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)andbit varying(n), wherenis a positive integer.

bittype data must match the lengthnexactly; it is an error to attempt to store shorter or longer bit strings.bit varyingdata is of variable length up to the maximum lengthn; longer strings will be rejected. Writingbitwithout a length is equivalent tobit(1), whilebit varyingwithout a length specification means unlimited length.

Note:If one explicitly casts a bit-string value tobit(n), it will be truncated or zero-padded on the right to be exactlynbits, without raising an error. Similarly, if one explicitly casts a bit-string value tobit varying(n), it will be truncated on the right if it is more thannbits.

Refer toSection 4.1.2.5for information about the syntax of bit string constants. Bit-logical operators and string manipulation functions are available; seeSection 9.6.

Example 8-3. Using the Bit String Types

  1. CREATE TABLE test (a BIT(3), b BIT VARYING(5));
  2. INSERT INTO test VALUES (B'101', B'00');
  3. INSERT INTO test VALUES (B'10', B'101');
  5. ERROR:  bit string length 2 does not match type bit(3)
  7. INSERT INTO test VALUES (B'10'::bit(3), B'101');
  8. SELECT * FROM test;
  10.   a  |  b
  11. -----+-----
  12.  101 | 00
  13.  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 inSection 8.3for character strings).