Syntax

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

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Key syntax elements

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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.

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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`)

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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

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Functions and operators

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

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Expressions and aliases

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

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Examples

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Basic SELECT query

SELECT taco_name, sauce_name
FROM taco_orders
WHERE taco_type = 'spicy'
ORDER BY taco_name DESC
LIMIT 10;

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Using functions and operators

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;

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Complex expression with alias

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;

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Frequently asked questions