指定要返回的文档
概述
在本指南中，您可以通过以下操作了解如何使用读取操作返回哪些文档：
limit：指定查询返回的最大文档数sort：指定返回文档的排序顺序skip：指定返回查询结果前要跳过的文档数
示例数据
本指南中的示例使用来自Atlas示例数据集的restaurants集合在sample_restaurants数据库中。要了解如何创建免费的MongoDB Atlas集群并加载示例数据集，请参阅Atlas入门指南。
限制
要指定从读取操作返回的最大文档数，将 limit 选项传递给您的 mongoc_collection_find_with_opts() 函数调用。
以下示例查找所有具有 cuisine 字段值为 "Italian" 的餐厅，并将结果限制为 5 个文档
const bson_t *doc; bson_t *filter = BCON_NEW ("cuisine", BCON_UTF8 ("Italian")); bson_t *opts = BCON_NEW ("limit", BCON_INT64 (5)); mongoc_cursor_t *results = mongoc_collection_find_with_opts (collection, filter, opts, NULL); while (mongoc_cursor_next (results, &doc)) { char *str = bson_as_canonical_extended_json (doc, NULL); printf ("%s\n", str); bson_free (str); } bson_destroy (filter); bson_destroy (opts); mongoc_cursor_destroy (results);
{ "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "V & T Restaurant", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Mimis Restaurant & Bar", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Venice Restaurant", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Areo Restaurant", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Tre Giovani Pizza & Pasta", ... }
提示
前面的示例返回了查询返回的前五个文档，按照 自然顺序。以下部分将描述如何按照指定的排序顺序返回文档。
排序
要按指定顺序返回文档，请使用 sort 选项。该 sort 选项接受一个排序方向作为参数。要指定排序方向，传递 1 进行升序排序或传递 -1 进行降序排序。升序排序按从低到高的顺序排序值，而降序排序按从高到低的顺序排序值。
以下示例返回所有具有 cuisine 字段值为 "Italian" 的文档，并按 name 字段的值进行升序排序
const bson_t *doc; bson_t *filter = BCON_NEW ("cuisine", BCON_UTF8 ("Italian")); bson_t *opts = BCON_NEW ("sort", "{", "name", BCON_INT32 (1), "}"); mongoc_cursor_t *results = mongoc_collection_find_with_opts (collection, filter, opts, NULL); while (mongoc_cursor_next (results, &doc)) { char *str = bson_as_canonical_extended_json (doc, NULL); printf ("%s\n", str); bson_free (str); } bson_destroy (filter); bson_destroy (opts); mongoc_cursor_destroy (results);
{ "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "(Lewis Drug Store) Locanda Vini E Olii", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "101 Restaurant And Bar", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "44 Sw Ristorante & Bar", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "900 Park", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "A Voce", ... } ...
跳过
要在返回查询结果之前跳过指定数量的文档，请使用 skip 选项并传入要跳过的文档数量。skip 选项会忽略查询结果中的指定数量文档，然后返回其余结果。
以下示例返回所有具有 cuisine 字段值为 "意大利菜" 的文档，并跳过前10个文档。
const bson_t *doc; bson_t *filter = BCON_NEW ("cuisine", BCON_UTF8 ("Italian")); bson_t *opts = BCON_NEW ("skip", BCON_INT64 (10)); mongoc_cursor_t *results = mongoc_collection_find_with_opts (collection, filter, opts, NULL); while (mongoc_cursor_next (results, &doc)) { char *str = bson_as_canonical_extended_json (doc, NULL); printf ("%s\n", str); bson_free (str); } bson_destroy (filter); bson_destroy (opts); mongoc_cursor_destroy (results);
{ "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Trattoria Alba", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Da Umberto Restaurant", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "La Strada Restaurant", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Pasta Lovers Trattoria", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Nanni Restaurant", ... } ...
合并限制、排序和跳过
您可以在单个操作中合并 limit、sort 和 skip 选项。这允许您设置返回的排序文档的最大数量，在返回之前跳过指定数量的文档。
以下示例返回具有 cuisine 字段值为 "意大利菜" 的文档。结果按字母顺序排序，跳过前10个文档，并限制结果为5个文档。
const bson_t *doc; bson_t *filter = BCON_NEW ("cuisine", BCON_UTF8 ("Italian")); bson_t *opts = BCON_NEW ("limit", BCON_INT64 (5), "skip", BCON_INT64 (10), "sort", "{", "name", BCON_INT32 (1), "}"); mongoc_cursor_t *results = mongoc_collection_find_with_opts (collection, filter, opts, NULL); while (mongoc_cursor_next (results, &doc)) { char *str = bson_as_canonical_extended_json (doc, NULL); printf ("%s\n", str); bson_free (str); } bson_destroy (filter); bson_destroy (opts); mongoc_cursor_destroy (results);
{ "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Acqua", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Acqua Restaurant", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Acqua Santa", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Acquista Trattoria", ... } { "_id" : { "$oid" : "..." }, "cuisine" : "Italian", "name" : "Acquolina Catering", ... }
注意
调用这些方法的顺序不会改变返回的文档。驱动程序会自动重新排序调用以首先执行排序和跳过操作，然后执行限制操作。
附加信息
有关指定查询的更多信息，请参阅指定查询.
有关检索文档的更多信息，请参阅检索数据。
API 文档
有关 mongoc_collection_find_with_opts() 函数的更多信息，请参阅API 文档。