            SetExpr::Select(select) => &select.projection,
+            _ => unreachable!(),
+        },
+        _ => unreachable!(),
+    };
+
+    assert_eq!(
+        &Expr::Value(Value::DollarQuotedString(DollarQuotedString {
+            tag: None,
+            value: "hello".into()
+        })),
+        expr_from_projection(&projection[0])
+    );
+
+    assert_eq!(
+        &Expr::Value(Value::DollarQuotedString(DollarQuotedString {
+            tag: Some("tag_name".into()),
+            value: "world".into()
+        })),
+        expr_from_projection(&projection[1])
+    );
+
+    assert_eq!(
+        &Expr::Value(Value::DollarQuotedString(DollarQuotedString {
+            tag: None,
+            value: "Foo$Bar".into()
+        })),
+        expr_from_projection(&projection[2])
+    );
+
+    assert_eq!(
+        projection[3],
+        SelectItem::ExprWithAlias {
+            expr: Expr::Value(Value::DollarQuotedString(DollarQuotedString {
+                tag: None,
+                value: "Foo$Bar".into(),
+            })),
+            alias: Ident {
+                value: "col_name".into(),
+                quote_style: None,
+            },
+        }
+    );
+
+    assert_eq!(
+        expr_from_projection(&projection[4]),
+        &Expr::Value(Value::DollarQuotedString(DollarQuotedString {
+            tag: None,
+            value: "".into()
+        })),
+    );
+
+    assert_eq!(
+        expr_from_projection(&projection[5]),
+        &Expr::Value(Value::DollarQuotedString(DollarQuotedString {
+            tag: Some("tag_name".into()),
+            value: "".into()
+        })),
+    );
+}
+
+#[test]
+fn parse_incorrect_dollar_quoted_string() {
+    let sql = "SELECT $x$hello$$";
+    assert!(pg().parse_sql_statements(sql).is_err());
+
+    let sql = "SELECT $hello$$";
+    assert!(pg().parse_sql_statements(sql).is_err());
+
+    let sql = "SELECT $$$";
+    assert!(pg().parse_sql_statements(sql).is_err());
+}