# CONNECT ## Name CONNECT -- 建立数据库连接 ## Synopsis ``` CONNECT TO _connection_target_ [ AS _connection_name_ ] [ USER _connection_user_name_ ] CONNECT TO DEFAULT CONNECT _connection_user_name_ DATABASE _connection_target_ ``` ## 描述 `CONNECT`命令在客户端和PostgreSQL服务器之间 创建了连接。 ## 参数 `_connection_target_` `_connection_target_` 在几种形式之一上指定了连接的目标服务器。 [ `_database_name_` ] [ `@``_host_` ] [ `:``_port_` ] TCP/IP连接 `unix:postgresql://``_host_` [ `:``_port_` ] `/` [ `_database_name_` ] [ `?``_connection_option_` ] 连接Unix-域套接字 `tcp:postgresql://``_host_` [ `:``_port_` ] `/` [ `_database_name_` ] [ `?``_connection_option_` ] TCP/IP连接 SQL字符串常量 在上述形式之一中包含一个值 宿主变量 类型`char[]` 或者`VARCHAR[]`的宿主变量包含上述形式之一的值。 `_connection_object_` 用于连接可选标识符，这样就可以指向其他命令。 这可以是一个SQL标识符或者宿主变量。 `_connection_user_` 用于数据库连接的用户名。 该参数可以指定用户名和密码，使用下面形式之一 `_user_name_`/`_password_`, `_user_name_` IDENTIFIED BY `_password_`或者 `_user_name_` USING `_password_`. 用户名和密码可以是SQL标识符，字符串常量，或者宿主变量。 `DEFAULT` 使用所有缺省连接参数，正如通过libpq定义的。 ## 例子 指定连接参数的几种变形： ``` 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; ``` 这里有一个例子程序阐述了使用宿主变量指定连接参数： ``` int main(void) { EXEC SQL BEGIN DECLARE SECTION; char *dbname = "testdb"; /*数据库名称*/ char *user = "testuser"; /*连接用户名*/ char *connection = "tcp:postgresql://localhost:5432/testdb"; /* 连接字符串 */ char ver[256]; /*存储版本字符串的缓冲区*/ EXEC SQL END DECLARE SECTION; ECPGdebug(1, stderr); EXEC SQL CONNECT TO :dbname USER :user; EXEC SQL SELECT version() INTO :ver; EXEC SQL DISCONNECT; printf("version: %s\n", ver); EXEC SQL CONNECT TO :connection USER :user; EXEC SQL SELECT version() INTO :ver; EXEC SQL DISCONNECT; printf("version: %s\n", ver); return 0; } ``` ## 兼容性 在SQL标准中声明`CONNECT`，但是 连接参数的格式是具体实施的。 ## 参见 [DISCONNECT](#calibre_link-27), [SET CONNECTION](#calibre_link-2019)