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1. Data query language DQL

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What are the four categories of database languages?

青灯夜游

Jul 23, 2021 pm 03:41 PM

database language

Database languages are divided into four categories: 1. Data query language DQL; 2. Data manipulation language DML; 3. Data definition language DDL; 4. Data control language DCL.

What are the four categories of database languages?

The operating environment of this tutorial: windows7 system, mysql8 version, Dell G3 computer.

SQL language is divided into four categories: data query language DQL, data manipulation language DML, data definition language DDL, and data control language DCL.

1. Data query language DQL

The basic structure of data query language DQL is a query block composed of SELECT clause, FROM clause, and WHERE

clause ：

SELECT <field name table>

##FROM

WHERE

2. Data manipulation language DML

Data manipulation language DML mainly has three forms:

1) Insert: INSERT

2) Update: UPDATE

3) Delete: DELETE

3. Data Definition Language DDL

Data Definition Language DDL is used to create various objects in the database ----- tables, views,

indexes, synonyms, clusters, etc. such as:

CREATE TABLE/VIEW/INDEX/SYN/CLUSTER
Table view index synonym cluster

DDL operations are submitted implicitly! Cannot rollback

4. Data control language DCL

Data control language DCL is used to grant or revoke certain privileges to access the database, and control the time and effect of database manipulation transactions, and perform operations on the database. Surveillance etc. Such as:

1) GRANT: Authorization.

2) ROLLBACK [WORK] TO [SAVEPOINT]: Roll back to a certain point.

Rollback---ROLLBACK

The rollback command returns the database state to the last submitted state. The format is:

SQL>ROLLBACK;

3) COMMIT [WORK]: Submit.

There are three types of submitted data

In the insertion, deletion and modification operations of the database, the transaction is completed only when it is submitted to the database. Before the transaction is committed, only the person operating the database has the right to see what has been done. Others can only see it after the final commit is completed.

There are three types of submitted data: explicit submission, implicit submission and automatic submission. These three types are described below.

(1) Explicit submission

The submission completed directly using the COMMIT command is an explicit submission. The format is:

SQL>COMMIT;

(2) Implicit submissionThe submission completed indirectly using SQL commands is implicit. submission. These commands are:

ALTER, AUDIT, COMMENT, CONNECT, CREATE, DISCONNECT, DROP, EXIT, GRANT, NOAUDIT, QUIT, REVOKE, RENAME.

(3) Automatic submission

若把AUTOCOMMIT设置为ON，则在插入、修改、删除语句执行后，
系统将自动进行提交，这就是自动提交。其格式为：
SQL>SET AUTOCOMMIT ON；

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(Recommended tutorial:

mysql video tutorial)

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