> 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/datatype/datatype-boolean.md).

# 8.6. Boolean Type \#

PostgreSQL provides the standard SQL type `boolean`; see [Table 8.19](#DATATYPE-BOOLEAN-TABLE). 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 |

<br>

Boolean constants can be represented in SQL queries by the SQL key words `TRUE`, `FALSE`, and `NULL`.

The datatype input function for type `boolean` accepts these string representations for the “true” state:

| `true` |
| ------ |
| `yes`  |
| `on`   |
| `1`    |

and these representations for the “false” state:

| `false` |
| ------- |
| `no`    |
| `off`   |
| `0`     |

Unique prefixes of these strings are also accepted, for example `t` or `n`. Leading or trailing whitespace is ignored, and case does not matter.

The datatype output function for type `boolean` always emits either `t` or `f`, as shown in [Example 8.2](#DATATYPE-BOOLEAN-EXAMPLE).

**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
```

<br>

The key words `TRUE` and `FALSE` are the preferred (SQL-compliant) method for writing Boolean constants in SQL queries. But you can also use the string representations by following the generic string-literal constant syntax described in [Section 4.1.2.7](/the-sql-language/sql-syntax/sql-syntax-lexical.md#SQL-SYNTAX-CONSTANTS-GENERIC), for example `'yes'::boolean`.

Note that the parser automatically understands that `TRUE` and `FALSE` are of type `boolean`, but this is not so for `NULL` because that can have any type. So in some contexts you might have to cast `NULL` to `boolean` explicitly, for example `NULL::boolean`. Conversely, the cast can be omitted from a string-literal Boolean value in contexts where the parser can deduce that the literal must be of type `boolean`.

***

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


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