# 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。