> 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/ddl/ddl-default.md).

# 5.2. Default Values \#

A column can be assigned a default value. When a new row is created and no values are specified for some of the columns, those columns will be filled with their respective default values. A data manipulation command can also request explicitly that a column be set to its default value, without having to know what that value is. (Details about data manipulation commands are in [Chapter 6](/the-sql-language/dml.md).)

If no default value is declared explicitly, the default value is the null value. This usually makes sense because a null value can be considered to represent unknown data.

In a table definition, default values are listed after the column data type. For example:

```

CREATE TABLE products (
    product_no integer,
    name text,
    price numeric DEFAULT 9.99
);
```

The default value can be an expression, which will be evaluated whenever the default value is inserted (*not* when the table is created). A common example is for a `timestamp` column to have a default of `CURRENT_TIMESTAMP`, so that it gets set to the time of row insertion. Another common example is generating a “serial number” for each row. In PostgreSQL this is typically done by something like:

```

CREATE TABLE products (
    product_no integer DEFAULT nextval('products_product_no_seq'),
    ...
);
```

where the `nextval()` function supplies successive values from a *sequence object* (see [Section 9.17](/the-sql-language/functions/functions-sequence.md)). This arrangement is sufficiently common that there's a special shorthand for it:

```

CREATE TABLE products (
    product_no SERIAL,
    ...
);
```

The `SERIAL` shorthand is discussed further in [Section 8.1.4](/the-sql-language/datatype/datatype-numeric.md#DATATYPE-SERIAL).

***

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


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