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Home

Technical Articles

Database

Oracle

What is the difference between a procedure and a function in PL/SQL?

In PL/SQL, the core difference between a procedure and a function is its purpose and return value: 1. The procedure is used to perform operations, and does not force the return value, but can return multiple values ??through the OUT parameter; 2. The function is used to calculate and return a single value, which is often used in expressions or SQL statements; for example, the update_salary procedure can perform update operations, while the get_bonus function returns the calculated bonus amount; the procedure is suitable for data modification and multiple output scenarios, and the function is suitable for calculation logic that needs to be embedded in SQL; in addition, functions can be called in the process, but procedures cannot be called directly in the SQL statement.

Jun 21, 2025 am 12:05 AM

pl/sql

Can you explain the concept of an Oracle schema and its relationship to user accounts?

In Oracle, the schema is closely associated with the user account. When creating a user, the same-name mode will be automatically created and all database objects in that mode are owned. 1. When creating a user such as CREATEUSERjohn, create a schema named john at the same time; 2. The tables created by the user belong to their schema by default, such as john.employees; 3. Other users need authorization to access objects in other schemas, such as GRANTSELECTONsarah.departmentsTOjohn; 4. The schema provides logical separation, used to organize data from different departments or application modules.

Jun 20, 2025 am 12:11 AM

User Account

What is PL/SQL, and how does it extend SQL with procedural capabilities?

PL/SQLextendsSQLwithproceduralfeaturesbyaddingvariables,controlstructures,errorhandling,andmodularcode.1.Itallowsdeveloperstowritecomplexlogiclikeloopsandconditionalswithinthedatabase.2.PL/SQLenablesthedeclarationofvariablesandconstantsforstoringinte

Jun 19, 2025 am 12:03 AM

pl/sql SQL extensions

How do sequences generate unique numbers in Oracle, and what are their typical use cases?

Oracle sequences are independent database objects used to generate unique values ??across sessions and transactions, often used for primary keys or unique identifiers. Its core mechanism is to generate a unique value through NEXTVAL increment, and CURRVAL obtains the current value without incrementing. Sequences do not depend on tables or columns, and support custom start values, step sizes and loop behaviors. Common scenarios during use include: 1. Primary key generation; 2. Order number; 3. Batch task ID; 4. Temporary unique ID. Notes include: transaction rollback causes gaps, cache size affects availability, naming specifications and permission control. Compared to UUID or identity columns, sequences are suitable for high concurrency environments, but they need to be traded down based on the needs.

Jun 18, 2025 am 12:03 AM

oracle sequence Unique number

How does Oracle manage memory allocation for the SGA and PGA (Automatic Memory Management vs. Manual)?

OraclemanagesmemoryallocationfortheSGAandPGAintwoways:AutomaticMemoryManagement(AMM)andManualMemoryManagement.1.AMMallowsOracletoautomaticallydividememorybetweenSGAandPGAbasedonworkloadbysettingMEMORY_TARGETandMEMORY_MAX_TARGET,offeringeaseofmanageme

Jun 17, 2025 am 09:21 AM

SGA/PGA

How do subqueries (scalar, multi-row, correlated) enhance Oracle SQL capabilities?

SubqueriesinOracleSQL—scalar,multi-row,andcorrelated—enhancequeryflexibilitybyenablingmodularlogic,dynamicdatahandling,andcomplexfiltering.Scalarsubqueriesreturnasinglevalueandareidealforcomparisonsorexpressionssuchascomputingtheaveragesalary;1.theys

Jun 14, 2025 am 12:07 AM

Subquery

What is Automatic Storage Management (ASM), and what are its benefits for Oracle database storage?

AutomaticStorageManagement(ASM)isOracle’sbuilt-instoragesolutiondesignedtosimplifyandoptimizethemanagementofdatabasestorage.1.IteliminatestheneedforexternalvolumemanagersorRAIDconfigurations.2.ASMautomaticallybalancesI/Oacrossdisks,preventinghotspots

Jun 13, 2025 am 12:01 AM

asm

How are exceptions handled in PL/SQL (predefined, user-defined)?

InPL/SQL,exceptionsarecategorizedintotwotypes:predefinedanduser-defined.1.Predefinedexceptionsarebuilt-inerrorssuchasNO_DATA_FOUND,TOO_MANY_ROWS,VALUE_ERROR,ZERO_DIVIDE,andINVALID_NUMBER,whichareautomaticallyraisedduringspecificruntimeerrors.2.User-d

Jun 12, 2025 am 10:23 AM

What are the differences between physical, logical, and snapshot standby databases in Data Guard?

OracleDataGuard supports three standby databases: physical, logical, and snapshot. 1. The physical standby database is a byte-level copy of the main library, synchronized using RedoApply, suitable for disaster recovery; 2. The logical standby database applies changes through SQLApply, which can be structured different from the main library, suitable for reporting and selective replication; 3. The snapshot standby database is based on physical standby and can be converted into a writable state for testing, and FlashbackDatabase needs to be enabled. Select according to requirements: requires data consistency and quick switching of physics, requires flexibility and support for report selection logic, and select snapshots if you need to test the production environment copy.

Jun 11, 2025 am 12:01 AM

Standby database

What are the differences between physical and logical database structures in Oracle?

The logical structure of Oracle database focuses on how data is organized by users and developers, including tables, views, patterns and table spaces; the physical structure involves the actual storage of data on disk, including data files, redo logs, control files, etc. 1. The logical structure includes tables, views, indexes, patterns and table spaces, which determine how users access data; 2. The physical structure consists of data files, redo logs, control files and archive logs, which are responsible for the persistence and recovery of data; 3. The table space is a key bridge connecting logic and physics, and its capacity is limited by the underlying data files; 4. Different roles have different levels of attention, developers focus on logic optimization, and DBA pays more attention to physical management; 5. Understanding the differences between the two can help efficiently troubleshoot problems, optimize performance and reasonable management

Jun 10, 2025 am 12:01 AM

oracle Database structure

How can you diagnose and resolve ORA-01555 'snapshot too old' errors?

The ORA-01555 error originates from the overwrite of the historical data required in the Oracle database consistency reading mechanism, which includes insufficient UNDO table space, long transaction interference, poor SQL execution efficiency and delayed block cleaning mechanism. 1. Check and expand the UNDO table space, enable automatic expansion or increase UNDO_RETENTENT time; 2. Find and terminate transactions that have not been submitted for a long time to free UNDO resources; 3. Optimize the SQL statement execution plan, add appropriate indexes and process big data in batches; 4. Avoid executing large-scale queries immediately after large-scale DML operations, and manually refresh the buffer cache if necessary. The above measures can be effectively solved.

Jun 09, 2025 am 12:03 AM

What is the role of initialization parameters (pfile vs. spfile) in Oracle database startup?

Oracle database starts with initialization parameters to define instance behavior, which are stored in PFILE (text file) or SPFILE (binary file). 1.PFILE is a text file that can be edited manually, while SPFILE is managed by Oracle and supports dynamic modification. 2. SPFILE supports persistence and session-level changes. After PFILE is modified, it needs to be restarted to take effect. 3. When Oracle starts, please be given priority to looking for SPFILE. If not, use PFILE. 4. You can convert file types to each other through the CREATESPFILE/PFILE command. 5. Use SHOWPARAMETERspfile; you can view the file type currently used. Select the correct file type

Jun 08, 2025 am 12:04 AM

How can you monitor and manage tablespace usage in an Oracle database?

ToeffectivelymonitorandmanagetablespaceusageinanOracledatabase,beginbycheckingcurrentusagewithsystemviewslikeDBA_TABLESPACE_USAGE_METRICSandDBA_DATA_FILEStoidentifyutilizationlevelsandavailablespace.1)Regularlyexecutequeriestodetecttrendsandsetalerts

Jun 07, 2025 am 12:08 AM

What is the role of the DBA_ , ALL_ , and USER_ data dictionary views?

The DBA_, ALL_, and USER_ views in Oracle database represent data dictionary perspectives at different permission levels. 1. The USER_view displays objects owned by the user, such as USER_TABLES and USER_CONSTRAINTS, which are suitable for developers or ordinary users to view their own objects; 2. The ALL_view contains all objects that the user has access rights, whether owned or not, such as ALL_TABLES and ALL_VIEWS, which are suitable for confirming accessible data in shared mode; 3. The DBA_view displays all objects in the database, but is only accessible to DBA or users with advanced permissions, such as DBA_USERS and DBA_TABLES, which are often used at the system level.

Jun 06, 2025 am 11:25 AM

Data Dictionary DBA View