# 8.10. 位元字串型別

Bit strings are strings of 1's and 0's. They can be used to store or visualize bit masks. There are two SQL bit types: `bit(`*`n`*) and `bit varying(`*`n`*), where *`n`* is a positive integer.

`bit` type data must match the length *`n`* exactly; it is an error to attempt to store shorter or longer bit strings. `bit varying` data is of variable length up to the maximum length *`n`*; longer strings will be rejected. Writing `bit` without a length is equivalent to `bit(1)`, while `bit varying` without a length specification means unlimited length.

#### Note

If one explicitly casts a bit-string value to `bit(`*`n`*), it will be truncated or zero-padded on the right to be exactly *`n`* bits, without raising an error. Similarly, if one explicitly casts a bit-string value to `bit varying(`*`n`*), it will be truncated on the right if it is more than *`n`* bits.

Refer to [Section 4.1.2.5](https://www.postgresql.org/docs/12/sql-syntax-lexical.html#SQL-SYNTAX-BIT-STRINGS) for information about the syntax of bit string constants. Bit-logical operators and string manipulation functions are available; see [Section 9.6](https://www.postgresql.org/docs/12/functions-bitstring.html).

**Example 8.3. Using the Bit String Types**

```
CREATE TABLE test (a BIT(3), b BIT VARYING(5));
INSERT INTO test VALUES (B'101', B'00');
INSERT INTO test VALUES (B'10', B'101');

ERROR:  bit string length 2 does not match type bit(3)

INSERT INTO test VALUES (B'10'::bit(3), B'101');
SELECT * FROM test;

  a  |  b
-----+-----
 101 | 00
 100 | 101
```

A bit string value requires 1 byte for each group of 8 bits, plus 5 or 8 bytes overhead depending on the length of the string (but long values may be compressed or moved out-of-line, as explained in [Section 8.3](https://www.postgresql.org/docs/12/datatype-character.html) for character strings).