> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/the-sql-language/functions/functions-bitstring.md).

# 9.6. Bit String Functions and Operators \#

This section describes functions and operators for examining and manipulating bit strings, that is values of the types `bit` and `bit varying`. (While only type `bit` is mentioned in these tables, values of type `bit varying` can be used interchangeably.) Bit strings support the usual comparison operators shown in [Table 9.1](/the-sql-language/functions/functions-comparison.md#FUNCTIONS-COMPARISON-OP-TABLE), as well as the operators shown in [Table 9.14](#FUNCTIONS-BIT-STRING-OP-TABLE).

**Table 9.14. Bit String Operators**

| <p>Operator</p><p>Description</p><p>Example(s)</p>                                                                                                                                                 |                                                                                                                    |                                                                                      |   |                                           |
| -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------ | - | ----------------------------------------- |
| <p><code>bit</code> <code>                                                                                                                                                                         |                                                                                                                    | </code> <code>bit</code> → <code>bit</code></p><p>Concatenation</p><p><code>B'10001' |   | B'011'</code> → <code>10001011</code></p> |
| <p><code>bit</code> <code>&</code> <code>bit</code> → <code>bit</code></p><p>Bitwise AND (inputs must be of equal length)</p><p><code>B'10001' & B'01101'</code> → <code>00001</code></p>          |                                                                                                                    |                                                                                      |   |                                           |
| <p><code>bit</code> <code>                                                                                                                                                                         | </code> <code>bit</code> → <code>bit</code></p><p>Bitwise OR (inputs must be of equal length)</p><p><code>B'10001' | B'01101'</code> → <code>11101</code></p>                                             |   |                                           |
| <p><code>bit</code> <code>#</code> <code>bit</code> → <code>bit</code></p><p>Bitwise exclusive OR (inputs must be of equal length)</p><p><code>B'10001' # B'01101'</code> → <code>11100</code></p> |                                                                                                                    |                                                                                      |   |                                           |
| <p><code>~~</code> <code>bit</code> → <code>bit</code></p><p>Bitwise NOT</p><p><code>~~ B'10001'</code> → <code>01110</code></p>                                                                   |                                                                                                                    |                                                                                      |   |                                           |
| <p><code>bit</code> <code><<</code> <code>integer</code> → <code>bit</code></p><p>Bitwise shift left (string length is preserved)</p><p><code>B'10001' << 3</code> → <code>01000</code></p>        |                                                                                                                    |                                                                                      |   |                                           |
| <p><code>bit</code> <code>>></code> <code>integer</code> → <code>bit</code></p><p>Bitwise shift right (string length is preserved)</p><p><code>B'10001' >> 2</code> → <code>00100</code></p>       |                                                                                                                    |                                                                                      |   |                                           |

<br>

Some of the functions available for binary strings are also available for bit strings, as shown in [Table 9.15](#FUNCTIONS-BIT-STRING-TABLE).

**Table 9.15. Bit String Functions**

| <p>Function</p><p>Description</p><p>Example(s)</p>                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| <p><code>bit\_count</code> ( <code>bit</code> ) → <code>bigint</code></p><p>Returns the number of bits set in the bit string (also known as “popcount”).</p><p><code>bit\_count(B'10111')</code> → <code>4</code></p>                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
| <p><code>bit\_length</code> ( <code>bit</code> ) → <code>integer</code></p><p>Returns number of bits in the bit string.</p><p><code>bit\_length(B'10111')</code> → <code>5</code></p>                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
| <p><code>length</code> ( <code>bit</code> ) → <code>integer</code></p><p>Returns number of bits in the bit string.</p><p><code>length(B'10111')</code> → <code>5</code></p>                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| <p><code>octet\_length</code> ( <code>bit</code> ) → <code>integer</code></p><p>Returns number of bytes in the bit string.</p><p><code>octet\_length(B'1011111011')</code> → <code>2</code></p>                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| <p><code>overlay</code> ( <em><code>bits</code></em> <code>bit</code> <code>PLACING</code> <em><code>newsubstring</code></em> <code>bit</code> <code>FROM</code> <em><code>start</code></em> <code>integer</code> \[ <code>FOR</code> <em><code>count</code></em> <code>integer</code> ] ) → <code>bit</code></p><p>Replaces the substring of <em><code>bits</code></em> that starts at the <em><code>start</code></em>'th bit and extends for <em><code>count</code></em> bits with <em><code>newsubstring</code></em>. If <em><code>count</code></em> is omitted, it defaults to the length of <em><code>newsubstring</code></em>.</p><p><code>overlay(B'01010101010101010' placing B'11111' from 2 for 3)</code> → <code>0111110101010101010</code></p> |
| <p><code>position</code> ( <em><code>substring</code></em> <code>bit</code> <code>IN</code> <em><code>bits</code></em> <code>bit</code> ) → <code>integer</code></p><p>Returns first starting index of the specified <em><code>substring</code></em> within <em><code>bits</code></em>, or zero if it's not present.</p><p><code>position(B'010' in B'000001101011')</code> → <code>8</code></p>                                                                                                                                                                                                                                                                                                                                                           |
| <p><code>substring</code> ( <em><code>bits</code></em> <code>bit</code> \[ <code>FROM</code> <em><code>start</code></em> <code>integer</code> ] \[ <code>FOR</code> <em><code>count</code></em> <code>integer</code> ] ) → <code>bit</code></p><p>Extracts the substring of <em><code>bits</code></em> starting at the <em><code>start</code></em>'th bit if that is specified, and stopping after <em><code>count</code></em> bits if that is specified. Provide at least one of <em><code>start</code></em> and <em><code>count</code></em>.</p><p><code>substring(B'110010111111' from 3 for 2)</code> → <code>00</code></p>                                                                                                                            |
| <p><code>get\_bit</code> ( <em><code>bits</code></em> <code>bit</code>, <em><code>n</code></em> <code>integer</code> ) → <code>integer</code></p><p>Extracts <em><code>n</code></em>'th bit from bit string; the first (leftmost) bit is bit 0.</p><p><code>get\_bit(B'101010101010101010', 6)</code> → <code>1</code></p>                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| <p><code>set\_bit</code> ( <em><code>bits</code></em> <code>bit</code>, <em><code>n</code></em> <code>integer</code>, <em><code>newvalue</code></em> <code>integer</code> ) → <code>bit</code></p><p>Sets <em><code>n</code></em>'th bit in bit string to <em><code>newvalue</code></em>; the first (leftmost) bit is bit 0.</p><p><code>set\_bit(B'101010101010101010', 6, 0)</code> → <code>101010001010101010</code></p>                                                                                                                                                                                                                                                                                                                                |

<br>

In addition, it is possible to cast integral values to and from type `bit`. Casting an integer to `bit(n)` copies the rightmost `n` bits. Casting an integer to a bit string width wider than the integer itself will sign-extend on the left. Some examples:

```

44::bit(10)                    0000101100
44::bit(3)                     100
cast(-44 as bit(12))           111111010100
'1110'::bit(4)::integer        14
```

Note that casting to just “bit” means casting to `bit(1)`, and so will deliver only the least significant bit of the integer.

***

原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/functions-bitstring.html)（英文原文，待翻譯）


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