# Structure and Syntax


MatrixOne uses a MySQL-like SQL dialect. When writing statements, pay attention to comment syntax, identifier rules, and the distinction between **reserved keywords** and ordinary keywords.

## Comments

Three common comment forms are supported:

| Form | Description |
| --- | --- |
| `# ...` | From `#` to the end of the line |
| `-- ...` | Two hyphens followed by at least one space or control character, through the end of the line |
| `/* ... */` | C-style comments, which can span lines or appear inside an expression |

```sql
SELECT 100 - 99;   -- End-of-line comment
SELECT 100 - 99;   # Also an end-of-line comment
SELECT 100 /* Inline comment */ - 99;
```

## Identifiers

Identifiers such as database, table, and column names:

- Can generally contain letters, numbers, and underscores. Do not start an unquoted identifier with a number.
- Must be enclosed in backticks when a **reserved keyword** is used as an identifier, for example, `` `select` `` or `` `order` ``.
- Even in qualified names such as `database.table`, reserved keywords should be—and in MatrixOne usually must be—enclosed in backticks. Avoid reserved words when creating databases and tables.

```sql
-- Incorrect: select is a reserved keyword
CREATE TABLE select (a INT);

-- Correct
CREATE TABLE `select` (a INT);
```

**Non-reserved keywords**, including some MatrixOne extensions, can usually be used directly as identifiers. If you are unsure, enclosing them in backticks is the safest approach.

## Keywords

- **Keyword**: A word with special meaning in SQL, such as `SELECT`, `UPDATE`, or `DELETE`.
- **Reserved keyword**: Must be enclosed in backticks when used as an identifier, or parsing fails.
- **Non-reserved keyword**: Can generally be used directly as an identifier.

The MatrixOne reserved-word set is broadly similar to MySQL and includes some MatrixOne-specific reserved words. Source documentation often marks differences from MySQL, such as `CONFIG` and `CURRENT_ROLE`. The complete list can change slightly between versions. When writing DDL:

1. Prefer clear, business-specific names that are not reserved.
2. If you must use a reserved word, consistently enclose it in backticks.
3. If the client reports `SQL parser error` near a word, first check whether an unquoted reserved word is being used.

## Statement terminators and case

- Interactive clients typically terminate statements with `;`.
- Keywords are case-insensitive. Identifier case behavior is controlled by system variables such as `lower_case_table_names`. See [System Variables](system-variables.md). A common default is case-insensitive table names, with a value of 1.