Postgresql 中文操作指南
Synopsis
GET DESCRIPTOR descriptor_name :cvariable = descriptor_header_item [, ... ]
GET DESCRIPTOR descriptor_name VALUE column_number :cvariable = descriptor_item [, ... ]Description
GET DESCRIPTOR 从 SQL 描述符区域中检索有关查询结果集的信息，并将其存储到主机变量中。通常使用 FETCH 或 SELECT 填充描述符区域，然后再使用此命令将信息传输到主机语言变量中。
此命令有两种形式：第一种形式检索描述符“标题”项，该项适用于整个结果集。一个示例就是行数。第二种形式需要列号作为附加参数，它用于检索有关特定列的信息。示例包括列名和实际列值。
Examples
一个检索结果集中列数的示例：
EXEC SQL GET DESCRIPTOR d :d_count = COUNT;一个检索第一列中数据长度的示例：
EXEC SQL GET DESCRIPTOR d VALUE 1 :d_returned_octet_length = RETURNED_OCTET_LENGTH;一个将第二列的数据主体作为字符串检索的示例：
EXEC SQL GET DESCRIPTOR d VALUE 2 :d_data = DATA;这是一个执行 SELECT current_database(); 的整个过程的示例，它显示了列数、列数据长度和列数据：
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
int d_count;
char d_data[1024];
int d_returned_octet_length;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb AS con1 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL ALLOCATE DESCRIPTOR d;
/* Declare, open a cursor, and assign a descriptor to the cursor */
EXEC SQL DECLARE cur CURSOR FOR SELECT current_database();
EXEC SQL OPEN cur;
EXEC SQL FETCH NEXT FROM cur INTO SQL DESCRIPTOR d;
/* Get a number of total columns */
EXEC SQL GET DESCRIPTOR d :d_count = COUNT;
printf("d_count = %d\n", d_count);
/* Get length of a returned column */
EXEC SQL GET DESCRIPTOR d VALUE 1 :d_returned_octet_length = RETURNED_OCTET_LENGTH;
printf("d_returned_octet_length = %d\n", d_returned_octet_length);
/* Fetch the returned column as a string */
EXEC SQL GET DESCRIPTOR d VALUE 1 :d_data = DATA;
printf("d_data = %s\n", d_data);
/* Closing */
EXEC SQL CLOSE cur;
EXEC SQL COMMIT;
EXEC SQL DEALLOCATE DESCRIPTOR d;
EXEC SQL DISCONNECT ALL;
return 0;
}执行示例后，结果将如下所示：
d_count = 1
d_returned_octet_length = 6
d_data = testdb