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Technical Articles

Database

How to find constraints on a table in SQL?

To find the constraints of a table, different methods need to be used according to the database system: 1. Use INFORMATION_SCHEMA.TABLE_CONSTRAINTS to query the constraint type, which is suitable for MySQL, SQLServer and PostgreSQL; 2. Use SHOWCREATETABLE in MySQL to quickly view all constraints, or query INFORMATION_SCHEMA to obtain primary key, foreign key and other information; 3. It is recommended to use pg_constraint system table in PostgreSQL, and query constraint details through conrelid='table name'::regclass; 4. You can use it in SQLServer to query the constraint details through conrelid='table name'::regclass; 4.

Sep 06, 2025 am 05:48 AM

What is the difference between DELETE and TRUNCATE in Oracle?

DELETEisaDMLcommandthatremovesrowsoneatatimewithindividuallogging,whileTRUNCATEisaDDLcommandthatdeallocatesdatapagesforfasterexecution.2.DELETEcanberolledbackwithinatransactionusingundologs,whereasTRUNCATEisgenerallynon-transactionalandirreversiblewi

Sep 06, 2025 am 05:45 AM

How to grant and revoke permissions in SQL

GrantingandrevokingpermissionsinSQLisdoneusingGRANTandREVOKEstatementstocontroluserorroleaccesstodatabaseobjects,withGRANTprovidingprivilegeslikeSELECT,INSERT,UPDATE,DELETE,ALTER,DROP,EXECUTE,orALLPRIVILEGESontables,views,orprocedures,andREVOKEremovi

Sep 06, 2025 am 05:35 AM

How to perform a FULL OUTER JOIN in SQL

AFULLOUTERJOINreturnsallrowsfrombothtables,withNULLsfornon-matchingrecords,andissupportedinPostgreSQL,SQLServer,Oracle,andSQLitebutnotinMySQL,whereitcanbeemulatedusingLEFTandRIGHTJOINscombinedwithUNIONtoincludealldatafrombothsideswhileremovingduplica

Sep 06, 2025 am 05:15 AM

How to publish a message to a channel using PUBLISH?

To publish a message through Redis's PUBLISH command, you need to clarify the channel name and message content. 1. Use the format PUBLISHchannelmessage; 2. Make sure that a client is listening to the channel, otherwise the message will be discarded; 3. It is recommended to use string or JSON format for messages to avoid excessive impact on performance; 4. Pay attention to the limitations of the PUB/SUB mode, such as no persistence, no playback support, etc.; 5. If you need reliable queue function, you can use it with RedisStreams. Master the above points to effectively use the PUBLISH command.

Sep 06, 2025 am 04:53 AM

How to use the MAX function in MySQL

TheMAX()functionreturnsthehighestvalueinaspecifiedcolumn.2.Itcanbeusedwithnumericdata,dates,orstrings,returningthelatestdateoralphabeticallylaststring.3.UseMAX()withWHEREtofilterrowsbeforefindingthemaximum.4.UseMAX()withGROUPBYtofindthemaximumvaluepe

Sep 06, 2025 am 04:47 AM

mysql max function

How to duplicate a table with data?

To copy a table with data, you need to choose the appropriate method according to different scenarios. ① In database operations, use SQL commands such as CREATETABLEnew_tableASSELECT*FROMoriginal_table to copy the table structure and data, or add WHERE1=0 to copy the structure only; ② After selecting the table area in Excel, copy and paste it, and use "Paste Special" to retain the format; ③ Copy the tag content through developer tools in web development, and dynamic data requires additional processing scripts. Each method requires attention to whether the target location exists and functional integrity verification.

Sep 06, 2025 am 04:46 AM

How to duplicate a row in phpMyAdmin

To copy rows in phpMyAdmin, first use the copy function of the editing interface or SQL query. 1. Find the target row in the "Browse" tab, click the "Edit" icon, click the "Insert/Copy as new row" button at the bottom of the form, and save it after modifying the primary key and other unique fields. 2. Or enter the "SQL" tab and execute the statement with the INSERTINTO table name SELECT*FROM table name WHERE condition. If the primary key is not automatically incremented, you need to manually specify the field to exclude the primary key to avoid conflicts. 3. You can also view the query statement in this line by clicking "Show" and manually construct the INSERT statement. The easiest way is to use the "Insert/Copy as new line" button of the edit page, but you have to make sure the primary key does not conflict, and eventually

Sep 06, 2025 am 04:27 AM

database

How to get the week number from a date in SQL

To get the number of weeks from dates, different functions must be used according to the database system, because each system has different definitions of the week's start date and the first week; MySQL uses WEEK() or WEEKOFYEAR(), PostgreSQL uses EXTRACT (WEEKFROMdate) to support ISO weeks by default, SQLServer uses DATEPART (WEEK, date) to start from Sunday by default, and ISO weeks can be obtained through DATEPART (ISO_WEEK, date). Oracle uses TO_CHAR (date,'IW'), SQLite needs to use strftime ('%W', date) and no native ISO

Sep 06, 2025 am 03:45 AM

sql date function

How to view binary data in Navicat?

ToviewandworkwithbinarydatainNavicat,youcanusebuilt-intoolsandmethodsdependingontheformatandpurpose.1.Double-clickthebinaryfieldtoopenthedataviewerandswitchbetweenHexorTextdisplaymodesforinspection.2.ExportBLOBdatatodiskwiththecorrectfileextensionfor

Sep 06, 2025 am 03:43 AM

navicat binary data

What is a composite primary key in MySQL?

AcompositeprimarykeyinMySQLusesmultiplecolumnstouniquelyidentifyarow,suchas(student_id,course_id)inanenrollmentstable,wherethecombinationensuresuniquenessbecauseneithercolumnalonecan;thisenforcesNOTNULLconstraintsonbothcolumns,createsasingleclustered

Sep 06, 2025 am 03:03 AM

How to Use Transactions in MongoDB

MongoDB supports multi-document transactions since version 4.0 and needs to be used in replica sets or sharded cluster environments; when using it, transactions must be started through a session, operations must be performed and submissions or rollbacks are properly handled. At the same time, transactions should be kept short, high concurrency scenarios should be avoided, and command restrictions and timeout control should be paid attention to.

Sep 06, 2025 am 02:38 AM

How to use STRING_AGG in SQL

STRING_AGG is a SQL aggregate function for concatenating multiple rows of values ??into a single string, supporting specified delimiters and optional sorting. Its basic syntax is STRING_AGG (expression, delimiter) and can be combined with ORDERBY or WITHINGROUP control order. It is often used to merge related data such as employee names, labels, etc. in GROUPBY queries. This function automatically ignores NULL values ??and supports custom delimiters such as commas, arrows or newlines. It is suitable for SQLServer2017, PostgreSQL, etc., but MySQL uses GROUP_CONCAT, and Oracle uses LISTAGG. When using it, you need to pay attention to correct grouping and explicit sorting to ensure the results

Sep 06, 2025 am 02:34 AM

sql

How to insert data from one table to another in SQL?

To insert data from one table into another table, you should use the INSERTINTO...SELECT statement, 1. Ensure that the target table already exists and the column data types are compatible, 2. Use the basic syntax of INSERTINTO target table (column name) SELECT corresponding column FROM source table WHERE conditions, 3. You can filter specific rows through WHERE, 4. If the target table has fewer columns, only matching columns are selected, 5. Pay attention to the database specific syntax when inserting across databases; this method does not create a new table. If you need to create a table at the same time, you can use SELECTINTO or CREATETABLE...ASSELECT. In the end, you must ensure that the constraints and data types of the target table can accept input data.

Sep 06, 2025 am 02:26 AM