---
title: FixedString Data Type reference
meta:
    description: Fixed-length string of N bytes.
---

# FixedString(N)

The `FixedString(N)` data type stores strings of a precise, fixed length of `N` bytes. Unlike variable-length strings, `FixedString` values always occupy `N` bytes, regardless of the actual content.

When a Data Source column is defined as `FixedString(N)`, it means the column is designed to hold string values that are exactly `N` bytes long. `N` must be a positive integer.

This type is most efficient when the data naturally fits the exact `N` byte length. Using `FixedString` for data that frequently varies in length can lead to inefficiencies.

Common use cases for `FixedString(N)` include:

*   Binary representations of IP addresses (e.g., `FixedString(16)` for IPv6).
*   Standardized codes like language codes (`en_US`) or currency codes (`USD`).
*   Binary representations of cryptographic hashes (e.g., `FixedString(16)` for MD5, `FixedString(32)` for SHA256).

For storing UUID values, it is recommended to use the dedicated `UUID` data type.

When a string value is inserted into a `FixedString(N)` column:

*   If the string contains fewer than `N` bytes, it is padded with null bytes (`\0`) at the end until it reaches `N` bytes.
*   If the string contains more than `N` bytes, an error is thrown, as the value exceeds the defined fixed length.

When querying data from a `FixedString(N)` column, the trailing null bytes are not automatically removed. This behavior is important when filtering data. If you use a `WHERE` clause to match a `FixedString` value that was padded, you must include the null bytes in your filter pattern.

### Example: Filtering FixedString values

Consider a `FixedString(2)` column. If you insert the string `'b'`, it will be stored as `'b\0'`.

```sql
SELECT CAST('b' AS FixedString(2)) AS my_fixed_string
WHERE my_fixed_string = 'b'
```

This query returns an empty result because `'b'` does not exactly match `'b\0'`. To match the stored value, you must include the padding:

```sql
SELECT CAST('b' AS FixedString(2)) AS my_fixed_string
WHERE my_fixed_string = 'b\0'
```

Result:

```result
┌─my_fixed_string─┐
│ b\0             │
└─────────────────┘
```

The `length` function will always return `N` for a `FixedString(N)` value, even if it's mostly filled with null bytes. However, the `empty` function will return `1` (true) if the `FixedString` value consists entirely of null bytes.