Why is My PHP Showing \'Warning: mysql_query(): 3 is not a valid MySQL-Link Resource\'?

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.

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.

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.

MySQL supports date operation through built-in functions and operators. 1. Use DATE_ADD() and DATE_SUB() to increase and decrease dates according to specified units (such as DAY, MONTH, etc.); 2. Use INTERVAL and -INTERVAL to simplify syntax to implement date addition and subtraction; 3. Use DATEDIFF() to calculate the difference in the number of days during the two-day period, or use TIMESTAMPDIFF() to obtain more accurate time unit differences (such as hours and minutes); 4. Common application scenarios include querying orders for the last 7 days, calculating the expiration date and user age, and ensuring that the date field type is correct and avoiding invalid date input, and finally, various date operation needs are efficiently handled through these functions and operators.

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