數據庫如何處理數據庫太大

Before learning the data manipulation in a network model, we are discussing data manipulation language, so what is the data manipulation language?

在學習網絡模型中的數據操作之前，我們正在討論數據操作語言，那么什么是數據操作語言？

數據處理語言(DML) (Data Manipulation Language (DML))

A data manipulation language (DML) is used to perform tasks such as to search and retrieve, insert, delete, modify records from the database and to connect and disconnect records from the set occurrences.

數據操作語言(DML)用于執行任務，例如從數據庫中搜索和檢索，插入，刪除，修改記錄，以及將記錄與所設置的事件連接和斷開連接。

Host languages are a general-purpose programming language. The data manipulation language that is associated with the network model consists of record-at-a-time commands that are embedded in host languages. These embedded commands are also termed as data sublanguage in DML.

宿主語言是一種通用的編程語言。 與網絡模型關聯的數據操作語言由嵌入在主機語言中的一次記錄命令組成。 這些嵌入式命令在DML中也稱為數據子語言。

COBOL and PL/I are the most commonly used host languages. On the other hand, to show the program segments, PASCAL notation is augmented with network DML commands.

COBOL和PL / I是最常用的宿主語言。 另一方面，為了顯示程序段，使用網絡DML命令來增強PASCAL表示法。

網絡數據庫操作 (Network Database Manipulation)

Basic Concepts for Network Database Manipulation...

網絡數據庫操作的基本概念...

In order to understand network data manipulation, we must get familiar with the basic concepts that show us, how data manipulation programs are written?

為了理解網絡數據操縱 ，我們必須熟悉向我們展示的基本概念， 如何編寫數據操縱程序？

There are two separate software systems, the database system, and the host programming language. Both the software systems are linked together by a common interface and only through this interface, they are allowed to communicate. It is important to identify specific records of the database as current records because DML commands are record-at-a-time.

有兩種獨立的軟件系統，即數據庫系統和主機編程語言。 這兩個軟件系統都通過一個公共接口鏈接在一起，并且只有通過該接口才能進行通信。 將數據庫的特定記錄標識為當前記錄非常重要，因為DML命令是一次記錄。

To keep track of the number of current records and set occurrences, DBMS uses the mechanism known as currency indicators. Also, to hold the records of different recode types, the host programming language requires local program variables, so that the host program can manipulate their contents. The set of these local variables in the program is termed as the User Work Area (UWA).

為了跟蹤當前記錄的數量并設置發生次數，DBMS使用稱為貨幣指標的機制。 同樣，為了保存不同記錄類型的記錄，主機編程語言需要本地程序變量，以便主機程序可以操縱其內容。 程序中這些局部變量的集合稱為用戶工作區(UWA)。

UWA is a set of program variables that are declared in the host program in order to communicate the contents of individual records between the DBMS and the host program. A program variable with the same format must be declared in the program, for each record type in the database schema.

UWA是在主機程序中聲明的一組程序變量，以便在DBMS和主機程序之間傳遞各個記錄的內容。 對于數據庫模式中的每種記錄類型，必須在程序中聲明具有相同格式的程序變量。

Currency Indicators

貨幣指標

In the network DML, to keep the trace of the search is critical as retrievals and updates are controlled by moving or navigating through the database records. Currency indicators keep the track of most recently accessed records and the set occurrences by DBMS. Each currency indicator can be termed as record pointer (or record address), that points to a single database record. Many currency indicators are used in network DBMS:

在網絡DML中，保持跟蹤是至關重要的，因為通過移動或瀏覽數據庫記錄來控制檢索和更新。 貨幣指示器跟蹤最近訪問的記錄以及DBMS設置的事件。 每個貨幣指標都可以稱為記錄指針(或記錄地址)，它指向單個數據庫記錄。 網絡DBMS中使用了許多貨幣指標：

• Current of record type:

  當前記錄類型：

  DBMS keeps the track of most recently accessed record of each record type. The current record becomes undefined if no record gets accessed from that record type.

  DBMS跟蹤每種記錄類型的最新訪問記錄。 如果未從該記錄類型訪問任何記錄，則當前記錄將變為未定義。

• Current of set type:

  設定類型的電流：

  DBMS keeps the track of most recently accessed set occurrence of each record type. Set which is the owner of the 16 member records, the set occurrence is stated by a single record from that set. Therefore the current set points to a record, although it is used to keep track of a set occurrence. The current set is undefined if the program has not accessed any record from that set type.

  DBMS跟蹤每種記錄類型的最近訪問集的發生情況。 集合是16個成員記錄的所有者，集合的出現由該集合中的單個記錄表示。 因此，當前集合指向記錄，盡管它用于跟蹤集合出現。 如果程序尚未訪問該集合類型的任何記錄，則當前集合未定義。

• Current of the run unit (CRU):

  運行單元(CRU)的電流：

  CRU is a database access program and gets executed on the computer system. In the database, CRU keeps track of the record most recently accessed by the program for each run unit such that this record can be from any record type.

  CRU是數據庫訪問程序，可在計算機系統上執行。 在數據庫中，CRU跟蹤程序最近為每個運行單元訪問的記錄，以便該記錄可以來自任何記錄類型。

Whenever a DML command get executed by a program, the currency indicators for record types and set types that are affected by the command get updated by DBMS.

每當程序執行DML命令時，DBMS都會更新該命令所影響的記錄類型和集合類型的貨幣指標。

Status Indicators

狀態指示燈

After each DML command gets executed, several status indicators return an indication of success or failure. With such indication, the program can check the values of these status indicators and take appropriate action, either to continue execution or to transfer to an error-handling routine.

在執行每個DML命令之后，幾個狀態指示器將返回成功或失敗的指示。 有了這樣的指示，程序可以檢查這些狀態指示器的值并采取適當的措施，以繼續執行或轉移到錯誤處理例程。

Assuming DB_STATUS to be implicitly declared in the host program, we call the main status variable. The value of DB_STATUS indicates whether the command was successful or whether an exception occurred after each DML command is executed. The most common exception to occur in any DML command is the END_OF_SET (EOS) exception.

假設在主機程序中隱式聲明了DB_STATUS ，我們將調用主狀態變量。 DB_STATUS的值指示命令是否成功執行或在執行每個DML命令之后是否發生異常。 在任何DML命令中最常見的異常是END_OF_SET(EOS)異常。

翻譯自: https://www.includehelp.com/dbms/data-manipulation-in-a-network-database.aspx

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