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
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:
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 to control the time and effect of database manipulation transactions. Monitor the database, 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.
During database insertion, deletion and modification operations, 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 submission
Submit completed indirectly using SQL commands is implicit submission. These commands are:
If AUTOCOMMIT is set to ON, the system will automatically submit after the insert, modify, and delete statements are executed. This is automatic submission.
The format is: SQL>SET AUTOCOMMIT ON;
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