关于SQL|DDL, DML, DCL and TCL 命令

目前网上讲解五花八门，这里援引一下Wiki和某公网的总结

援引Wiki（不贴地址了）

1.A data definition language or data description language (DDL) is a syntax similar to a computer programming language for defining data structures, especially database schemas.

2.A data manipulation language (DML) is a computer programming language used for adding (inserting), deleting, and modifying (updating) data in a database. A DML is often a sublanguage of a broader database language such as SQL, with the DML comprising some of the operators in the language.[1] Read-only selecting of data is sometimes distinguished as being part of a separate data query language (DQL), but it is closely related and sometimes also considered a component of a DML; some operators may perform both selecting (reading) and writing.

3.A data control language (DCL) is a syntax similar to a computer programming language used to control access to data stored in a database (Authorization). In particular, it is a component of Structured Query Language (SQL).

 

援引网站：https://www.geeksforgeeks.org/sql-ddl-dml-dcl-tcl-commands/

Structured Query Language(SQL) as we all know is the database language by the use of which we can perform certain operations on the existing database and also we can use this language to create a database. SQL uses certain commands like Create, Drop, Insert etc. to carry out the required tasks.

These SQL commands are mainly categorized into four categories as discussed below:

 

1. DDL(Data Definition Language) : DDL or Data Definition Language actually consists of the SQL commands that can be used to define the database schema. It simply deals with descriptions of the database schema and is used to create and modify the structure of database objects in database.

   Examples of DDL commands:

   • CREATE – is used to create the database or its objects (like table, index, function, views, store procedure and triggers).
   • DROP – is used to delete objects from the database.
   • ALTER-is used to alter the structure of the database.
   • TRUNCATE–is used to remove all records from a table, including all spaces allocated for the records are removed.
   • COMMENT –is used to add comments to the data dictionary.
   • RENAME –is used to rename an object existing in the database.
2. DML(Data Manipulation Language) : The SQL commands that deals with the manipulation of data present in database belong to DML or Data Manipulation Language and this includes most of the SQL statements.

   Examples of DML:

   • SELECT – is used to retrieve data from the a database.
   • INSERT – is used to insert data into a table.
   • UPDATE – is used to update existing data within a table.
   • DELETE – is used to delete records from a database table.
3. DCL(Data Control Language) : DCL includes commands such as GRANT and REVOKE which mainly deals with the rights, permissions and other controls of the database system.

   Examples of DCL commands:

   • GRANT-gives user’s access privileges to database.
   • REVOKE-withdraw user’s access privileges given by using the GRANT command.
4. TCL(transaction Control Language) : TCL commands deals with the transaction within the database.

   Examples of TCL commands:

   • COMMIT– commits a Transaction.
   • ROLLBACK– rollbacks a transaction in case of any error occurs.
   • SAVEPOINT–sets a savepoint within a transaction.
   • SET TRANSACTION–specify characteristics for the transaction.

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