> 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/queries/queries-overview.md).

# 7.1. Overview \#

The process of retrieving or the command to retrieve data from a database is called a *query*. In SQL the [`SELECT`](/reference/sql-commands/sql-select.md) command is used to specify queries. The general syntax of the `SELECT` command is

```

[WITH with_queries] SELECT select_list FROM table_expression [sort_specification]
```

The following sections describe the details of the select list, the table expression, and the sort specification. `WITH` queries are treated last since they are an advanced feature.

A simple kind of query has the form:

```

SELECT * FROM table1;
```

Assuming that there is a table called `table1`, this command would retrieve all rows and all user-defined columns from `table1`. (The method of retrieval depends on the client application. For example, the psql program will display an ASCII-art table on the screen, while client libraries will offer functions to extract individual values from the query result.) The select list specification `*` means all columns that the table expression happens to provide. A select list can also select a subset of the available columns or make calculations using the columns. For example, if `table1` has columns named `a`, `b`, and `c` (and perhaps others) you can make the following query:

```

SELECT a, b + c FROM table1;
```

(assuming that `b` and `c` are of a numerical data type). See [Section 7.3](/the-sql-language/queries/queries-select-lists.md) for more details.

`FROM table1` is a simple kind of table expression: it reads just one table. In general, table expressions can be complex constructs of base tables, joins, and subqueries. But you can also omit the table expression entirely and use the `SELECT` command as a calculator:

```

SELECT 3 * 4;
```

This is more useful if the expressions in the select list return varying results. For example, you could call a function this way:

```

SELECT random();
```

***

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


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