How to remove Oracle RAC

Usethe||operatortoconcatenatemultiplecolumnsinOracle,asitismorepracticalandflexiblethanCONCAT();2.Addseparatorslikespacesorcommasdirectlywithintheexpressionusingquotes;3.HandleNULLvaluessafelysinceOracletreatsthemasemptystringsduringconcatenation;4.U

OracleSQL's CASE statement is used to implement conditional logic in queries, supporting two forms: 1. Simple CASE is used to compare a single expression with multiple values, such as returning department names according to department_id; 2. Search CASE is used to evaluate multiple boolean conditions, suitable for scope or complex logic, such as classified by salary level; 3. CASE can be used in SELECT, ORDERBY, WHERE (indirect), GROUPBY and HAVING clauses to implement data conversion, sorting, filtering, and grouping; 4. Best practices include always using ELSE to prevent NULL, ensure ending in END, adding alias to the result columns, and avoiding excessive nesting; 5. Compared with the old DECOD

TheWITHclauseinOracle,alsoknownassubqueryfactoring,enablesdefiningcommontableexpressions(CTEs)forimprovedqueryreadabilityandperformance.1.ThebasicsyntaxusesWITHcte_nameAS(SELECT...)followedbyamainqueryreferencingtheCTE.2.AsingleCTEexamplecomputesaver

First, confirm whether the listener on the database server has been started, use lsnrctlstatus to check, if it is not running, execute lsnrctlstart to start; 2. Check whether the HOST and PORT settings in the listener.ora configuration file are correct, avoid using localhost, and restart the listener after modification; 3. Use the netstat or lsof command to verify whether the listener is listening on the specified port (such as 1521). The client can test port connectivity through telnet or nc; 4. Ensure that the server and network firewall allow the listening port communication, the Linux system needs to be configured with firewalld or iptables, and Windows needs to enable inbound

Usejdbc:oracle:thin:@hostname:port:sidforSID-basedconnections,e.g.,jdbc:oracle:thin:@localhost:1521:ORCL.2.Usejdbc:oracle:thin:@//hostname:port/service_nameforservicenames,requiredforOracle12c multitenant,e.g.,jdbc:oracle:thin:@//localhost:1521/XEPDB

Aviewdoesnotstoredataphysicallyandexecutestheunderlyingqueryeachtimeitisaccessed,whileamaterializedviewstoresthequeryresultasaphysicaltable.2.Materializedviewsgenerallyofferfasterqueryperformancebecausetheyaccessprecomputeddata,whereasviewscanbeslowe

When encountering an ORA-01017 error, it means that the login is denied. The main reason is that the user name or password is wrong or the account status is abnormal. 1. First, manually check the user name and password, and note that the upper and lower case and special characters must be wrapped in double quotes; 2. Confirm that the connected service name or SID is correct, and you can connect through tnsping test; 3. Check whether the account is locked or the password expires, and the DBA needs to query the dba_users view to confirm the status; 4. If the account is locked or expired, you need to execute the ALTERUSER command to unlock and reset the password; 5. Note that Oracle11g and above versions are case-sensitive by default, and you need to ensure that the input is accurate. 6. When logging in to special users such as SYS, you should use the assysdba method to ensure the password.

Use the CREATESEQUENCE statement to create sequences, which are used to generate unique values, often used for primary or proxy keys; 2. Common options include STARTWITH, INCREMENTBY, MAXVALUE/MINVALUE, CYCLE/NOCYCLE and CACHE/NOCACHE; 3. Get the next value through NEXTVAL, and CURRVAL gets the current value; 4. You can use sequence values to insert data in the INSERT statement; 5. It is recommended to avoid cache to prevent the loss of values due to crashes, and the sequence values will not be released due to transaction rollback; 6. Use DROPSEQUENCE to delete sequences when no longer needed.