NoSQL, which stands for "Not Only SQL," is a type of database management system that provides a flexible and scalable approach to storing and retrieving data. It emerged as a response to the limitations of traditional relational databases, especially when dealing with large-scale and complex data sets. Unlike relational databases, NoSQL databases do not rely on a fixed schema and use different data models to organize and access data.?

Key features of NoSQL databases include:

1. Schema flexibility: NoSQL databases allow for dynamic and flexible schema designs, where data structures can vary from one record to another. This enables easy adaptation to evolving data requirements and simplifies data integration.

2. Scalability: NoSQL databases are designed to handle large-scale data sets and high-volume read and write operations. They provide horizontal scalability by distributing data across multiple servers or clusters.

3. High performance: NoSQL databases offer fast and efficient data access and processing capabilities. They optimize for specific use cases, such as real-time analytics, high-speed transactions, or content management, by leveraging data models and indexing techniques tailored to those scenarios.

4. Availability and fault tolerance: NoSQL databases often employ replication and distributed architecture to ensure high availability and fault tolerance. Data is replicated across multiple nodes, allowing for automatic failover and continuous operation even in the event of hardware or network failures.

Advantages of using semi-structured data storage (often associated with NoSQL databases) over traditional relational databases include:

1. Flexibility: Semi-structured data storage allows for storing and querying data without the need for predefined schemas. This flexibility is particularly useful when dealing with unstructured or evolving data, such as social media posts, sensor data, or log files.

2. Scalability: NoSQL databases excel at horizontal scalability, allowing for seamless distribution of data across multiple nodes. This enables handling massive amounts of data and accommodating high traffic loads without sacrificing performance.

3. Performance: By optimizing data storage and retrieval for specific use cases, NoSQL databases can achieve high-performance levels. They leverage data models and indexing techniques tailored to the application requirements, resulting in faster query response times.

4. Agility: Semi-structured data storage allows for agile development and quick iterations. Developers can easily add, modify, or remove fields in the data model without worrying about schema migrations or data inconsistencies.

5. Cost-effectiveness: NoSQL databases often offer a more cost-effective solution for large-scale data storage compared to traditional relational databases. They can leverage commodity hardware and distributed architectures, reducing infrastructure costs.

It's important to note that the choice between a traditional relational database and a NoSQL database depends on the specific requirements of the application, the nature of the data, scalability needs, and development considerations. Each type of database has its strengths and trade-offs, and the selection should align with the specific use case at hand.

NoSQL代表“不僅僅是SQL”，是一種數(shù)據(jù)庫管理系統(tǒng)，它為存儲和檢索數(shù)據(jù)提供了靈活且可擴(kuò)展的方法。它是對傳統(tǒng)關(guān)系數(shù)據(jù)庫局限性的回應(yīng)，尤其是在處理大規(guī)模復(fù)雜的數(shù)據(jù)集時(shí)。與關(guān)系數(shù)據(jù)庫不同，NoSQL數(shù)據(jù)庫不依賴于固定的模式，而是使用不同的數(shù)據(jù)模型來組織和訪問數(shù)據(jù)。

NoSQL數(shù)據(jù)庫的主要功能包括：

1.模式靈活性：NoSQL數(shù)據(jù)庫允許動態(tài)和靈活的模式設(shè)計(jì)，其中數(shù)據(jù)結(jié)構(gòu)可以因記錄而異。這使得能夠輕松適應(yīng)不斷變化的數(shù)據(jù)需求，并簡化數(shù)據(jù)集成。

2.可擴(kuò)展性：NoSQL數(shù)據(jù)庫設(shè)計(jì)用于處理大規(guī)模數(shù)據(jù)集和高容量讀寫操作。它們通過在多個(gè)服務(wù)器或集群之間分布數(shù)據(jù)來提供橫向可擴(kuò)展性。

3.高性能：NoSQL數(shù)據(jù)庫提供快速高效的數(shù)據(jù)訪問和處理能力。他們通過利用針對這些場景定制的數(shù)據(jù)模型和索引技術(shù)，針對特定用例進(jìn)行優(yōu)化，如實(shí)時(shí)分析、高速事務(wù)或內(nèi)容管理。

4.可用性和容錯(cuò)：NoSQL數(shù)據(jù)庫通常采用復(fù)制和分布式體系結(jié)構(gòu)來確保高可用性和容錯(cuò)性。數(shù)據(jù)跨多個(gè)節(jié)點(diǎn)進(jìn)行復(fù)制，即使在發(fā)生硬件或網(wǎng)絡(luò)故障的情況下也可以實(shí)現(xiàn)自動故障切換和連續(xù)操作。

與傳統(tǒng)關(guān)系數(shù)據(jù)庫相比，使用半結(jié)構(gòu)化數(shù)據(jù)存儲（通常與NoSQL數(shù)據(jù)庫相關(guān)）的優(yōu)勢包括：

1.靈活性：半結(jié)構(gòu)化數(shù)據(jù)存儲允許在不需要預(yù)定義模式的情況下存儲和查詢數(shù)據(jù)。這種靈活性在處理非結(jié)構(gòu)化或不斷發(fā)展的數(shù)據(jù)時(shí)尤其有用，例如社交媒體帖子、傳感器數(shù)據(jù)或日志文件。

2.可擴(kuò)展性：NoSQL數(shù)據(jù)庫擅長橫向可擴(kuò)展性，允許數(shù)據(jù)在多個(gè)節(jié)點(diǎn)之間無縫分布。這使得能夠在不犧牲性能的情況下處理大量數(shù)據(jù)并適應(yīng)高流量負(fù)載。

3.性能：通過針對特定用例優(yōu)化數(shù)據(jù)存儲和檢索，NoSQL數(shù)據(jù)庫可以達(dá)到高性能級別。它們利用了根據(jù)應(yīng)用程序需求定制的數(shù)據(jù)模型和索引技術(shù)，從而加快了查詢響應(yīng)時(shí)間。

4.敏捷性：半結(jié)構(gòu)化數(shù)據(jù)存儲允許敏捷開發(fā)和快速迭代。開發(fā)人員可以輕松地添加、修改或刪除數(shù)據(jù)模型中的字段，而無需擔(dān)心架構(gòu)遷移或數(shù)據(jù)不一致。

5.成本效益：與傳統(tǒng)的關(guān)系數(shù)據(jù)庫相比，NoSQL數(shù)據(jù)庫通常為大規(guī)模數(shù)據(jù)存儲提供更具成本效益的解決方案。他們可以利用商品硬件和分布式體系結(jié)構(gòu)，降低基礎(chǔ)設(shè)施成本。

需要注意的是，傳統(tǒng)關(guān)系數(shù)據(jù)庫和NoSQL數(shù)據(jù)庫之間的選擇取決于應(yīng)用程序的特定需求、數(shù)據(jù)的性質(zhì)、可擴(kuò)展性需求和開發(fā)考慮因素。每種類型的數(shù)據(jù)庫都有其優(yōu)勢和權(quán)衡，選擇應(yīng)該與手頭的特定用例保持一致。

兩階段鎖定協(xié)議（2PL）是數(shù)據(jù)庫系統(tǒng)中使用的一種并發(fā)控制機(jī)制，用于確保可序列化性和防止事務(wù)之間的沖突。它包括兩個(gè)階段：成長階段和萎縮階段。

1.生長階段：

-鎖定獲?。寒?dāng)事務(wù)開始時(shí)，它可以獲取需要訪問的數(shù)據(jù)庫對象（如表、記錄）的鎖定。鎖有兩種類型：共享（S）鎖和獨(dú)占（X）鎖。

-共享（S）鎖：允許多個(gè)事務(wù)同時(shí)讀取同一數(shù)據(jù)，但防止任何事務(wù)修改數(shù)據(jù)。

-獨(dú)占（X）鎖：授予對事務(wù)的獨(dú)占訪問權(quán)限，防止其他事務(wù)讀取或?qū)懭腈i定的數(shù)據(jù)。

-嚴(yán)格兩階段鎖定：在增長階段，事務(wù)遵循嚴(yán)格的兩階段鎖定規(guī)則，該規(guī)則規(guī)定事務(wù)在釋放任何鎖之前必須獲取所有所需的鎖。這樣可以確保在事務(wù)執(zhí)行過程中不會發(fā)生沖突。

2.收縮階段：

-鎖釋放：一旦事務(wù)完成了對數(shù)據(jù)項(xiàng)的訪問，它就可以釋放所持有的鎖。收縮階段發(fā)生在事務(wù)釋放了所有鎖之后。

-無鎖獲取：在收縮階段，交易無法獲取任何新鎖。它只能釋放以前獲取的鎖。

兩相鎖定協(xié)議（2PL）的優(yōu)點(diǎn)：

1.可串行性：2PL協(xié)議通過防止事務(wù)之間的沖突來保證可串行性。它確保事務(wù)的交錯(cuò)執(zhí)行產(chǎn)生與串行執(zhí)行相同的結(jié)果，從而保持?jǐn)?shù)據(jù)一致性。

2.死鎖預(yù)防：通過強(qiáng)制執(zhí)行在釋放任何鎖之前獲取所有所需鎖的嚴(yán)格規(guī)則，2PL協(xié)議可以防止死鎖。當(dāng)事務(wù)無限期地等待其他事務(wù)持有的資源時(shí)，就會發(fā)生死鎖，2PL避免了這種情況。

3.簡單性：2PL協(xié)議的實(shí)現(xiàn)和理解相對簡單。它為管理鎖和控制對共享數(shù)據(jù)的并發(fā)訪問提供了一種簡單的機(jī)制。

4.并發(fā)控制：2PL允許多個(gè)事務(wù)同時(shí)讀取共享數(shù)據(jù)，從而實(shí)現(xiàn)更高程度的并發(fā)。它優(yōu)化了系統(tǒng)資源的利用率，同時(shí)確保了數(shù)據(jù)的完整性。

5.與基于磁盤的系統(tǒng)的兼容性：2PL協(xié)議與數(shù)據(jù)存儲在非易失性存儲器上的基于磁盤的數(shù)據(jù)庫系統(tǒng)兼容。它確保在考慮磁盤I/O操作的情況下正確獲取和釋放必要的鎖。

盡管2PL協(xié)議有其優(yōu)點(diǎn)，但它也有局限性，例如在事務(wù)頻繁相互沖突的情況下，潛在的鎖爭用和并發(fā)性降低。已經(jīng)開發(fā)了高級并發(fā)控制機(jī)制，如多版本并發(fā)控制（MVCC）和樂觀并發(fā)控制（OCC）來解決這些限制。

__________________________________________________________________________

(a) SQL查詢語句以返回partnum為'P001'的數(shù)量：

```

SELECT quantity

FROM Part-schema

WHERE partnum = 'P001';

```

(b) SQL查詢語句以返回至少提供了2個(gè)熔巖燈零件的供應(yīng)商名稱：

```

SELECT suppliername

FROM Supplies-schema

GROUP BY suppliername

HAVING COUNT(partnum) >= 2;

```

(c) SQL查詢語句以返回提供partnum為'P001'的最低價(jià)格和公司存儲的數(shù)量的供應(yīng)商名稱：

```

SELECT suppliername, quantity

FROM Part-schema, Supplies-schema

WHERE Part-schema.partnum = Supplies-schema.partnum

AND partnum = 'P001'

AND cost <= ALL(SELECT cost FROM Supplies-schema WHERE partnum = 'P001');

```

請注意，這些查詢語句基于提供的關(guān)系模式和要求進(jìn)行了編寫，但實(shí)際運(yùn)行時(shí)可能需要根據(jù)具體的數(shù)據(jù)庫系統(tǒng)和數(shù)據(jù)存儲結(jié)構(gòu)進(jìn)行微調(diào)。

(d) 查找只提供1個(gè)零件給該公司的供應(yīng)商的供應(yīng)商名稱：

```sql

SELECT suppliername

FROM Supplies-schema

GROUP BY suppliername

HAVING COUNT(partnum) = 1;

```

(e) 查找為該公司提供partnum為'P001'但不提供partnum為'P002'的供應(yīng)商名稱：

```sql

SELECT suppliername

FROM Supplies-schema

WHERE suppliername IN (

? ? SELECT suppliername

? ? FROM Supplies-schema Sc1

? ? WHERE NOT EXISTS (

? ? ? ? SELECT *

? ? ? ? FROM Supplies-schema Sc2

? ? ? ? WHERE Sc2.partnum = 'P002' AND Sc2.suppliername = Sc1.suppliername

? ? )

? ? AND Sc1.partnum = 'P001'

);

```

(f) 查找為該公司提供最多零件的供應(yīng)商的供應(yīng)商名稱：

```sql

SELECT suppliername

FROM Supplies-schema

GROUP BY suppliername

HAVING COUNT(partnum) = (

? ? SELECT MAX(partCount)

? ? FROM (

? ? ? ? SELECT COUNT(partnum) AS partCount

? ? ? ? FROM Supplies-schema

? ? ? ? GROUP BY suppliername

? ? ) AS subquery

);

```

請注意，這些查詢語句基于提供的關(guān)系模式和要求進(jìn)行了編寫，但實(shí)際運(yùn)行時(shí)可能需要根據(jù)具體的數(shù)據(jù)庫系統(tǒng)和數(shù)據(jù)存儲結(jié)構(gòu)進(jìn)行微調(diào)。