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Technical Articles

Database

How does SQL handle data type precedence in expressions?

SQLusesdatatypeprecedencetodetermineimplicitconversions,wherehigher-precedencetypesforcetheconversionoflower-precedenceones;forexample,INThashigherprecedencethanVARCHAR,soin'123' 456,thestring'123'isconvertedtoaninteger,resultingin579,but'abc' 123fai

Sep 01, 2025 am 07:56 AM

How to use hints in Oracle queries

HintsinOracleSQLareinstructionsembeddedinqueriestoinfluencetheoptimizer’sexecutionplan,usedwhenthedefaultplanissuboptimal.1.HintsareplacedincommentsrightafterSELECT,UPDATE,INSERT,orMERGEusing/* HINT_NAME*/syntax.2.TheFULLhintforcesafulltablescan,usef

Sep 01, 2025 am 07:47 AM

How to combine multiple columns into one in SQL?

To merge multiple columns in SQL into one column, you need to use a string splicing function and process NULL values. The specific method depends on the database: 1. MySQL uses the CONCAT() function; 2. PostgreSQL supports the CONCAT() or || operator; 3. SQLServer supports the CONCAT() or operator; 4. Oracle and SQLite use the || operator, where Oracle's CONCAT() only supports two parameters; 5. To avoid NULL resulting in NULL, COALESCE(), IFNULL() or ISNULL() should be used to convert NULL into an empty string, or directly use CONC that can automatically handle NULL.

Sep 01, 2025 am 07:12 AM

What is the difference between ENUM and SET data types in MySQL?

The ENUM type only allows the selection of a single value from a predefined list, which is suitable for single-select scenarios such as state or gender; the SET type allows the selection of zero or more values, which is suitable for multiple-select scenarios such as permissions or tags. ENUM supports up to 65,535 members, and is stored internally with indexes starting at 1; SET supports up to 64 members, and is stored internally in bitmap form, with each value corresponding to a binary bit. Inserting an invalid value in ENUM will report an error or be saved as an empty string, and SET will automatically ignore the invalid value or process it according to SQL mode. For example, ENUM('active','inactive') can only store one state, while SET('read','write') can store the 'read, write' combination. because

Sep 01, 2025 am 07:03 AM

set enum

phpMyAdmin search for a value in a table

TosearchforavalueinatableusingphpMyAdmin,selectthedatabaseandtable,clickthe"Search"tab,choosecolumns,enterthevalue,andclick"Go"toretrievematchingrows.2.Formorecontrol,usethe"SQL"tabandrunacustomquerylikeSELECT*FROMyour_t

Sep 01, 2025 am 06:34 AM

How to use ROLLUP and CUBE operators in SQL

ROLLUP generates hierarchical subtotals and totals, which are suitable for scenarios such as summary by region and product step by step, while CUBE generates aggregation of all column combinations, which is suitable for multi-dimensional analysis; both enhance SQL expression by reducing the use of UNIONALL, which is often used in reports, but attention should be paid to performance and NULL value processing. The final choice depends on whether full combination (CUBE) or hierarchical summary (ROLLUP) is required.

Sep 01, 2025 am 06:27 AM

How to handle 'division by zero' errors in SQL?

UseNULLIF(denominator,0)tosafelyreturnNULLinsteadofcausingadivisionbyzeroerror.2.CombinewithCOALESCE()toreplaceNULLresultswithadefaultvaluelike0.3.UseCASEstatementsforcomplexlogic,suchashandlingzero,NULL,ornegativedenominatorswithcustomrules.4.Accoun

Sep 01, 2025 am 06:25 AM

How to understand the difference between COALESCE and ISNULL in SQL

COALESCE and ISNULL are both used to process NULL values, but COALESCE is a standard SQL function that supports multiple databases and multiple parameters, and returns results based on data type priority. It is suitable for complex scenarios and cross-platform use; ISNULL is a SQLServer-specific function that supports only two parameters. The return type is determined by the first parameter. It has slightly better performance but lacks portability. It is suitable for NULL replacement in simple and single database environments. The choice of both should be based on database platform and functional requirements.

Sep 01, 2025 am 05:22 AM

How to export a database in phpMyAdmin

LogintophpMyAdminviayourbrowserusingyourMySQLcredentials.2.Selectthedesireddatabasefromtheleft-handsidebar.3.ClicktheExporttabatthetopofthepage.4.ChooseeitherQuickorCustomexportmethod,withCustomrecommendedformorecontrol.5.InCustomsettings,selectSQLfo

Sep 01, 2025 am 04:47 AM

How to backup only specific tables using Navicat?

Navicat supports backup of specific tables only. The operation steps are: 1. Open Navicat and expand the target database, right-click on one or more selected tables (Ctrl can be held multiple selections), select "Dump SQL File" or "Export Wizard" to back up only the selected tables; 2. Use the "Export Wizard" to further control the export content, such as only structure, data only, or both, and set the format and query conditions; 3. If you need to automatically and regularly backup, you can add the export task through scheduled tasks and manually check the tables to be backed up, and set the execution frequency to achieve automated backup; precautions include ensuring that the account has read permissions, the export path can be written, and enabling logging to facilitate troubleshooting.

Sep 01, 2025 am 04:17 AM

navicat database backup

Implementing MySQL Data Archiving with Partitioning

MySQL data archive can be implemented through partitioning to improve performance and maintenance efficiency. 1. Select the appropriate partitioning strategy: Priority is given to using RANGE partitions to archive by time, such as dividing order data by month; or use LIST partitions to archive by classification. 2. Notes should be paid attention to when designing table structure: Partition fields must be included in primary keys or unique constraints, and queries should be equipped with partition fields to enable partition cropping. 3. Automatic archives can regularly perform deletion of old partitions through scripts, and record logs and check the existence of partitions to avoid mistaken deletion. 4. Limited applicable scenarios: small tables, query without partition fields, cloud database restriction partitioning function, etc., other archive solutions should be considered, such as timed migration of archive tables. Under reasonable design, partition archives can efficiently manage historical data, otherwise it is easy to induce

Sep 01, 2025 am 04:12 AM

How to check the status of a MySQL server

Usemysqladmin-uroot-pstatustogetkeymetricslikeuptime,threads,andqueries,ormysqladminpingtocheckiftheserverisalive;2.Loginwithmysql-uroot-pandrunSHOWSTATUSLIKE'variable_name'toviewspecificserverstatusvariablessuchasUptime,Threads_connected,Queries,and

Sep 01, 2025 am 04:10 AM

How to export a MySQL table to a CSV file?

Use SELECTINTOOUTFILE to export tables as CSV files on MySQL server. It requires FILE permissions and the target path is writable. For example: SELECT*FROMusersINTOOUTFILE'/tmp/users.csv'FIELDSTERMINATEDBY','ENCLOSEDBY'"'LINESTERMINATEDBY'\n'; If there is no server access permission, you can export them through client commands combined with shell redirection or using scripting languages ??such as Python. The scripting method can better handle special characters, encodings and references to ensure data integrity. Therefore, it is recommended to replication

Sep 01, 2025 am 04:08 AM

What is the purpose of the requirepass configuration directive?

TherequirepassdirectiveinRedisisusedtosetapasswordforclientauthentication.1.ItensuresthatonlyuserswhoknowthepasswordcaninteractwiththeRedisinstanceafterconnecting.2.Itisconfiguredbyeditingtheredis.conffileandspecifyingthepasswordwithrequirepassyour_s

Sep 01, 2025 am 03:42 AM

redis