> 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/15/client-interfaces/ecpg-embedded-sql-in-c/35.14.-embedded-sql-commands/ecpg-sql-connect.md).

# CONNECT

## CONNECT

CONNECT — establish a database connection

### Synopsis

```

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

### Description

The `CONNECT` command establishes a connection between the client and the PostgreSQL server.

### Parameters

*`connection_target`*

*`connection_target`* specifies the target server of the connection on one of several forms.

\[ *`database_name`* ] \[ `@`*`host`* ] \[ `:`*`port`* ]

Connect over TCP/IP

`unix:postgresql://`*`host`* \[ `:`*`port`* ] `/` \[ *`database_name`* ] \[ `?`*`connection_option`* ]

Connect over Unix-domain sockets

`tcp:postgresql://`*`host`* \[ `:`*`port`* ] `/` \[ *`database_name`* ] \[ `?`*`connection_option`* ]

Connect over TCP/IP

SQL string constant

containing a value in one of the above forms

host variable

host variable of type `char[]` or `VARCHAR[]` containing a value in one of the above forms

*`connection_name`*

An optional identifier for the connection, so that it can be referred to in other commands. This can be an SQL identifier or a host variable.

*`connection_user`*

The user name for the database connection.

This parameter can also specify user name and password, using one the forms *`user_name`*`/`*`password`*, *`user_name`*` ``IDENTIFIED BY`` `*`password`*, or *`user_name`*` ``USING`` `*`password`*.

User name and password can be SQL identifiers, string constants, or host variables.

`DEFAULT`

Use all default connection parameters, as defined by libpq.

### Examples

Here a several variants for specifying connection parameters:

```

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;
```

Here is an example program that illustrates the use of host variables to specify connection parameters:

```

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;
}
```

### Compatibility

`CONNECT` is specified in the SQL standard, but the format of the connection parameters is implementation-specific.

### See Also

[DISCONNECT](/15/client-interfaces/ecpg-embedded-sql-in-c/35.14.-embedded-sql-commands/ecpg-sql-disconnect.md), [SET CONNECTION](/15/client-interfaces/ecpg-embedded-sql-in-c/35.14.-embedded-sql-commands/ecpg-sql-set-connection.md)

***

原文：[PostgreSQL 15.19 Documentation](/15/client-interfaces/ecpg-embedded-sql-in-c/35.14.-embedded-sql-commands/ecpg-sql-connect.md)（英文原文，待翻譯）


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