如何使用 C API 处理"SELECT *" Mysql 预准备语句查询后的结果集？

Question

https://dev.mysql.com/doc/c-api/8.0/en/mysql-stmt-fetch.html.

在代码示例中，它提到必须声明单独的出站参数，以便能够访问SQL查询中列出的列。

是否可以运行"SELECT * WHERE column = ?"仅通过准备语句查询子句的WHERE部分，并且在不知道SELECT列的情况下处理结果，因此没有将它们声明为出站参数?

如果有，有没有例子?

感谢

Answer 1

我能够找到一个示例，该示例显示需要生成出站绑定参数。php ->Pdo_mysql扩展源代码:

php-8.1.12/ext/pdo_mysql/mysql_statement.c

static bool pdo_mysql_stmt_after_execute_prepared(pdo_stmt_t *stmt) {
        pdo_mysql_stmt *S = stmt->driver_data;
        pdo_mysql_db_handle *H = S->H;
#ifdef PDO_USE_MYSQLND
        /* For SHOW/DESCRIBE and others the column/field count is not available before execute. */
        php_pdo_stmt_set_column_count(stmt, mysql_stmt_field_count(S->stmt));
        for (int i = 0; i < stmt->column_count; i++) {
                mysqlnd_stmt_bind_one_result(S->stmt, i);
        }
        S->result = mysqlnd_stmt_result_metadata(S->stmt);
        if (S->result) {
                S->fields = mysql_fetch_fields(S->result);
                /* If buffered, pre-fetch all the data */
                if (H->buffered) {
                        if (mysql_stmt_store_result(S->stmt)) {
                                pdo_mysql_error_stmt(stmt);
                                return false;
                        }
                }
        }
#else
        /* figure out the result set format, if any */
        S->result = mysql_stmt_result_metadata(S->stmt);
        if (S->result) {
                int calc_max_length = H->buffered && S->max_length == 1;
                S->fields = mysql_fetch_fields(S->result);
                php_pdo_stmt_set_column_count(stmt, (int)mysql_num_fields(S->result));
                S->bound_result = ecalloc(stmt->column_count, sizeof(MYSQL_BIND));
                S->out_null = ecalloc(stmt->column_count, sizeof(my_bool));
                S->out_length = ecalloc(stmt->column_count, sizeof(zend_ulong));
                /* summon memory to hold the row */
                for (int i = 0; i < stmt->column_count; i++) {
                        if (calc_max_length && S->fields[i].type == FIELD_TYPE_BLOB) {
                                my_bool on = 1;
                                mysql_stmt_attr_set(S->stmt, STMT_ATTR_UPDATE_MAX_LENGTH, &on);
                                calc_max_length = 0;
                        }
                        switch (S->fields[i].type) {
                                case FIELD_TYPE_INT24:
                                        S->bound_result[i].buffer_length = MAX_MEDIUMINT_WIDTH + 1;
                                        break;
                                case FIELD_TYPE_LONG:
                                        S->bound_result[i].buffer_length = MAX_INT_WIDTH + 1;
                                        break;
                                case FIELD_TYPE_LONGLONG:
                                        S->bound_result[i].buffer_length = MAX_BIGINT_WIDTH + 1;
                                        break;
                                case FIELD_TYPE_TINY:
                                        S->bound_result[i].buffer_length = MAX_TINYINT_WIDTH + 1;
                                        break;
                                case FIELD_TYPE_SHORT:
                                        S->bound_result[i].buffer_length = MAX_SMALLINT_WIDTH + 1;
                                        break;
                                default:
                                        S->bound_result[i].buffer_length =
                                                S->fields[i].max_length? S->fields[i].max_length:
                                                S->fields[i].length;
                                        /* work-around for longtext and alike */
                                        if (S->bound_result[i].buffer_length > H->max_buffer_size) {
                                                S->bound_result[i].buffer_length = H->max_buffer_size;
                                        }
                        }
                        /* there are cases where the length reported by mysql is too short.
                         * eg: when describing a table that contains an enum column. Since
                         * we have no way of knowing the true length either, we'll bump up
                         * our buffer size to a reasonable size, just in case */
                        if (S->fields[i].max_length == 0 && S->bound_result[i].buffer_length < 128 && MYSQL_TYPE_VAR_STRING) {
                                S->bound_result[i].buffer_length = 128;
                        }
                        S->out_length[i] = 0;
                        S->bound_result[i].buffer = emalloc(S->bound_result[i].buffer_length);
                        S->bound_result[i].is_null = &S->out_null[i];
                        S->bound_result[i].length = &S->out_length[i];
                        S->bound_result[i].buffer_type = MYSQL_TYPE_STRING;
                }
                if (mysql_stmt_bind_result(S->stmt, S->bound_result)) {
                        pdo_mysql_error_stmt(stmt);
                        PDO_DBG_RETURN(0);
                }
                /* if buffered, pre-fetch all the data */
                if (H->buffered) {
                        if (mysql_stmt_store_result(S->stmt)) {
                                pdo_mysql_error_stmt(stmt);
                                PDO_DBG_RETURN(0);
                        }
                }
        }
#endif
        pdo_mysql_stmt_set_row_count(stmt);
        return true;
}