What is data definition language

Data Definition Language (DDL) is a language responsible for data structure definition and database object definition in the SQL language. It consists of three syntaxes: CREATE, ALTER and DROP.

was originally started by the Codasyl (Conference on Data Systems Languages) data model, and is now incorporated into SQL instructions as a subset of it .

Most DBMS currently support DDL operations on database objects. Some databases (such as PostgreSQL) can put DDL in transaction instructions, which means it can be withdrawn (Rollback). Newer versions of DBMS will add DDL-specific triggers so that database administrators can track modifications from DDL.

Sql statements are divided into three categories:

Data definition language, responsible for creating, modifying, and deleting tables, indexes, views, functions, Objects such as stored procedures and triggers;

Data manipulation language, responsible for inserting, modifying, deleting and other operations of data in the database;

Data control language, used to grant and revoke user permissions.

Example

##CREATE

For example, the command of CREATE DATABASE (create database) is:

CREATE DATABASE Sales
ON ( NAME = Sales_dat, FILENAME = 'saledat.mdf', SIZE = 10, MAXSIZE = 50, FILEGROWTH = 5 )
LOG ON ( NAME = Sales_log, FILENAME = 'salelog.ldf', SIZE = 5MB, MAXSIZE = 25MB, FILEGROWTH = 5MB )

Another example, the command of CREATE TABLE (create data table) is:

CREATE TABLE [dbo].[PurchaseOrderDetail]
(
[PurchaseOrderID] [int] NOT NULL REFERENCES Purchasing.PurchaseOrderHeader(PurchaseOrderID), -- 具引用完整性限制字段
[LineNumber] [smallint] NOT NULL,
[ProductID] [int] NULL REFERENCES Production.Product(ProductID), -- 具引用完整性限制字段
[UnitPrice] [money] NULL,
[OrderQty] [smallint] NULL,
[ReceivedQty] [float] NULL,
[RejectedQty] [float] NULL,
[DueDate] [datetime] NULL,
[rowguid] [uniqueidentifier] ROWGUIDCOL NOT NULL CONSTRAINT [DF_PurchaseOrderDetail_rowguid] DEFAULT (newid()), -- 具限制字段，并有默认值
[ModifiedDate] [datetime] NOT NULL CONSTRAINT [DF_PurchaseOrderDetail_ModifiedDate] DEFAULT (getdate()), -- 具限制字段，并有默认值
[LineTotal] AS (([UnitPrice]*[OrderQty])),
[StockedQty] AS (([ReceivedQty]-[RejectedQty])),
CONSTRAINT [PK_PurchaseOrderDetail_PurchaseOrderID_LineNumber] -- 主键宣告
PRIMARY KEY CLUSTERED ([PurchaseOrderID], [LineNumber])
WITH (IGNORE_DUP_KEY = OFF)
)
ON [PRIMARY]

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