> ## Documentation Index
> Fetch the complete documentation index at: https://www.propeldata.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Syntax

> Learn the key elements of ClickHouse SQL syntax.

<div />

ClickHouse SQL syntax follows standard SQL conventions with some unique features. This guide covers the essential elements of ClickHouse SQL syntax.

## Key syntax elements

### Spaces and comments

* Spaces between syntactical constructions are optional but recommended for readability.
* SQL-style comments: Start with `--`, `#!`, or `#` and continue to the end of the line.
* C-style comments: Enclosed between `/*` and `*/`, can be multiline.

### Keywords and identifiers

* Keywords are case-insensitive for SQL standard terms (e.g., `SELECT`, `select`, `SeLeCt`).
* Other keywords, including function names, are case-sensitive.
* Identifiers (e.g., table names, column names) can be quoted or non-quoted.
  * Non-quoted: Must match `^[a-zA-Z_][0-9a-zA-Z_]*$`
  * Quoted: Use double quotes or backticks (e.g., `"id"`, `` `id` ``)

### Literals

* Numeric: Integers, decimals, exponential notation, hex, and binary (e.g., `1`, `0.1`, `1e100`, `0xc0fe`, `0b1101`)
* String: Enclosed in single quotes (e.g., `'Hello, world'`)
* Array: Square brackets (e.g., `[1, 2, 3]`)
* Tuple: Round brackets (e.g., `(1, 'Hello', 2)`)
* NULL: Represents a missing value

### Functions and operators

* Function calls: `function_name(arg1, arg2, ...)`
* Operators are converted to their corresponding functions during parsing

### Expressions and aliases

* Expressions can include functions, identifiers, literals, operators, and subqueries.
* Aliases: `expression AS alias_name`

## Examples

### Basic SELECT query

```sql theme={"system"}
SELECT taco_name, sauce_name
FROM taco_orders
WHERE taco_type = 'spicy'
ORDER BY taco_name DESC
LIMIT 10;
```

### Using functions and operators

```sql theme={"system"}
SELECT
  toDate(order_time) AS order_date,
  count() AS taco_count,
  avg(taco_price) AS average_price
FROM taco_sales
GROUP BY order_date
HAVING taco_count > 100
ORDER BY order_date;
```

### Complex expression with alias

```sql theme={"system"}
SELECT
  (count() * 100 / sum(count()) OVER ()) AS taco_percentage,
  toStartOfMonth(order_time) AS month
FROM taco_orders
GROUP BY month
ORDER BY taco_percentage DESC;
```

## Frequently asked questions

<AccordionGroup>
  <Accordion title="Are ClickHouse keywords case-sensitive?">
    Keywords corresponding to SQL standards are case-insensitive (e.g., SELECT, FROM, WHERE). However, other keywords, including function names, are case-sensitive.
  </Accordion>

  <Accordion title="How do I comment my ClickHouse SQL queries?">
    You can use SQL-style comments (starting with `--`, `#!`, or `#`) for single-line comments, or C-style comments (`/* */`) for multi-line comments.
  </Accordion>

  <Accordion title="Can I use spaces freely in my ClickHouse queries?">
    Yes, spaces between syntactical constructions are optional. You can use them freely to improve the readability of your queries.
  </Accordion>

  <Accordion title="What types of literals does ClickHouse support?">
    ClickHouse supports numeric literals (integers, decimals, exponential notation, hex, and binary), string literals (enclosed in single quotes), array literals (using square brackets), tuple literals (using round brackets), and NULL.
  </Accordion>
</AccordionGroup>