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* DOCAPI-5436: Some restructurization in FixedString EN text. * DOCAPI-5436: EN review and RU translation of FixedString description. * DOCAPI-5436: Minor fixes in the FixedString docs. * DOCAPI-5436: Markup fix.
57 lines
2.2 KiB
Markdown
57 lines
2.2 KiB
Markdown
# FixedString
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A fixed-length string of `N` bytes (neither characters nor code points).
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To declare a column of `FixedString` type, use the following syntax:
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```
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<column_name> FixedString(N)
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```
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Where `N` is a natural number.
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The `FixedString` type is efficient when data has the length of precisely `N` bytes. In all other cases, it is likely to reduce efficiency.
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Examples of the values that can be efficiently stored in `FixedString`-typed columns:
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- Binary representation of IP addresses (`FixedString(16)` for IPv6).
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- Language codes (ru_RU, en_US ... ).
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- Currency codes (USD, RUB ... ).
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- Binary representation of hashes (`FixedString(16)` for MD5, `FixedString(32)` for SHA256).
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To store UUID values, use the [UUID](uuid.md) data type.
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When inserting the data, ClickHouse:
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- Complements a string with null bytes if the string contains fewer than `N` bytes.
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- Throws the `Too large value for FixedString(N)` exception if the string contains more than `N` bytes.
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When selecting the data, ClickHouse does not remove the null bytes at the end of the string. If you use the `WHERE` clause, you should add null bytes manually to match the `FixedString` value. The following example illustrates how to use the `WHERE` clause with `FixedString`.
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Let's consider the following table with the single `FixedString(2)` column:
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```
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┌─name──┐
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│ b │
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└───────┘
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```
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The query `SELECT * FROM FixedStringTable WHERE a = 'b'` does not return any data as a result. We should complement the filter pattern with null bytes.
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```
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SELECT * FROM FixedStringTable
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WHERE a = 'b\0'
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┌─a─┐
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│ b │
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└───┘
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1 rows in set. Elapsed: 0.002 sec.
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```
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This behavior differs from MySQL behavior for the `CHAR` type (where strings are padded with spaces, and the spaces are removed for output).
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Note that the length of the `FixedString(N)` value is constant. The [length](../query_language/functions/array_functions.md#array_functions-length) function returns `N` even if the `FixedString(N)` value is filled only with null bytes, but the [empty](../query_language/functions/string_functions.md#string_functions-empty) function returns `1` in this case.
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[Original article](https://clickhouse.yandex/docs/en/data_types/fixedstring/) <!--hide-->
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