UNIONremovesduplicateswhileUNIONALLkeepsallrowsincludingduplicates;1.UNIONperformsdeduplicationbysortingandcomparingrows,returningonlyuniqueresults,whichmakesitsloweronlargedatasets;2.UNIONALLincludeseveryrowfromeachquerywithoutcheckingforduplicates,

Removeunusedpackagesanddependencieswithsudoaptautoremove,cleanpackagecacheusingsudoaptcleanorautoclean,andremoveoldkernelsviasudoaptautoremove--purge.2.Clearsystemlogswithsudojournalctl--vacuum-time=7d,deletearchivedlogsin/var/log,andempty/tmpand/var

You can customize the separator by using the SEPARATOR keyword in the GROUP_CONCAT() function; 1. Use SEPARATOR to specify a custom separator, such as SEPARATOR'; 'The separator can be changed to a semicolon and plus space; 2. Common examples include using the pipe character '|', space'', line break character '\n' or custom string '->' as the separator; 3. Note that the separator must be a string literal or expression, and the result length is limited by the group_concat_max_len variable, which can be adjusted by SETSESSIONgroup_concat_max_len=10000; 4. SEPARATOR is optional

The table can be locked manually using LOCKTABLES. The READ lock allows multiple sessions to read but cannot be written. The WRITE lock provides exclusive read and write permissions for the current session and other sessions cannot read and write. 2. The lock is only for the current connection. Execution of STARTTRANSACTION and other commands will implicitly release the lock. After locking, it can only access the locked table; 3. Only use it in specific scenarios such as MyISAM table maintenance and data backup. InnoDB should give priority to using transaction and row-level locks such as SELECT...FORUPDATE to avoid performance problems; 4. After the operation is completed, UNLOCKTABLES must be explicitly released, otherwise resource blockage may occur.

Use MySQL to process JSON data to directly store, query and operate semi-structured data in relational databases. Since version 5.7, JSON types are supported; columns are defined through JSON data types and legal JSON values are inserted, MySQL will automatically verify the syntax; data can be extracted using JSON_EXTRACT() or -> (returns quoted strings) and ->> (returns unquoted values), such as profile->> "$.city" to obtain city names; support filtering JSON values through WHERE clauses, and it is recommended to use generated columns and indexes to improve performance, such as ADDcityVARCHAR(50)GENERA

To select data from MySQL table, you should use SELECT statement, 1. Use SELECTcolumn1, column2FROMtable_name to obtain the specified column, or use SELECT* to obtain all columns; 2. Use WHERE clause to filter rows, such as SELECTname, ageFROMusersWHEREage>25; 3. Use ORDERBY to sort the results, such as ORDERBYageDESC, representing descending order of age; 4. Use LIMIT to limit the number of rows, such as LIMIT5 to return the first 5 rows, or use LIMIT10OFFSET20 to implement paging; 5. Use AND, OR and parentheses to combine

The steps to set up alias in Linux are as follows: 1. Temporarily set the use of the alias command such as aliasll='ls-la'; 2. Permanently set the shell configuration file, such as ~/.bashrc, and then execute the source to take effect; 3. Be careful to avoid overwriting the original command and the different shell configurations are independent. Alias can simplify complex commands and improve efficiency, but only after the current shell environment takes effect and closes the terminal, it needs to be reasonably defined and regularly checked for configuration.

IFNULL()inMySQLreturnsthefirstexpressionifitisnotNULL,otherwisereturnsthesecondexpression,makingitidealforreplacingNULLvalueswithdefaults;forexample,IFNULL(middle_name,'N/A')displays'N/A'whenmiddle_nameisNULL,IFNULL(discount,0)ensurescalculationslike