# 9.6. 二元字串函式及運算子

This section describes functions and operators for examining and manipulating bit strings, that is values of the types `bit` and `bit varying`. Aside from the usual comparison operators, the operators shown in [Table 9.14](https://www.postgresql.org/docs/12/functions-bitstring.html#FUNCTIONS-BIT-STRING-OP-TABLE) can be used. Bit string operands of `&`, `|`, and `#` must be of equal length. When bit shifting, the original length of the string is preserved, as shown in the examples.

#### **Table 9.14. Bit String Operators**

| Operator | Description         | Example                | Result     |
| -------- | ------------------- | ---------------------- | ---------- |
| `\|\|`   | concatenation       | `B'10001' \|\| B'011'` | `10001011` |
| `&`      | bitwise AND         | `B'10001' & B'01101'`  | `00001`    |
| `\|`     | bitwise OR          | `B'10001' \| B'01101'` | `11101`    |
| `#`      | bitwise XOR         | `B'10001' # B'01101'`  | `11100`    |
| `~`      | bitwise NOT         | `~ B'10001'`           | `01110`    |
| `<<`     | bitwise shift left  | `B'10001' << 3`        | `01000`    |
| `>>`     | bitwise shift right | `B'10001' >> 2`        | `00100`    |

The following SQL-standard functions work on bit strings as well as character strings: `length`, `bit_length`, `octet_length`, `position`, `substring`, `overlay`.

The following functions work on bit strings as well as binary strings: `get_bit`, `set_bit`. When working with a bit string, these functions number the first (leftmost) bit of the string as bit 0.

In addition, it is possible to cast integral values to and from type `bit`. 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.

#### Note

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.\\