opensearch-docs-cn/_search-plugins/sql/identifiers.md

---
layout: default
title: Identifiers
parent: SQL and PPL
nav_order: 6
redirect_from:
  - /search-plugins/ppl/identifiers/
---


# Identifiers

An identifier is an ID to name your database objects, such as index names, field names, aliases, and so on.
OpenSearch supports two types of identifiers: regular identifiers and delimited identifiers.

## Regular identifiers

A regular identifier is a string of characters that starts with an ASCII letter (lower or upper case).
The next character can either be a letter, digit, or underscore (_). It can't be a reserved keyword.
Whitespace and other special characters are also not allowed.

OpenSearch supports the following regular identifiers:

1. Identifiers prefixed by a dot `.` sign. Use to hide an index. For example `.opensearch-dashboards`.
2. Identifiers prefixed by an `@` sign. Use for meta fields generated by Logstash ingestion.
3. Identifiers with hyphen `-` in the middle. Use for index names with date information.
4. Identifiers with star `*` present. Use for wildcard match of index patterns.

For regular identifiers, you can use the name without any back tick or escape characters.
In this example, `source`, `fields`, `account_number`, `firstname`, and `lastname` are all identifiers. Out of these, the `source` field is a reserved identifier.

```sql
SELECT account_number, firstname, lastname FROM accounts;
```

| account_number | firstname | lastname |
:--- | :--- |
| 1  | Amber | Duke       
| 6  | Hattie | Bond
| 13 | Nanette | Bates
| 18 | Dale | Adams


## Delimited identifiers

A delimited identifier can contain special characters not allowed by a regular identifier.
You must enclose delimited identifiers with back ticks (\`\`). Back ticks differentiate the identifier from special characters.

If the index name includes a dot (`.`), for example, `log-2021.01.11`, use delimited identifiers with back ticks to escape it \``log-2021.01.11`\`.

Typical examples of using delimited identifiers:

1. Identifiers with reserved keywords.
2. Identifiers with a `.` present. Similarly, `-` to include date information.
3. Identifiers with other special characters. For example, Unicode characters.

To quote an index name with back ticks:

```sql
source=`accounts` | fields `account_number`;
```

| account_number |
:--- |
| 1  |       
| 6  |
| 13 |
| 18 |

## Case sensitivity

Identifiers are case sensitive. They must be exactly the same as what's stored in OpenSearch.

For example, if you run `source=Accounts`, you'll get an index not found exception because the actual index name is in lower case.