What is the difference between PL/SQL and SQL in Oracle?
The main differences between PL/SQL and SQL in Oracle include:
1. Programming ability:PL/SQL is a programmatic language that supports variables, conditional statements and loops, while SQL is mainly used for data query and update.
2. Execution method:PL/SQL can execute a whole piece of code, and SQL statements are executed one by one.
3. Purpose:PL/SQL is suitable for writing complex database applications, and SQL is used for simple data operations and queries.
What is the table partition in an Oracle database? What are its advantages?
Table partitioning is a technique in Oracle databases that break down tables and indexes into smaller, more manageable parts. Its advantages include:
1. Performance improvement:It can improve query performance, especially for large tables.
2. Manageability:Partition tables are easier to manage and maintain.
3. High availability:Partitioning helps improve data availability and recovery performance.
The difference between index organization tables and heap organization tables in Oracle.
The differences between index organization tables (IOT) and heap organization tables in Oracle mainly include:
1. Storage structure:IOT is an index-based table structure, where data is stored in primary key order; heap organization tables are caused by data in physical storage without specific order.
2. Performance:IOT performs better on some queries, especially primary key searches.
3. Use scenarios:IOT is suitable for scenarios with frequent primary key access modes, while heap organization tables are suitable for a wider range of scenarios.
What are the sequences in Oracle databases? What are its uses?
A sequence is a database object in Oracle that generates unique numerical values. Uses include:
1. Generate unique identifier:Used to automatically generate unique row identifiers.
2. Automatic numbering:Numbers are automatically generated when inserting records.
What are triggers in Oracle? What types of them are there?
A trigger is a special type of stored procedure in Oracle databases that is automatically executed when a specific database operation occurs. Types include:
1. Row-level trigger:Flashed when modifying a certain line.
2. Statement-level trigger:Flashed when a specific SQL statement is executed.
3. BEFORE and AFTER triggers:The operation is triggered before and after it is executed.
What parts does the logical storage structure of Oracle database include?
The logical storage structure of Oracle database mainly includes:
1. Data block:Minimum storage unit.
2. Paragraph:A set of data blocks with specific purposes, such as table segments and index segments.
3. Area (Extent):It consists of continuous data blocks and is used to store specific segments.
4. Table space:The highest logical level of data in a database consists of one or more data files.
MVC (multi-version concurrency control) mechanism in Oracle.
The MVCC mechanism in Oracle is used to achieve efficient concurrent control, and its core principles include:
1. Data versioning:Provides a snapshot version of the data for each transaction to support simultaneous read and write operations.
2. Non-blocking reading:The read operation will not block the write operation and vice versa.
3. Isolation level:Support different transaction isolation levels to ensure data consistency and concurrency.
What is the Explain Plan command in Oracle used for?
The Explain Plan command is used in Oracle to display the execution plan of SQL statements. Its main functions include:
1. Query optimization:Help developers understand and optimize SQL query performance.
2. Analyze the execution path:Shows how the database performs specific SQL queries, including the index and connection methods used.
What are V$ views and DBA_views in Oracle?
V$ view and DBA_view are used in Oracle to provide dynamic and static information of the database:
1. V$ view:Provides dynamic performance and statistical information of the database, such as V$SESSION and V$SQL.
2. DBA_View:Provides global static information of the database, such as DBA_TABLES and DBA_USERS.
What is a data pump in an Oracle database?
Data Pump is a tool for Oracle databases for efficient data import and export. Its features include:
1. High performance:Compared with traditional exp and imp tools, data pumps provide faster data movement.
2. Flexibility:Supports multiple data extraction and loading methods.
3. Network transmission:Supports direct transmission of data over the network.
What is the PL/SQL package in Oracle? What are its advantagessome?
The PL/SQL package is a collection of logically related PL/SQL program units, such as procedures and functions. Its advantages include:
1. Modular:Easy to organize and manage complex PL/SQL code.
2. Performance:It can improve code execution efficiency.
3. Encapsulation:Provides better data hiding and encapsulation.
What is Flashback Query in Oracle?
Flashback query is a function in Oracle that can view the data status of a certain point in the past. It allows users to query historical data without restoring or rolling back the database.
What is Automatic Storage Management (ASM) in Oracle?
Automatic Storage Management (ASM) is a database file system and disk manager provided by Oracle to simplify the layout and management of database files. Its features include:
1. Simplify management:Automatically manage database storage structure.
2. Improve performance:Improve database performance by balancing I/O allocation.
3. Enhance reliability:Supports disk mirroring and striping to improve data reliability.
Connection pooling technology and its advantages in Oracle.
Connection pooling technology is used in Oracle to manage database connection resources. Its advantages include:
1. Resource reuse:Connections can be shared by multiple users, avoiding the overhead of frequent creation and destruction of connections.
2. Improve performance:Reduces database connection and disconnection time and improves application response speed.
3. Better scalability:Supports the performance stability of applications under high loads.
What are cursors in Oracle databases? What is its function?
Cursors are used in Oracle databases to process the result set returned by SQL statements. Its functions include:
1. Processing line by line:Cursors allow line-by-line access and processing of query results.
2. More accurate data operations:Through cursors, complex logical operations can be performed on each row in the dataset.
What is the tablespace in Oracle? How does it work?
Tablespace is the logical unit of data stored in Oracle database. It works:
1. Data organization:The tablespace contains one or more data files that store the actual data in the database.
2. Space management:The storage structure of the database can be effectively managed through table space.
What is the difference between a snapshot and a materialized view in Oracle?
Although snapshots (now commonly referred to as materialized views) and materialized views are often used alternately in Oracle, they have the following differences:
1. Update method:Materialized views can be refreshed regularly, while snapshots are a more static representation of data.
2. Application scenarios:Materialized views are suitable for scenarios that require regular updates and efficient queries, such as data warehouses.
What is the difference between NVL and COALESCE functions in Oracle database?
NVL and COALESCE are both functions in Oracle that are used to handle NULL values, but there are the following differences:
1. Number of parameters:NVL accepts two parameters, while COALESCE can accept two or more parameters.
2. Evaluation method:NVL always evaluates two parameters, while COALESCE only evaluates parameters until the first non-NULL value is found.
What is dynamic SQL in Oracle? What are its advantages?
Dynamic SQL refers to SQL statements built and executed at runtime. Its advantages in Oracle include:
1. Flexibility:Complex SQL statements can be dynamically constructed based on program logic.
2. Adaptability:Can deal with database operations that cannot be determined at compile time.
The difference between archived log mode and non-archived log mode in Oracle database.
Archive log mode and non-archive log mode are two operating modes of Oracle database:
1. Archive log mode:In this mode, all redo log files will be saved before being overwritten.
2. Non-archive log mode:In this mode, the redo log file can be overwritten when space needs it, without saving historical logs.
3. Data recovery:Archive log mode supports more complete data recovery options.
What is the synonym in Oracle? What is its usage scenario?
Synonym is an alias in Oracle that points to another object (such as tables, views, etc.). Its usage scenarios include:
1. Simplified name:Provides simple alias for complex object names.
2. Security:Enhance security by hiding the real name and location of an object through synonyms.
What is the difference between REDO logs and UNDO data in Oracle databases?
The main differences between REDO logs and UNDO data in Oracle databases include:
1. Purpose:REDO logs are used to record all committed changes in the database for recovery in the event of a system failure; UNDO data is used to store uncommitted changes to support rollback operations.
2. Persistence:REDO logs are persisted to disk, and UNDO data is usually stored in UNDO tablespace.
What is a data dictionary in Oracle? What is its main purpose?
A data dictionary is a collection of metadata stored in an Oracle database about the database structure. Its main uses include:
1. Storage table structure:Including definition information for tables, columns, indexes, constraints, etc.
2. Store system permissions and role information:Contains detailed information about database users, permissions and roles.
3. Query optimization:Used for query optimization during the optimizer decision process.
How does PL/SQL exception handling work in Oracle?
PL/SQL's exception handling mechanism allows for graceful handling of errors and exceptions in a program. It works:
1. Abnormal recognition:Identify potential error points in the code.
2. Exception handling:Use the EXCEPTION clause to handle errors to prevent program crashes.
3. Custom exception:Custom exceptions can be defined and thrown.
What is a global temporary table in Oracle? What are its characteristics and uses?
Global temporary table is a special table in Oracle, and its features and uses include:
1. Data range:The data is only visible in the current session.
2. Performance optimization:Used to store temporary data and improve processing speed.
3. Reduce the impact on the main database:Since the data is not permanently stored, it has a smaller impact on the database.
The locking mechanism and its type in Oracle.
Oracle database lock mechanism is used to control concurrent access to shared resources. The types of locks include:
1. DML lock:Used to control concurrent access to data manipulation languages (such as INSERT, UPDATE).
2. DDL lock:Used to control concurrent access to data definition languages (such as CREATE, ALTER).
3. Internal locks and latches:Automatically managed by Oracle to protect the internal structure of the database.
What are distributed transactions in Oracle? How does it work?
Distributed transactions refer to transactions across multiple database nodes. In Oracle, it works by:
1. Two-stage submission:In order to ensure data consistency across nodes, a two-stage submission protocol is used.
2. Transaction Coordinator:Responsible for coordinating transaction submission or rollback between each node.
What are the common methods for Oracle database optimization?
Common methods for Oracle database optimization include:
1. SQL optimization:Rewrite inefficient SQL queries.
2. Use index:Rationally create and use indexes to speed up querying.
3. Memory management:Adjust SGA and PGA to optimize memory usage.
4. Physical design optimization:Optimize data files, tablespaces and disk layout.
What is RMAN in Oracle? What is its main purpose?
RMAN (Recovery Manager) is a database backup and recovery solution provided by Oracle. Its main uses include:
1. Database backup:Provides backups at the full library, tablespace or data file level.
2. Database recovery:Supports full and point-time recovery.
3. Backup management:Manage and maintain backup sets and recovery strategies for databases.
The difference between logical backup and physical backup in Oracle.
Logical backup and physical backup are two different types of backup methods in Oracle:
1. Logical backup:Export logical data of specific objects (such as tables, views, stored procedures) in the database.
2. Physical backup:Copy database files, such as data files, control files and redo log files.
3. Use scenarios:Logical backups are suitable for data migration and lightweight backups; physical backups are suitable for disaster recovery and full backups.
How does the locking mechanism in Oracle database work?
The locking mechanism of Oracle database works in the following ways:
1. Type of lock:Oracle provides a variety of lock types, including row-level locks, table-level locks and higher-level locks to control concurrent access to data.
2. Automatic row-level locking:When the data is modified, Oracle automatically places row-level locks on the modified row to prevent other transactions from modifying the same row at the same time.
3. Intention lock:Oracle uses intent locks to indicate the locking intent to a data row in a data structure (such as a table).
4. Deadlock detection:Oracle monitors and solves the deadlock problem. When a deadlock is detected, one of the transactions will be automatically rolled back to unlock.
5. Lock upgrade:In high concurrency environments, Oracle may upgrade row-level locks to table-level locks to reduce the overhead of lock management.
How does MVCC (Multi-version Concurrency Control) in Oracle Database work?
The working principle of MVCC in Oracle is as follows:
1. Create a copy of the data:When a transaction wants to modify data, Oracle creates a new version for the modified row, retaining the old version.
2. Read consistency:Read operations always access the data version at the beginning of the transaction to ensure the consistency of the data without being affected by the write operations performed simultaneously.
3. Rollback segment:The old data version is stored in the rollback segment to support the access and transaction rollback of the old version.
4. Isolation of different transactions:Different transactions can access different versions of the same data at the same time to achieve transaction isolation.
5. Reduce lock competition:Since read and write operations work on different versions of data, MVCC significantly reduces lock competition.
What is the tablespace in Oracle and what is its function?
Table spaces in Oracle are logical divisions of data files, and their functions include:
1. Data management:Table space provides a way to organize database objects (such as tables and indexes) for easy management and maintenance.
2. Storage management:The distribution and storage of data files can be controlled through table space.
3. Performance optimization:Different tablespaces can be placed on different physical media to optimize access performance.
4. Data isolation:Different users or applications can be allocated to different table spaces as needed to achieve data isolation.
5. Flexible space allocation:Table space allows dynamic resize and provides flexible space allocation methods.
What is the difference between the archive log mode and the non-archive log mode of Oracle database?
The main difference between archived log mode and non-archived log mode is:
1. Data recovery capability:Archive log mode can perform complete data recovery, while non-archive log mode cannot recover data in some cases.
2. Log file processing:In archived log mode, when the redo log is full, the log file will be archived; in non-archive mode, the log file will be overwritten before reuse.
3. Backup method:Archive mode supports hot backup (online backup), while non-archive mode usually only allows cold backups.
4. Space requirements:Archive mode requires more disk space to store archived log files.
5. Applicable scenarios:Archive mode is suitable for environments with high data recovery requirements, while non-archive mode is suitable for environments with low data recovery requirements.
What is the role of sequences in Oracle, and how to create and use sequences?
The role of sequences in Oracle and their creation and usage are as follows:
1. Function:Sequences are used to generate unique numeric values, usually used to automatically generate primary key values.
2. Create a sequence:
CREATE SEQUENCE seq_name START WITH 1 INCREMENT BY 1 NOCACHE;
This example creates a sequence starting from 1, incrementing by 1 each time.
3. Use sequence:useNEXTVALAttributes get the next value of the sequence, for example:
INSERT INTO table_name (id, name) VALUES (seq_name.NEXTVAL, 'Example');
This inserts the next value of the sequence into the id column of the table.
4. Cache mechanism:The cache size of the sequence can be set to improve the efficiency of obtaining sequence values.
5. The properties of the sequence:The sequence can also set attributes such as maximum value, minimum value, and whether to cycle to meet different needs.
What is PL/SQL in Oracle and how is it different from SQL?
PL/SQL is a processed SQL language for Oracle databases. The main differences between it and SQL include:
1. Program structure:PL/SQL supports program structures such as variables, conditional statements, and loops, while SQL is mainly used for data query and update.
2. Process characteristics:PL/SQL allows the creation of stored procedures, functions, triggers, etc., and can encapsulate complex logic, but SQL does not have these functions.
3. Error handling:PL/SQL provides an exception handling mechanism that can catch and handle runtime errors, while error handling in SQL is not so flexible.
4. Performance optimization:PL/SQL can optimize performance by reducing the interaction between the application server and the database.
5. Bind variables:PL/SQL supports the use of binding variables to improve code efficiency and security.
How do indexes in Oracle databases work and what are their types?
How indexes in Oracle databases work and their types include:
1. Working principle:An index is a separate data structure (usually B-Tree) that can quickly access specific information in tables and reduce the need for full table scanning.
2. B-Tree index:Standard index type, suitable for equal value query and range query.
3. Bitmap index:Applicable to columns with a small number of different values, such as gender or status fields.
4. Composite index:Contains two or more columns, suitable for queries involving multiple columns.
5. Functional index:Create indexes based on function results of data in tables, which are suitable for cases where function results are frequently queried.
What is a table partition in an Oracle database and what are its benefits?
Table partitioning in Oracle databases is to distribute the data of the table in different physical parts, and its benefits include:
1. Improve performance:By querying only relevant partitions, the amount of query data is reduced, thereby improving query performance.
2. Manage a large amount of data:Partitioning helps to manage and maintain large tables more efficiently.
3. Backup and restore:It can independently back up and restore each partition to improve efficiency.
4. Improve usability:When some partitions have problems, other partitions are still available.
5. Optimized storage:Different partitions can be stored on different types of storage media.
What are the views of Oracle databases, what are their functions and advantages and disadvantages?
Views in Oracle are virtual tables based on SQL statements, and their functions, advantages and disadvantages include:
1. Function:Views can encapsulate complex query statements to simplify user operations.
2. Data security:Views can restrict users from accessing specific data and enhance data security.
3. Logical independence:Changes to the view structure will not affect the underlying table and provide logical independence.
4. Advantages:Views can simplify complex data operations and facilitate management and maintenance.
5. Disadvantages:Views can affect performance, especially when it involves large amounts of data and complex operations.
What are the triggers for Oracle databases, what are its types, and how to use them?
The trigger in the Oracle database is an automatically executed database object, and its types and usage methods include:
1. Definition:A trigger is a PL/SQL code block that is automatically executed in response to a specific database event (such as insertion, update, or delete).
2. Type:
Row-level trigger:Execute once for each affected row.
Statement-level trigger:Executes once for each triggering SQL statement.
3. How to use:
CREATE OR REPLACE TRIGGER trigger_name BEFORE INSERT ON table_name FOR EACH ROW BEGIN -- Trigger logic END;
This example creates a row-level trigger that is executed before the insertion operation.
4. Use scenarios:Triggers can be used to automatically maintain data integrity, automatically update statistics, write audit logs, etc.
5. Notes:Triggers need to be used with caution as they can affect performance and make error debugging more complicated.
How to implement partitioning of tables in Oracle database?
Oracle database optimizes query performance and manages large tables through table partitioning. The steps to implement table partitioning include:
1. Select the partition type:Select the appropriate partition type according to the data characteristics, such as range partition (Range), list partition (List), hash partition (Hash), or composite partition (Composite).
2. Define the partition key:Select one or more columns as partition keys. The values of these keys will determine which partition the data is stored in.
3. Create a partition table:Use the CREATE TABLE statement and specify partition options. For example, using RANGE partitions can specify the range of values for each partition.
4. Manage partitions:Partitions can be added, deleted, or merged to adapt to data changes.
5. Maintain partition table:Regularly perform maintenance tasks for partition tables, such as reorganizing partitions to optimize performance.
Sample code:
CREATE TABLE sales (
sale_date DATE,
product_id NUMBER,
amount NUMBER
)
PARTITION BY RANGE (sale_date) (
PARTITION p1 VALUES LESS THAN (TO_DATE('2022-01-01', 'YYYY-MM-DD')),
PARTITION p2 VALUES LESS THAN (TO_DATE('2022-07-01', 'YYYY-MM-DD')),
PARTITION p3 VALUES LESS THAN (TO_DATE('2023-01-01', 'YYYY-MM-DD'))
);
How to optimize SQL queries in Oracle?
Optimizing SQL queries in Oracle usually involves the following steps:
1. Use EXPLAIN PLAN:Use EXPLAIN PLAN to analyze the execution plan of the query and identify potential performance bottlenecks.
2. Index optimization:Create appropriate indexes to speed up queries while avoiding increased maintenance costs caused by over-index.
3. SQL rewrite:Rewrite inefficient SQL statements, such as replacing subqueries with joins (JOIN), and optimizing WHERE clause conditions.
4. Use binding variables:Avoid hard-coded values, use binding variables to reduce parsing overhead and improve SQL reuse.
5. Data partition:Partition data on large tables to improve query efficiency.
6. Avoid full table scanning:Reduce the occurrence of full table scans through appropriate indexing and query conditions.
7. Use Oracle Optimizer prompts:Use optimizer prompts to affect Oracle's query optimization decisions.
How to use triggers in Oracle?
Using triggers in Oracle includes the following steps:
1. Determine the trigger type:Select whether it is a row-level trigger or a statement-level trigger, and whether it is a BEFORE or AFTER trigger.
2. Write trigger logic:Defines the action to be performed when the trigger event occurs.
3. Create trigger:Use the CREATE TRIGGER statement to create a trigger and specify the trigger conditions and PL/SQL code executed at the time of triggering.
4. Test trigger:Test the trigger to make sure it works as expected.
5. Management triggers:Enable or disable triggers, modify or delete them as needed.
Sample code:
CREATE OR REPLACE TRIGGER audit_salary_changes
BEFORE UPDATE OF salary ON employees
FOR EACH ROW
BEGIN
INSERT INTO salary_audit (employee_id, old_salary, new_salary, change_date)
VALUES (:OLD.employee_id, :, :, SYSDATE);
END;
How does MVCC (Multi-version Concurrency Control) work in Oracle?
Oracle's multi-version concurrency control (MVCC) mechanism works in the following ways:
1. Data versioning:Each time the data is updated, Oracle will retain a version of the old data instead of directly overwriting it.
2. Read consistency:Read operations always access the data version at the moment when the transaction starts, ensuring data consistency.
3. Non-blocking reading:Read operations will not be blocked by write locks because they can access older versions of the data.
4. Row-level locking:The update operation only locks the rows involved, reducing lock competition.
5. Transaction ID and revocation data:Each transaction has a unique ID, and Oracle uses the revocation data to provide the old version of the data.
This mechanism supports high concurrent access while maintaining data consistency and integrity.
What is PL/SQL in Oracle database and what are its characteristics?
PL/SQL is a procedural language extension of Oracle databases, with the following features:
1. Block structure language:PL/SQL is a block-based language that consists of a declaration part, an execution part and an exception handling part.
2. Support process programming:Provides components such as procedures, functions and packages to support complex business logic.
3. Integrated database operations:SQL statements can be directly executed, supporting Data Manipulation Language (DML) and Data Definition Language (DDL).
4. Exception handling:Powerful exception handling mechanism, which can define and handle various runtime errors.
5. High performance:The code is executed on the server side, reducing the overhead of network transmission and environment switching.
6. Security:Support role-based secure access control to improve data security.
Sample code:
CREATE OR REPLACE FUNCTION get_employee_name (employee_id NUMBER) RETURN VARCHAR2 IS
employee_name VARCHAR2(100);
BEGIN
SELECT first_name || ' ' || last_name
INTO employee_name
FROM employees
WHERE id = employee_id;
RETURN employee_name;
EXCEPTION
WHEN NO_DATA_FOUND THEN
RETURN 'Not Found';
END;
How to backup and restore data in Oracle database?
Data backup and recovery of Oracle databases usually involve the following key steps:
1. Select the backup type:Select a full backup (backup of the entire database) or incremental backup according to your needs (backup only the data changed since the last backup).
2. Use the RMAN tool:Use Oracle's Recovery Manager (RMAN) for backup. RMAN provides powerful backup, recovery and maintenance capabilities.
3. Configure the backup policy:Set the frequency and type of backup, such as regular full backups and frequent incremental backups.
4. Perform backup operations:Use the RMAN command or Oracle Enterprise Manager to perform backup operations.
5. Data recovery:In the event of data loss or corruption, use RMAN to perform data recovery. Depending on the type of backup and the degree of damage, a full or partial recovery may be performed.
6. Backup verification and testing:Regularly verify the integrity of the backup and test the recovery process to ensure that the backup can effectively restore data when needed.
Implement high availability in Oracle?
Implementing high availability of Oracle databases usually involves the following policies:
1. Use Oracle Real Application Clusters (RAC):RAC allows multiple database servers to share the same database, providing failover and load balancing.
2. Data copy:Use Oracle Data Guard to realize real-time data replication and failover.
3. Backup and recovery strategies:Establish effective backup and recovery strategies to quickly restore data.
4. Use Oracle Flashback technology:Flashback technology can quickly restore the database to a past state, reducing data recovery time.
5. Monitoring and maintenance:Regularly monitor the health status and performance of the database, and promptly maintain and tune.
How to use PL/SQL to process cursors in Oracle database?
Using PL/SQL to process cursors in Oracle databases usually involves the following steps:
1. Declare cursor:Use the CURSOR keyword to declare a cursor and specify the corresponding SELECT query.
2. Open the cursor:Use the OPEN statement to open the cursor and start traversing the result set.
3. Traversing the result set:Use the FETCH statement to retrieve the data pointed to by the cursor line by line.
4. Close the cursor:After traversing the result set, use the CLOSE statement to close the cursor.
5. Exception handling:Use the exception handling mechanism rationally when handling cursors to ensure that resources are released correctly.
Sample code:
DECLARE
CURSOR employee_cursor IS SELECT id, name FROM employees;
employee_record employee_cursor%ROWTYPE;
BEGIN
OPEN employee_cursor;
LOOP
FETCH employee_cursor INTO employee_record;
EXIT WHEN employee_cursor%NOTFOUND;
-- Process each row of data
END LOOP;
CLOSE employee_cursor;
END;
What are the functions of views in Oracle databases and how do you create and use them?
The functions and creation and usage of views in Oracle database include:
1. Function:Views can simplify complex SQL operations, provide logical display of data, and limit access to data.
2. Create a view:Use the CREATE VIEW statement to create a view and specify the data to be displayed.
3. Use the view:Views can be queried like ordinary tables, but do not store actual data.
4. Update the view:When certain conditions are met, the view can be updated and reflected in the basic table.
5. Management View:You can change the definition of a view (using CREATE OR REPLACE VIEW) or delete the view (using DROP VIEW).
Sample code:
CREATE VIEW employee_view AS SELECT id, name, department FROM employees WHERE department = 'IT';
What are sequences in Oracle databases, and how do you create and use sequences?
The concepts and creation and usage of sequences in Oracle database include:
1. Sequence concept:Sequences are database objects in Oracle that generate unique sequences of numbers, and are usually used to generate primary key values.
2. Create a sequence:Use the CREATE SEQUENCE statement to create a sequence, and you can specify starting value, increment and other parameters.
3. Use sequence:Use the NEXTVAL and CURRVAL pseudo-columns of the sequence to obtain the sequence value.
4. Sequence management:The sequence can be modified (ALTER SEQUENCE) and deleted (DROP SEQUENCE).
5. Application scenarios:Sequences are widely used in scenarios where unique identifiers are required, such as the automatically growing ID field.
Sample code:
CREATE SEQUENCE employee_seq
START WITH 1
INCREMENT BY 1;
Use sequence:
INSERT INTO employees (id, name) VALUES (employee_seq.NEXTVAL, 'John Doe');
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