Functions for Working with Strings

Functions for searching in strings and for replacing in strings are described separately.

empty

Checks whether the input string is empty.

A string is considered non-empty if it contains at least one byte, even if this byte is a space or the null byte.

The function is also available for arrays and UUIDs.

Syntax

empty(x)

Arguments

• x — Input value. String.

Returned value

• Returns 1 for an empty string or 0 for a non-empty string. UInt8.

Example

SELECT empty('');

Result:

┌─empty('')─┐
│         1 │
└───────────┘

notEmpty

Checks whether the input string is non-empty.

A string is considered non-empty if it contains at least one byte, even if this byte is a space or the null byte.

The function is also available for arrays and UUIDs.

Syntax

notEmpty(x)

Arguments

• x — Input value. String.

Returned value

• Returns 1 for a non-empty string or 0 for an empty string string. UInt8.

Example

SELECT notEmpty('text');

Result:

┌─notEmpty('text')─┐
│                1 │
└──────────────────┘

length

Returns the length of a string in bytes rather than in characters or Unicode code points. The function also works for arrays.

Alias: OCTET_LENGTH

Syntax

length(s)

Parameters

• s: An input string or array. String/Array.

Returned value

• Length of the string or array s in bytes. UInt64.

Example

Query:

SELECT length('Hello, world!');

Result:

┌─length('Hello, world!')─┐
│                      13 │
└─────────────────────────┘

Query:

SELECT length([1, 2, 3, 4]);

Result:

┌─length([1, 2, 3, 4])─┐
│                    4 │
└──────────────────────┘

lengthUTF8

Returns the length of a string in Unicode code points rather than in bytes or characters. It assumes that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.

Aliases:

• CHAR_LENGTH
• CHARACTER_LENGTH

Syntax

lengthUTF8(s)

Parameters

• s: String containing valid UTF-8 encoded text. String.

Returned value

• Length of the string s in Unicode code points. UInt64.

Example

Query:

SELECT lengthUTF8('Здравствуй, мир!');

Result:

┌─lengthUTF8('Здравствуй, мир!')─┐
│                             16 │
└────────────────────────────────┘

left

Returns a substring of string s with a specified offset starting from the left.

Syntax

left(s, offset)

Parameters

• s: The string to calculate a substring from. String or FixedString.
• offset: The number of bytes of the offset. UInt*.

Returned value

• For positive offset: A substring of s with offset many bytes, starting from the left of the string.
• For negative offset: A substring of s with length(s) - |offset| bytes, starting from the left of the string.
• An empty string if length is 0.

Example

Query:

SELECT left('Hello', 3);

Result:

Hel

Query:

SELECT left('Hello', -3);

Result:

He

leftUTF8

Returns a substring of a UTF-8 encoded string s with a specified offset starting from the left.

Syntax

leftUTF8(s, offset)

Parameters

• s: The UTF-8 encoded string to calculate a substring from. String or FixedString.
• offset: The number of bytes of the offset. UInt*.

Returned value

• For positive offset: A substring of s with offset many bytes, starting from the left of the string.
• For negative offset: A substring of s with length(s) - |offset| bytes, starting from the left of the string.
• An empty string if length is 0.

Example

Query:

SELECT leftUTF8('Привет', 4);

Result:

Прив

Query:

SELECT leftUTF8('Привет', -4);

Result:

Пр

leftPad

Pads a string from the left with spaces or with a specified string (multiple times, if needed) until the resulting string reaches the specified length.

Syntax

leftPad(string, length[, pad_string])

Alias: LPAD

Arguments

• string — Input string that should be padded. String.
• length — The length of the resulting string. UInt or Int. If the value is smaller than the input string length, then the input string is shortened to length characters.
• pad_string — The string to pad the input string with. String. Optional. If not specified, then the input string is padded with spaces.

Returned value

• A left-padded string of the given length. String.

Example

SELECT leftPad('abc', 7, '*'), leftPad('def', 7);

Result:

┌─leftPad('abc', 7, '*')─┬─leftPad('def', 7)─┐
│ ****abc                │     def           │
└────────────────────────┴───────────────────┘

leftPadUTF8

Pads the string from the left with spaces or a specified string (multiple times, if needed) until the resulting string reaches the given length. Unlike leftPad which measures the string length in bytes, the string length is measured in code points.

Syntax

leftPadUTF8(string, length[, pad_string])

Arguments

• string — Input string that should be padded. String.
• length — The length of the resulting string. UInt or Int. If the value is smaller than the input string length, then the input string is shortened to length characters.
• pad_string — The string to pad the input string with. String. Optional. If not specified, then the input string is padded with spaces.

Returned value

• A left-padded string of the given length. String.

Example

SELECT leftPadUTF8('абвг', 7, '*'), leftPadUTF8('дежз', 7);

Result:

┌─leftPadUTF8('абвг', 7, '*')─┬─leftPadUTF8('дежз', 7)─┐
│ ***абвг                     │    дежз                │
└─────────────────────────────┴────────────────────────┘

right

Returns a substring of string s with a specified offset starting from the right.

Syntax

right(s, offset)

Parameters

• s: The string to calculate a substring from. String or FixedString.
• offset: The number of bytes of the offset. UInt*.

Returned value

• For positive offset: A substring of s with offset many bytes, starting from the right of the string.
• For negative offset: A substring of s with length(s) - |offset| bytes, starting from the right of the string.
• An empty string if length is 0.

Example

Query:

SELECT right('Hello', 3);

Result:

llo

Query:

SELECT right('Hello', -3);

Result:

lo

rightUTF8

Returns a substring of UTF-8 encoded string s with a specified offset starting from the right.

Syntax

rightUTF8(s, offset)

Parameters

• s: The UTF-8 encoded string to calculate a substring from. String or FixedString.
• offset: The number of bytes of the offset. UInt*.

Returned value

• For positive offset: A substring of s with offset many bytes, starting from the right of the string.
• For negative offset: A substring of s with length(s) - |offset| bytes, starting from the right of the string.
• An empty string if length is 0.

Example

Query:

SELECT rightUTF8('Привет', 4);

Result:

ивет

Query:

SELECT rightUTF8('Привет', -4);

Result:

ет

rightPad

Pads a string from the right with spaces or with a specified string (multiple times, if needed) until the resulting string reaches the specified length.

Syntax

rightPad(string, length[, pad_string])

Alias: RPAD

Arguments

• string — Input string that should be padded. String.
• length — The length of the resulting string. UInt or Int. If the value is smaller than the input string length, then the input string is shortened to length characters.
• pad_string — The string to pad the input string with. String. Optional. If not specified, then the input string is padded with spaces.

Returned value

• A left-padded string of the given length. String.

Example

SELECT rightPad('abc', 7, '*'), rightPad('abc', 7);

Result:

┌─rightPad('abc', 7, '*')─┬─rightPad('abc', 7)─┐
│ abc****                 │ abc                │
└─────────────────────────┴────────────────────┘

rightPadUTF8

Pads the string from the right with spaces or a specified string (multiple times, if needed) until the resulting string reaches the given length. Unlike rightPad which measures the string length in bytes, the string length is measured in code points.

Syntax

rightPadUTF8(string, length[, pad_string])

Arguments

• string — Input string that should be padded. String.
• length — The length of the resulting string. UInt or Int. If the value is smaller than the input string length, then the input string is shortened to length characters.
• pad_string — The string to pad the input string with. String. Optional. If not specified, then the input string is padded with spaces.

Returned value

• A right-padded string of the given length. String.

Example

SELECT rightPadUTF8('абвг', 7, '*'), rightPadUTF8('абвг', 7);

Result:

┌─rightPadUTF8('абвг', 7, '*')─┬─rightPadUTF8('абвг', 7)─┐
│ абвг***                      │ абвг                    │
└──────────────────────────────┴─────────────────────────┘

lower

Converts the ASCII Latin symbols in a string to lowercase.

*Syntax**

lower(input)

Alias: lcase

Parameters

• input: A string type String.

Returned value

• A String data type value.

Example

Query:

SELECT lower('CLICKHOUSE');

┌─lower('CLICKHOUSE')─┐
│ clickhouse          │
└─────────────────────┘

upper

Converts the ASCII Latin symbols in a string to uppercase.

Syntax

upper(input)

Alias: ucase

Parameters

• input: A string type String.

Returned value

• A String data type value.

Examples

Query:

SELECT upper('clickhouse');

┌─upper('clickhouse')─┐
│ CLICKHOUSE          │
└─────────────────────┘

lowerUTF8

Converts a string to lowercase, assuming that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.

Does not detect the language, e.g. for Turkish the result might not be exactly correct (i/İ vs. i/I).

If the length of the UTF-8 byte sequence is different for upper and lower case of a code point, the result may be incorrect for this code point.

upperUTF8

Converts a string to uppercase, assuming that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.

Does not detect the language, e.g. for Turkish the result might not be exactly correct (i/İ vs. i/I).

If the length of the UTF-8 byte sequence is different for upper and lower case of a code point, the result may be incorrect for this code point.

Syntax

upperUTF8(input)

Parameters

• input: A string type String.

Returned value

• A String data type value.

Example

Query:

SELECT upperUTF8('München') as Upperutf8;

┌─Upperutf8─┐
│ MÜNCHEN   │
└───────────┘

isValidUTF8

Returns 1, if the set of bytes constitutes valid UTF-8-encoded text, otherwise 0.

toValidUTF8

Replaces invalid UTF-8 characters by the � (U+FFFD) character. All running in a row invalid characters are collapsed into the one replacement character.

Syntax

toValidUTF8(input_string)

Arguments

• input_string — Any set of bytes represented as the String data type object.

Returned value

• A valid UTF-8 string.

Example

SELECT toValidUTF8('\x61\xF0\x80\x80\x80b');

┌─toValidUTF8('a����b')─┐
│ a�b                   │
└───────────────────────┘

repeat

Concatenates a string as many times with itself as specified.

Syntax

repeat(s, n)

Alias: REPEAT

Arguments

• s — The string to repeat. String.
• n — The number of times to repeat the string. UInt or Int.

Returned value

A string containing string s repeated n times. If n <= 0, the function returns the empty string. String.

Example

SELECT repeat('abc', 10);

Result:

┌─repeat('abc', 10)──────────────┐
│ abcabcabcabcabcabcabcabcabcabc │
└────────────────────────────────┘

space

Concatenates a space ( ) as many times with itself as specified.

Syntax

space(n)

Alias: SPACE.

Arguments

• n — The number of times to repeat the space. UInt or Int.

Returned value

The string containing string repeated n times. If n <= 0, the function returns the empty string. String.

Example

Query:

SELECT space(3);

Result:

┌─space(3) ────┐
│              │
└──────────────┘

reverse

Reverses the sequence of bytes in a string.

reverseUTF8

Reverses a sequence of Unicode code points in a string. Assumes that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.

concat

Concatenates the given arguments.

Syntax

concat(s1, s2, ...)

Arguments

At least one value of arbitrary type.

Arguments which are not of types String or FixedString are converted to strings using their default serialization. As this decreases performance, it is not recommended to use non-String/FixedString arguments.

Returned values

The String created by concatenating the arguments.

If any of arguments is NULL, the function returns NULL.

Example

Query:

SELECT concat('Hello, ', 'World!');

Result:

┌─concat('Hello, ', 'World!')─┐
│ Hello, World!               │
└─────────────────────────────┘

Query:

SELECT concat(42, 144);

Result:

┌─concat(42, 144)─┐
│ 42144           │
└─────────────────┘

concatAssumeInjective

Like concat but assumes that concat(s1, s2, ...) → sn is injective. Can be used for optimization of GROUP BY.

A function is called injective if it returns for different arguments different results. In other words: different arguments never produce identical result.

Syntax

concatAssumeInjective(s1, s2, ...)

Arguments

Values of type String or FixedString.

Returned values

The String created by concatenating the arguments.

If any of argument values is NULL, the function returns NULL.

Example

Input table:

CREATE TABLE key_val(`key1` String, `key2` String, `value` UInt32) ENGINE = TinyLog;
INSERT INTO key_val VALUES ('Hello, ','World',1), ('Hello, ','World',2), ('Hello, ','World!',3), ('Hello',', World!',2);
SELECT * from key_val;

┌─key1────┬─key2─────┬─value─┐
│ Hello,  │ World    │     1 │
│ Hello,  │ World    │     2 │
│ Hello,  │ World!   │     3 │
│ Hello   │ , World! │     2 │
└─────────┴──────────┴───────┘

SELECT concat(key1, key2), sum(value) FROM key_val GROUP BY concatAssumeInjective(key1, key2);

Result:

┌─concat(key1, key2)─┬─sum(value)─┐
│ Hello, World!      │          3 │
│ Hello, World!      │          2 │
│ Hello, World       │          3 │
└────────────────────┴────────────┘

concatWithSeparator

Concatenates the given strings with a given separator.

Syntax

concatWithSeparator(sep, expr1, expr2, expr3...)

Alias: concat_ws

Arguments

• sep — separator. Const String or FixedString.
• exprN — expression to be concatenated. Arguments which are not of types String or FixedString are converted to strings using their default serialization. As this decreases performance, it is not recommended to use non-String/FixedString arguments.

Returned values

The String created by concatenating the arguments.

If any of the argument values is NULL, the function returns NULL.

Example

SELECT concatWithSeparator('a', '1', '2', '3', '4')

Result:

┌─concatWithSeparator('a', '1', '2', '3', '4')─┐
│ 1a2a3a4                                      │
└──────────────────────────────────────────────┘

concatWithSeparatorAssumeInjective

Like concatWithSeparator but assumes that concatWithSeparator(sep, expr1, expr2, expr3...) → result is injective. Can be used for optimization of GROUP BY.

A function is called injective if it returns for different arguments different results. In other words: different arguments never produce identical result.

substring

Returns the substring of a string s which starts at the specified byte index offset. Byte counting starts from 1. If offset is 0, an empty string is returned. If offset is negative, the substring starts pos characters from the end of the string, rather than from the beginning. An optional argument length specifies the maximum number of bytes the returned substring may have.

Syntax

substring(s, offset[, length])

Alias:

• substr
• mid
• byteSlice

Arguments

• s — The string to calculate a substring from. String, FixedString or Enum
• offset — The starting position of the substring in s . (U)Int*.
• length — The maximum length of the substring. (U)Int*. Optional.

Returned value

A substring of s with length many bytes, starting at index offset. String.

Example

SELECT 'database' AS db, substr(db, 5), substr(db, 5, 1)

Result:

┌─db───────┬─substring('database', 5)─┬─substring('database', 5, 1)─┐
│ database │ base                     │ b                           │
└──────────┴──────────────────────────┴─────────────────────────────┘

substringUTF8

Returns the substring of a string s which starts at the specified byte index offset for Unicode code points. Byte counting starts from 1. If offset is 0, an empty string is returned. If offset is negative, the substring starts pos characters from the end of the string, rather than from the beginning. An optional argument length specifies the maximum number of bytes the returned substring may have.

Assumes that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.

Syntax

substringUTF8(s, offset[, length])

Arguments

• s: The string to calculate a substring from. String, FixedString or Enum
• offset: The starting position of the substring in s . (U)Int*.
• length: The maximum length of the substring. (U)Int*. Optional.

Returned value

A substring of s with length many bytes, starting at index offset.

Implementation details

Assumes that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.

Example

SELECT 'Täglich grüßt das Murmeltier.' AS str,
       substringUTF8(str, 9),
       substringUTF8(str, 9, 5)

Täglich grüßt das Murmeltier.   grüßt das Murmeltier.   grüßt

substringIndex

Returns the substring of s before count occurrences of the delimiter delim, as in Spark or MySQL.

Syntax

substringIndex(s, delim, count)

Alias: SUBSTRING_INDEX

Arguments

• s: The string to extract substring from. String.
• delim: The character to split. String.
• count: The number of occurrences of the delimiter to count before extracting the substring. If count is positive, everything to the left of the final delimiter (counting from the left) is returned. If count is negative, everything to the right of the final delimiter (counting from the right) is returned. UInt or Int

Example

SELECT substringIndex('www.clickhouse.com', '.', 2)

Result:

┌─substringIndex('www.clickhouse.com', '.', 2)─┐
│ www.clickhouse                               │
└──────────────────────────────────────────────┘

substringIndexUTF8

Returns the substring of s before count occurrences of the delimiter delim, specifically for Unicode code points.

Assumes that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.

Syntax

substringIndexUTF8(s, delim, count)

Arguments

• s: The string to extract substring from. String.
• delim: The character to split. String.
• count: The number of occurrences of the delimiter to count before extracting the substring. If count is positive, everything to the left of the final delimiter (counting from the left) is returned. If count is negative, everything to the right of the final delimiter (counting from the right) is returned. UInt or Int

Returned value

A substring String of s before count occurrences of delim.

Implementation details

Assumes that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.

Example

SELECT substringIndexUTF8('www.straßen-in-europa.de', '.', 2)

www.straßen-in-europa

appendTrailingCharIfAbsent

Appends character c to string s if s is non-empty and does not end with character c.

Syntax

appendTrailingCharIfAbsent(s, c)

convertCharset

Returns string s converted from the encoding from to encoding to.

Syntax

convertCharset(s, from, to)

base58Encode

Encodes a String using Base58 in the "Bitcoin" alphabet.

Syntax

base58Encode(plaintext)

Arguments

• plaintext — String column or constant.

Returned value

• A string containing the encoded value of the argument. String.

Example

SELECT base58Encode('Encoded');

Result:

┌─base58Encode('Encoded')─┐
│ 3dc8KtHrwM              │
└─────────────────────────┘

base58Decode

Accepts a String and decodes it using Base58 encoding scheme using "Bitcoin" alphabet.

Syntax

base58Decode(encoded)

Arguments

• encoded — String column or constant. If the string is not a valid Base58-encoded value, an exception is thrown.

Returned value

• A string containing the decoded value of the argument. String.

Example

SELECT base58Decode('3dc8KtHrwM');

Result:

┌─base58Decode('3dc8KtHrwM')─┐
│ Encoded                    │
└────────────────────────────┘

tryBase58Decode

Like base58Decode but returns an empty string in case of error.

Syntax

tryBase58Decode(encoded)

Parameters

• encoded: String column or constant. If the string is not a valid Base58-encoded value, returns an empty string in case of error.

Returned value

• A string containing the decoded value of the argument.

Examples

Query:

SELECT tryBase58Decode('3dc8KtHrwM') as res, tryBase58Decode('invalid') as res_invalid;

┌─res─────┬─res_invalid─┐
│ Encoded │             │
└─────────┴─────────────┘

base64Encode

Encodes a String or FixedString as base64.

Alias: TO_BASE64.

base64Decode

Decodes a base64-encoded String or FixedString. Throws an exception in case of error.

Alias: FROM_BASE64.

tryBase64Decode

Like base64Decode but returns an empty string in case of error.

Syntax

tryBase64Decode(encoded)

Parameters

• encoded: String column or constant. If the string is not a valid Base58-encoded value, returns an empty string in case of error.

Examples

Query:

SELECT tryBase64Decode('RW5jb2RlZA==') as res, tryBase64Decode('invalid') as res_invalid;

┌─res─────┬─res_invalid─┐
│ Encoded │             │
└─────────┴─────────────┘

endsWith

Returns whether string str ends with suffix.

Syntax

endsWith(str, suffix)

endsWithUTF8

Returns whether string str ends with suffix, the difference between endsWithUTF8 and endsWith is that endsWithUTF8 match str and suffix by UTF-8 characters.

Syntax

endsWithUTF8(str, suffix)

Example

SELECT endsWithUTF8('中国', '\xbd'), endsWith('中国', '\xbd')

Result:

┌─endsWithUTF8('中国', '½')─┬─endsWith('中国', '½')─┐
│                        0 │                    1 │
└──────────────────────────┴──────────────────────┘

startsWith

Returns whether string str starts with prefix.

Syntax

startsWith(str, prefix)

Example

SELECT startsWith('Spider-Man', 'Spi');

startsWithUTF8

Available in version 23.8 and later

Returns whether string str starts with prefix, the difference between startsWithUTF8 and startsWith is that startsWithUTF8 match str and suffix by UTF-8 characters.

Example

SELECT startsWithUTF8('中国', '\xe4'), startsWith('中国', '\xe4')

Result:

┌─startsWithUTF8('中国', '⥩─┬─startsWith('中国', '⥩─┐
│                          0 │                      1 │
└────────────────────────────┴────────────────────────┘

trim

Removes the specified characters from the start or end of a string. If not specified otherwise, the function removes whitespace (ASCII-character 32).

Syntax

trim([[LEADING|TRAILING|BOTH] trim_character FROM] input_string)

Arguments

• trim_character — Specified characters for trim. String.
• input_string — String for trim. String.

Returned value

A string without leading and/or trailing specified characters. String.

Example

SELECT trim(BOTH ' ()' FROM '(   Hello, world!   )');

Result:

┌─trim(BOTH ' ()' FROM '(   Hello, world!   )')─┐
│ Hello, world!                                 │
└───────────────────────────────────────────────┘

trimLeft

Removes the consecutive occurrences of whitespace (ASCII-character 32) from the start of a string.

Syntax

trimLeft(input_string)

Alias: ltrim(input_string).

Arguments

• input_string — string to trim. String.

Returned value

A string without leading common whitespaces. String.

Example

SELECT trimLeft('     Hello, world!     ');

Result:

┌─trimLeft('     Hello, world!     ')─┐
│ Hello, world!                       │
└─────────────────────────────────────┘

trimRight

Removes the consecutive occurrences of whitespace (ASCII-character 32) from the end of a string.

Syntax

trimRight(input_string)

Alias: rtrim(input_string).

Arguments

• input_string — string to trim. String.

Returned value

A string without trailing common whitespaces. String.

Example

SELECT trimRight('     Hello, world!     ');

Result:

┌─trimRight('     Hello, world!     ')─┐
│      Hello, world!                   │
└──────────────────────────────────────┘

trimBoth

Removes the consecutive occurrences of whitespace (ASCII-character 32) from both ends of a string.

Syntax

trimBoth(input_string)

Alias: trim(input_string).

Arguments

• input_string — string to trim. String.

Returned value

A string without leading and trailing common whitespaces. String.

Example

SELECT trimBoth('     Hello, world!     ');

Result:

┌─trimBoth('     Hello, world!     ')─┐
│ Hello, world!                       │
└─────────────────────────────────────┘

CRC32

Returns the CRC32 checksum of a string using CRC-32-IEEE 802.3 polynomial and initial value 0xffffffff (zlib implementation).

The result type is UInt32.

CRC32IEEE

Returns the CRC32 checksum of a string, using CRC-32-IEEE 802.3 polynomial.

The result type is UInt32.

CRC64

Returns the CRC64 checksum of a string, using CRC-64-ECMA polynomial.

The result type is UInt64.

normalizeQuery

Replaces literals, sequences of literals and complex aliases with placeholders.

Syntax

normalizeQuery(x)

Arguments

• x — Sequence of characters. String.

Returned value

• Sequence of characters with placeholders. String.

Example

SELECT normalizeQuery('[1, 2, 3, x]') AS query;

Result:

┌─query────┐
│ [?.., x] │
└──────────┘

normalizedQueryHash

Returns identical 64bit hash values without the values of literals for similar queries. Can be helpful to analyze query log.

Syntax

normalizedQueryHash(x)

Arguments

• x — Sequence of characters. String.

Returned value

• Hash value. UInt64.

Example

SELECT normalizedQueryHash('SELECT 1 AS `xyz`') != normalizedQueryHash('SELECT 1 AS `abc`') AS res;

Result:

┌─res─┐
│   1 │
└─────┘

normalizeUTF8NFC

Converts a string to NFC normalized form, assuming the string is valid UTF8-encoded text.

Syntax

normalizeUTF8NFC(words)

Arguments

• words — UTF8-encoded input string. String.

Returned value

• String transformed to NFC normalization form. String.

Example

SELECT length('â'), normalizeUTF8NFC('â') AS nfc, length(nfc) AS nfc_len;

Result:

┌─length('â')─┬─nfc─┬─nfc_len─┐
│           2 │ â   │       2 │
└─────────────┴─────┴─────────┘

normalizeUTF8NFD

Converts a string to NFD normalized form, assuming the string is valid UTF8-encoded text.

Syntax

normalizeUTF8NFD(words)

Arguments

• words — UTF8-encoded input string. String.

Returned value

• String transformed to NFD normalization form. String.

Example

SELECT length('â'), normalizeUTF8NFD('â') AS nfd, length(nfd) AS nfd_len;

Result:

┌─length('â')─┬─nfd─┬─nfd_len─┐
│           2 │ â   │       3 │
└─────────────┴─────┴─────────┘

normalizeUTF8NFKC

Converts a string to NFKC normalized form, assuming the string is valid UTF8-encoded text.

Syntax

normalizeUTF8NFKC(words)

Arguments

• words — UTF8-encoded input string. String.

Returned value

• String transformed to NFKC normalization form. String.

Example

SELECT length('â'), normalizeUTF8NFKC('â') AS nfkc, length(nfkc) AS nfkc_len;

Result:

┌─length('â')─┬─nfkc─┬─nfkc_len─┐
│           2 │ â    │        2 │
└─────────────┴──────┴──────────┘

normalizeUTF8NFKD

Converts a string to NFKD normalized form, assuming the string is valid UTF8-encoded text.

Syntax

normalizeUTF8NFKD(words)

Arguments

• words — UTF8-encoded input string. String.

Returned value

• String transformed to NFKD normalization form. String.

Example

SELECT length('â'), normalizeUTF8NFKD('â') AS nfkd, length(nfkd) AS nfkd_len;

Result:

┌─length('â')─┬─nfkd─┬─nfkd_len─┐
│           2 │ â    │        3 │
└─────────────┴──────┴──────────┘

encodeXMLComponent

Escapes characters with special meaning in XML such that they can afterwards be place into a XML text node or attribute.

The following characters are replaced: <, &, >, ", '. Also see the list of XML and HTML character entity references.

Syntax

encodeXMLComponent(x)

Arguments

• x — An input string. String.

Returned value

• The escaped string. String.

Example

SELECT encodeXMLComponent('Hello, "world"!');
SELECT encodeXMLComponent('<123>');
SELECT encodeXMLComponent('&clickhouse');
SELECT encodeXMLComponent('\'foo\'');

Result:

Hello, "world"!
<123>
&clickhouse
'foo'

decodeXMLComponent

Un-escapes substrings with special meaning in XML. These substrings are: " & ' > <

This function also replaces numeric character references with Unicode characters. Both decimal (like ✓) and hexadecimal (✓) forms are supported.

Syntax

decodeXMLComponent(x)

Arguments

• x — An input string. String.

Returned value

• The un-escaped string. String.

Example

SELECT decodeXMLComponent(''foo'');
SELECT decodeXMLComponent('< Σ >');

Result:

'foo'
< Σ >

decodeHTMLComponent

Un-escapes substrings with special meaning in HTML. For example: ℏ > ♦ ♥ < etc.

This function also replaces numeric character references with Unicode characters. Both decimal (like ✓) and hexadecimal (✓) forms are supported.

Syntax

decodeHTMLComponent(x)

Arguments

• x — An input string. String.

Returned value

• The un-escaped string. String.

Example

SELECT decodeHTMLComponent(''CH');
SELECT decodeHTMLComponent('I♥ClickHouse');

Result:

'CH'
I♥ClickHouse'

extractTextFromHTML

This function extracts plain text from HTML or XHTML.

It does not conform 100% to the HTML, XML or XHTML specification but the implementation is reasonably accurate and fast. The rules are the following:

1. Comments are skipped. Example: <!-- test -->. Comment must end with -->. Nested comments are disallowed. Note: constructions like <!--> and <!---> are not valid comments in HTML but they are skipped by other rules.
2. CDATA is pasted verbatim. Note: CDATA is XML/XHTML-specific and processed on a "best-effort" basis.
3. script and style elements are removed with all their content. Note: it is assumed that closing tag cannot appear inside content. For example, in JS string literal has to be escaped like "<\/script>". Note: comments and CDATA are possible inside script or style - then closing tags are not searched inside CDATA. Example: <script><![CDATA[</script>]]></script>. But they are still searched inside comments. Sometimes it becomes complicated: <script>var x = "<!--"; </script> var y = "-->"; alert(x + y);</script> Note: script and style can be the names of XML namespaces - then they are not treated like usual script or style elements. Example: <script:a>Hello</script:a>. Note: whitespaces are possible after closing tag name: </script > but not before: < / script>.
4. Other tags or tag-like elements are skipped without inner content. Example: <a>.</a> Note: it is expected that this HTML is illegal: <a test=">"></a> Note: it also skips something like tags: <>, <!>, etc. Note: tag without end is skipped to the end of input: <hello
5. HTML and XML entities are not decoded. They must be processed by separate function.
6. Whitespaces in the text are collapsed or inserted by specific rules.
   • Whitespaces at the beginning and at the end are removed.
   • Consecutive whitespaces are collapsed.
   • But if the text is separated by other elements and there is no whitespace, it is inserted.
   • It may cause unnatural examples: Hello<b>world</b>, Hello<!-- -->world - there is no whitespace in HTML, but the function inserts it. Also consider: Hello<p>world</p>, Hello<br>world. This behavior is reasonable for data analysis, e.g. to convert HTML to a bag of words.
7. Also note that correct handling of whitespaces requires the support of <pre></pre> and CSS display and white-space properties.

Syntax

extractTextFromHTML(x)

Arguments

• x — input text. String.

Returned value

• Extracted text. String.

Example

The first example contains several tags and a comment and also shows whitespace processing. The second example shows CDATA and script tag processing. In the third example text is extracted from the full HTML response received by the url function.

SELECT extractTextFromHTML(' <p> A text <i>with</i><b>tags</b>. <!-- comments --> </p> ');
SELECT extractTextFromHTML('<![CDATA[The content within <b>CDATA</b>]]> <script>alert("Script");</script>');
SELECT extractTextFromHTML(html) FROM url('http://www.donothingfor2minutes.com/', RawBLOB, 'html String');

Result:

A text with tags .
The content within <b>CDATA</b>
Do Nothing for 2 Minutes 2:00 &nbsp;

ascii

Returns the ASCII code point (as Int32) of the first character of string s.

If s is empty, the result is 0. If the first character is not an ASCII character or not part of the Latin-1 supplement range of UTF-16, the result is undefined.

Syntax

ascii(s)

soundex

Returns the Soundex code of a string.

Syntax

soundex(val)

Arguments

• val - Input value. String

Returned value

• The Soundex code of the input value. String

Example

select soundex('aksel');

Result:

┌─soundex('aksel')─┐
│ A240             │
└──────────────────┘

punycodeEncode

Returns the Punycode representation of a string. The string must be UTF8-encoded, otherwise the behavior is undefined.

Syntax

punycodeEncode(val)

Arguments

• val - Input value. String

Returned value

• A Punycode representation of the input value. String

Example

select punycodeEncode('München');

Result:

┌─punycodeEncode('München')─┐
│ Mnchen-3ya                │
└───────────────────────────┘

punycodeDecode

Returns the UTF8-encoded plaintext of a Punycode-encoded string. If no valid Punycode-encoded string is given, an exception is thrown.

Syntax

punycodeEncode(val)

Arguments

• val - Punycode-encoded string. String

Returned value

• The plaintext of the input value. String

Example

select punycodeDecode('Mnchen-3ya');

Result:

┌─punycodeDecode('Mnchen-3ya')─┐
│ München                      │
└──────────────────────────────┘

tryPunycodeDecode

Like punycodeDecode but returns an empty string if no valid Punycode-encoded string is given.

idnaEncode

Returns the the ASCII representation (ToASCII algorithm) of a domain name according to the Internationalized Domain Names in Applications (IDNA) mechanism. The input string must be UTF-encoded and translatable to an ASCII string, otherwise an exception is thrown. Note: No percent decoding or trimming of tabs, spaces or control characters is performed.

Syntax

idnaEncode(val)

Arguments

• val - Input value. String

Returned value

• A ASCII representation according to the IDNA mechanism of the input value. String

Example

select idnaEncode('straße.münchen.de');

Result:

┌─idnaEncode('straße.münchen.de')─────┐
│ xn--strae-oqa.xn--mnchen-3ya.de     │
└─────────────────────────────────────┘

tryIdnaEncode

Like idnaEncode but returns an empty string in case of an error instead of throwing an exception.

idnaDecode

Returns the the Unicode (UTF-8) representation (ToUnicode algorithm) of a domain name according to the Internationalized Domain Names in Applications (IDNA) mechanism. In case of an error (e.g. because the input is invalid), the input string is returned. Note that repeated application of idnaEncode() and idnaDecode() does not necessarily return the original string due to case normalization.

Syntax

idnaDecode(val)

Arguments

• val - Input value. String

Returned value

• A Unicode (UTF-8) representation according to the IDNA mechanism of the input value. String

Example

select idnaDecode('xn--strae-oqa.xn--mnchen-3ya.de');

Result:

┌─idnaDecode('xn--strae-oqa.xn--mnchen-3ya.de')─┐
│ straße.münchen.de                             │
└───────────────────────────────────────────────┘

byteHammingDistance

Calculates the hamming distance between two byte strings.

Syntax

byteHammingDistance(string1, string2)

Examples

SELECT byteHammingDistance('karolin', 'kathrin');

Result:

┌─byteHammingDistance('karolin', 'kathrin')─┐
│                                         3 │
└───────────────────────────────────────────┘

Alias: mismatches

stringJaccardIndex

Calculates the Jaccard similarity index between two byte strings.

Syntax

stringJaccardIndex(string1, string2)

Examples

SELECT stringJaccardIndex('clickhouse', 'mouse');

Result:

┌─stringJaccardIndex('clickhouse', 'mouse')─┐
│                                       0.4 │
└───────────────────────────────────────────┘

stringJaccardIndexUTF8

Like stringJaccardIndex but for UTF8-encoded strings.

editDistance

Calculates the edit distance between two byte strings.

Syntax

editDistance(string1, string2)

Examples

SELECT editDistance('clickhouse', 'mouse');

Result:

┌─editDistance('clickhouse', 'mouse')─┐
│                                   6 │
└─────────────────────────────────────┘

Alias: levenshteinDistance

damerauLevenshteinDistance

Calculates the Damerau-Levenshtein distance between two byte strings.

Syntax

damerauLevenshteinDistance(string1, string2)

Examples

SELECT damerauLevenshteinDistance('clickhouse', 'mouse');

Result:

┌─damerauLevenshteinDistance('clickhouse', 'mouse')─┐
│                                                 6 │
└───────────────────────────────────────────────────┘

jaroSimilarity

Calculates the Jaro similarity between two byte strings.

Syntax

jaroSimilarity(string1, string2)

Examples

SELECT jaroSimilarity('clickhouse', 'click');

Result:

┌─jaroSimilarity('clickhouse', 'click')─┐
│                    0.8333333333333333 │
└───────────────────────────────────────┘

jaroWinklerSimilarity

Calculates the Jaro-Winkler similarity between two byte strings.

Syntax

jaroWinklerSimilarity(string1, string2)

Examples

SELECT jaroWinklerSimilarity('clickhouse', 'click');

Result:

┌─jaroWinklerSimilarity('clickhouse', 'click')─┐
│                           0.8999999999999999 │
└──────────────────────────────────────────────┘

initcap

Convert the first letter of each word to upper case and the rest to lower case. Words are sequences of alphanumeric characters separated by non-alphanumeric characters.

initcapUTF8

Like initcap, assuming that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.

Does not detect the language, e.g. for Turkish the result might not be exactly correct (i/İ vs. i/I).

If the length of the UTF-8 byte sequence is different for upper and lower case of a code point, the result may be incorrect for this code point.

firstLine

Returns the first line from a multi-line string.

Syntax

firstLine(val)

Arguments

• val - Input value. String

Returned value

• The first line of the input value or the whole value if there is no line separators. String

Example

select firstLine('foo\nbar\nbaz');

Result:

┌─firstLine('foo\nbar\nbaz')─┐
│ foo                        │
└────────────────────────────┘