ClickHouse substring Function

The substring function in ClickHouse is used to extract a portion of a string based on specified start position and length. It's particularly useful for string manipulation and data extraction tasks within queries.

Syntax

substring(str, start[, length])

Official Documentation

Example usage

SELECT substring('Hello, World!', 1, 5) AS result;
-- Output: Hello

SELECT substring('ClickHouse', -5) AS result;
-- Output: House

Common issues

  • If the start position is beyond the string length, an empty string is returned.
  • Negative start positions count from the end of the string.
  • If length is omitted, the function returns the substring from the start position to the end of the string.

Best practices

  • Use substring in combination with other string functions for complex text processing.
  • Be cautious with large datasets as string operations can be resource-intensive.
  • Consider using materialized columns for frequently used substring operations to improve query performance.

Frequently Asked Questions

Q: Can I use substring with non-ASCII characters?
A: Yes, substring works with UTF-8 encoded strings, including non-ASCII characters.

Q: How does substring handle NULL values?
A: If any of the arguments are NULL, substring returns NULL.

Q: Is there a performance difference between substring and left/right functions?
A: For simple extractions from the beginning or end of a string, left and right functions might be slightly more efficient.

Q: Can substring be used in WHERE clauses?
A: Yes, substring can be used in WHERE clauses for filtering based on parts of strings.

Q: How does substring behave with empty strings?
A: When applied to an empty string, substring returns an empty string regardless of the start and length parameters.

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