How to delete not null restriction in mysql

The answer is: MySQL's CASE statement is used to implement conditional logic in query, and supports two forms: simple and search. Different values ​​can be dynamically returned in clauses such as SELECT, WHERE, and ORDERBY; for example, in SELECT, classification of scores by fractional segments, combining aggregate functions to count the number of states, or prioritizing specific roles in ORDERBY, it is necessary to always end with END and it is recommended to use ELSE to handle the default situation.

Create a shell script containing the database configuration and mysqldump command and save it as mysql_backup.sh; 2. Store MySQL credentials by creating ~/.my.cnf file and set 600 permissions to improve security, modify the script to use configuration file authentication; 3. Use chmod x to make the script executable and manually test whether the backup is successful; 4. Add timed tasks through crontab-e, such as 02/path/to/mysql_backup.sh>>/path/to/backup/backup.log2>&1, realize automatic backup and logging at 2 a.m. every day; 5.

Subqueries can be used in WHERE, FROM, SELECT, and HAVING clauses to implement filtering or calculation based on the result of another query. Operators such as IN, ANY, ALL are commonly used in WHERE; alias are required as derivative tables in FROM; single values ​​must be returned in SELECT; related subqueries rely on outer query to execute each row. For example, check employees whose average salary is higher than the department, or add the company average salary list. Subqueries improve logical clarity, but performance may be lower than JOIN, so you need to ensure that you return the expected results.

INSERT...ONDUPLICATEKEYUPDATE implementation will be updated if it exists, otherwise it will be inserted, and it requires unique or primary key constraints; 2. Reinsert after deletion of REPLACEINTO, which may cause changes in the auto-increment ID; 3. INSERTIGNORE only inserts and does not repetitive data, and does not update. It is recommended to use the first implementation of upsert.

EXPLAINinMySQLrevealsqueryexecutionplans,showingindexusage,tablereadorder,androwfilteringtooptimizeperformance;useitbeforeSELECTtoanalyzesteps,checkkeycolumnsliketypeandrows,identifyinefficienciesinExtra,andcombinewithindexingstrategiesforfasterqueri

Use the DISTINCT keyword to remove duplicate values ​​from the specified column and return unique values. 1. The basic syntax is SELECTDISTINCTcolumn_nameFROMtable_name; 2. Query the unique value of a single column, such as SELECTDISTINCTcityFROMcustomers; 3. Query the unique combination of multiple columns, such as SELECTDISTINCTcity, stateFROMcustomers; 4. Filter with the WHERE clause and get the unique value, such as SELECTDISTINCTproduct_nameFROMordersWHEREorder_date>'202

MySQL can calculate geographical distances through the Haversine formula or the ST_Distance_Sphere function. The former is suitable for all versions, and the latter provides easier and more accurate spherical distance calculations since 5.7.

Use UTC to store time, set the MySQL server time zone to UTC, use TIMESTAMP to realize automatic time zone conversion, adjust the time zone according to user needs in the session, display the local time through the CONVERT_TZ function, and ensure that the time zone table is loaded.