How Can I Create a Unique Index on a Column Allowing NULL Values in SQL Server?

MySQL's DATE_FORMAT() function is used to customize the date and time display format. The syntax is DATE_FORMAT(date, format), and supports a variety of format characters such as %Y, %M, %d, etc., which can realize date display, group statistics and other functions.

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.

AUTO_INCREMENT automatically generates unique values ​​for the primary key column of the MySQL table. When creating the table, define this attribute and ensure that the column is indexed. When inserting data, omit the column or set it to NULL to trigger automatic assignment. The most recently inserted ID can be obtained through the LAST_INSERT_ID() function. The start value and step size can be customized through ALTERTABLE or system variables, which is suitable for unique identification management.

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.

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.

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