GET DESCRIPTOR — 从 SQL 描述符区域获取信息
GET DESCRIPTORdescriptor_name
:cvariable
=descriptor_header_item
[, ... ] GET DESCRIPTORdescriptor_name
VALUEcolumn_number
:cvariable
=descriptor_item
[, ... ]
GET DESCRIPTOR
从 SQL 描述符区域检索有关查询结果集的信息,并将其存储到主机变量中。在使用此命令将信息传输到主机语言变量之前,通常使用 FETCH
或 SELECT
填充描述符区域。
此命令有两种形式:第一种形式检索描述符 “header” 项,这些项适用于整个结果集。行计数就是一个示例。第二种形式需要列号作为附加参数,检索有关特定列的信息。列名和实际列值就是示例。
检索结果集中列数量的示例
EXEC SQL GET DESCRIPTOR d :d_count = COUNT;
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;
检索第二列的数据正文作为字符串的示例
EXEC SQL GET DESCRIPTOR d VALUE 2 :d_data = DATA;
以下是执行 SELECT current_database();
的完整过程的示例,并显示列数、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
GET DESCRIPTOR
在 SQL 标准中指定。