PostgreSQL provides the
    standard SQL type boolean;
    see Table 8-19.
    The boolean type can have several states:
    "true", "false", and a third state,
    "unknown", which is represented by the
    SQL null value.
   
Table 8-19. Boolean Data Type
| Name | Storage Size | Description | 
|---|
| boolean | 1 byte | state of true or false | 
    Valid literal values for the "true" state are:
    
| TRUE | 
| 't' | 
| 'true' | 
| 'y' | 
| 'yes' | 
| 'on' | 
| '1' | 
    For the 
"false" state, the following values can be
    used:
    
| FALSE | 
| 'f' | 
| 'false' | 
| 'n' | 
| 'no' | 
| 'off' | 
| '0' | 
    Leading or trailing whitespace is ignored, and case does not matter.
    The key words
    
TRUE and 
FALSE are the preferred
    (
SQL-compliant) usage.
   
    Example 8-2 shows that
    boolean values are output using the letters
    t and f.
   
Example 8-2. Using the boolean Type
CREATE TABLE test1 (a boolean, b text);
INSERT INTO test1 VALUES (TRUE, 'sic est');
INSERT INTO test1 VALUES (FALSE, 'non est');
SELECT * FROM test1;
 a |    b
---+---------
 t | sic est
 f | non est
SELECT * FROM test1 WHERE a;
 a |    b
---+---------
 t | sic est