ecpg-sql-connect.md

## 连接

CONNECT — 建立数据库连接

## 概要

```
CONNECT TO connection_target [ AS connection_name ] [ USER connection_user ]
CONNECT TO DEFAULT
CONNECT connection_user
DATABASE connection_target
```

## 描述

这`连接`命令在客户端和 PostgreSQL 服务器之间建立连接。

## 参数

*`连接目标`*

*`连接目标`*以多种形式之一指定连接的目标服务器。

[ *`数据库名称`* ][ `@`*`host`* ] [ `:`*`：`*港口 ]

通过 TCP/IP 连接

`unix:postgresql://`*`主持人`* [ `:`*`：`*港口 ] `/` [ *`数据库名称`* ][ `?`*`connection_option`* ]

通过 Unix 域套接字连接

`tcp:postgresql://`*`主持人`* [ `:`*`：`*港口 ] `/` [ *`数据库名称`* ][ `?`*`connection_option`* ]

通过 TCP/IP 连接

SQL 字符串常量

包含上述形式之一的值

主变量

类型的主变量`字符[]`或者`VARCHAR[]`包含上述形式之一的值

*`连接名称`*

连接的可选标识符，以便可以在其他命令中引用。这可以是 SQL 标识符或主机变量。

*`连接用户`*

数据库连接的用户名。

该参数还可以指定用户名和密码，使用形式之一`*`用户名`*/*`密码`*`,`*`用户名`* 由 * 识别`密码`*`， 要么`*`用户名`* 使用 *`密码`*`.

用户名和密码可以是 SQL 标识符、字符串常量或主机变量。

`默认`

使用 libpq 定义的所有默认连接参数。

## 例子

这里有几个用于指定连接参数的变体：

```
EXEC SQL CONNECT TO "connectdb" AS main;
EXEC SQL CONNECT TO "connectdb" AS second;
EXEC SQL CONNECT TO "unix:postgresql://200.46.204.71/connectdb" AS main USER connectuser;
EXEC SQL CONNECT TO "unix:postgresql://localhost/connectdb" AS main USER connectuser;
EXEC SQL CONNECT TO 'connectdb' AS main;
EXEC SQL CONNECT TO 'unix:postgresql://localhost/connectdb' AS main USER :user;
EXEC SQL CONNECT TO :db AS :id;
EXEC SQL CONNECT TO :db USER connectuser USING :pw;
EXEC SQL CONNECT TO @localhost AS main USER connectdb;
EXEC SQL CONNECT TO REGRESSDB1 as main;
EXEC SQL CONNECT TO AS main USER connectdb;
EXEC SQL CONNECT TO connectdb AS :id;
EXEC SQL CONNECT TO connectdb AS main USER connectuser/connectdb;
EXEC SQL CONNECT TO connectdb AS main;
EXEC SQL CONNECT TO connectdb@localhost AS main;
EXEC SQL CONNECT TO tcp:postgresql://localhost/ USER connectdb;
EXEC SQL CONNECT TO tcp:postgresql://localhost/connectdb USER connectuser IDENTIFIED BY connectpw;
EXEC SQL CONNECT TO tcp:postgresql://localhost:20/connectdb USER connectuser IDENTIFIED BY connectpw;
EXEC SQL CONNECT TO unix:postgresql://localhost/ AS main USER connectdb;
EXEC SQL CONNECT TO unix:postgresql://localhost/connectdb AS main USER connectuser;
EXEC SQL CONNECT TO unix:postgresql://localhost/connectdb USER connectuser IDENTIFIED BY "connectpw";
EXEC SQL CONNECT TO unix:postgresql://localhost/connectdb USER connectuser USING "connectpw";
EXEC SQL CONNECT TO unix:postgresql://localhost/connectdb?connect_timeout=14 USER connectuser;
```

下面是一个示例程序，它说明了使用主机变量来指定连接参数：

```
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
    char *dbname     = "testdb";    /* database name */
    char *user       = "testuser";  /* connection user name */
    char *connection = "tcp:postgresql://localhost:5432/testdb";
                                    /* connection string */
    char ver[256];                  /* buffer to store the version string */
EXEC SQL END DECLARE SECTION;

    ECPGdebug(1, stderr);

    EXEC SQL CONNECT TO :dbname USER :user;
    EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
    EXEC SQL SELECT version() INTO :ver;
    EXEC SQL DISCONNECT;

    printf("version: %s\n", ver);

    EXEC SQL CONNECT TO :connection USER :user;
    EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
    EXEC SQL SELECT version() INTO :ver;
    EXEC SQL DISCONNECT;

    printf("version: %s\n", ver);

    return 0;
}
```

## 兼容性

`连接`在 SQL 标准中指定，但连接参数的格式是特定于实现的。

## 也可以看看

[断开](ecpg-sql-disconnect.html), [设置连接](ecpg-sql-set-connection.html)