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  • mysql st_distance function
    mysql st_distance function
    ST_Distance is a function in MySQL that calculates the shortest distance between two geometric objects and is suitable for geospatial data queries. 1. Its basic usage is ST_Distance(g1,g2), which returns the minimum distance between two geometric objects. The unit depends on whether SRS and coordinate system type is used; 2. If you use latitude and longitude (such as EPSG:4326), the default is "degrees". It is recommended to use ST_Distance_Sphere to obtain the distance in meters instead; 3. Query records within a certain range around a certain point, you can combine WHERE conditions, but the performance is poor. It is recommended to filter the rectangle range first and then calculate it accurately; 4. Notes include: Inconsistent units may lead to misunderstandings, poor index efficiency, and ST
    Mysql Tutorial . Database 287 2025-07-16 03:53:10
  • mysql trigger after insert
    mysql trigger after insert
    AFTERINSERT triggers are mechanisms used in MySQL to automatically perform operations after data is inserted, and are often used for logging, data synchronization, and auto-filling fields. It is different from BEFOREINSERT, because it is triggered after the insertion action is completed, and it can safely refer to the newly inserted data line content. Common uses include: 1. Data recording and auditing, such as recording the inserted user information to the log table; 2. Cascadingly update other tables, such as updating the total consumption amount of users after inserting the order; 3. Initializing the associated data, such as generating default configuration items after inserting the user. Create a syntax of CREATETRIGGER and use the NEW keyword to refer to the just-inserted record field. For example, when inserting a new user into the users table, a trigger can be used
    Mysql Tutorial . Database 290 2025-07-16 03:51:31
  • mysql convert int to varchar
    mysql convert int to varchar
    In MySQL, converting integers into strings can be implemented through CAST, CONVERT functions or implicit conversions. 1. Use CAST (column_nameASCHAR) to explicit conversion, such as SELECTCAST (123ASCHAR); 2. Use CONVERT (column_name,CHAR), such as SELECTCONVERT (456,CHAR); 3. MySQL will automatically perform implicit conversions during splicing or comparison, but it is not recommended to rely on this mechanism to avoid performance and logic problems; it is recommended to use explicit conversion functions when explicit string processing is required to ensure the accuracy and maintainability of the query.
    Mysql Tutorial . Database 222 2025-07-16 03:50:41
  • how to drop a database in mysql
    how to drop a database in mysql
    The key to deleting the MySQL database is to use the DROPDATABASE command, but the library name and permissions must be confirmed before execution; 1. Make sure to log in with a user or root user with DROP permissions, otherwise an error will be reported due to insufficient permissions; 2. Execute the command DROPDATABASEdatabase_name; be sure to carefully check the database name to avoid mistaken deletion; 3. The deletion operation is irreversible, the data will be completely cleared, and the recovery can only rely on backup, binlog log or third-party tools; 4. It is recommended to run SHOWDATABASES before deletion; confirm the target database, and notify the team or make a reminder before the formal environment operation; 5. You should develop the habit of regular backup to deal with the error deletion.
    Mysql Tutorial . Database 703 2025-07-16 03:48:50
  • mysql aggregate functions
    mysql aggregate functions
    MySQL aggregation function is used for data statistics and is suitable for reporting and analysis. 1. COUNT counts the number of rows, COUNT(*) includes NULL, COUNT(field) excludes NULL; 2. SUM and AVG are used to sum and average values, pay attention to type conversion and NULL processing; 3. MAX and MIN can process numbers, dates, and strings, and are often used with GROUPBY to improve efficiency. Mastering these details can avoid common errors and optimize query performance.
    Mysql Tutorial . Database 155 2025-07-16 03:47:51
  • mysql find_in_set function
    mysql find_in_set function
    FIND_IN_SET() is suitable for querying whether the specified value is included in the comma-separated string list. 1. Used to store multiple values in fields, such as SELECT*FROMusersWHEREFIND_IN_SET('apple',favorite_fruits) to find favorite_fruits containing 'apple' records; 2. The parameter str is a search string, strlist is a comma-separated string field, and returns a position or 0; 3. Note that the field value cannot have spaces or no indexes to affect performance, use big data with caution; 4. Alternative solutions include intermediate tables, JSON type fields or application layer processing; 5. Error writing methods such as FIND_IN_SET(
    Mysql Tutorial . Database 415 2025-07-16 03:44:51
  • Configuring and utilizing the MySQL query cache
    Configuring and utilizing the MySQL query cache
    Query caching is a mechanism in MySQL that improves query performance by caching SELECT results. It is suitable for scenarios where frequent readings and fewer data changes. It saves resources by skipping parsing and executing steps, and is suitable for static content, report query and other scenarios. However, once the table is updated, the relevant cache will be cleared, so it may backfire in a write-in environment. To enable query caching, query_cache_type (ON/OFF/DEMAND), query_cache_size (recommended 64M), and query_cache_limit (such as 2M). MySQL 8.0 has removed this feature and only supports version 5.x. Hit situations can be accessed through Qcache_hits and Qcac
    Mysql Tutorial . Database 604 2025-07-16 03:44:31
  • mysql string to date
    mysql string to date
    In MySQL, converting strings to dates mainly uses the STR_TO_DATE() function. 1. The basic usage of this function is STR_TO_DATE(str,format), which is used to parse string str into date or time value in the specified format; 2. The return value type can be DATE, DATETIME or TIME, depending on the input string and format; 3. If the format does not match, NULL or error results may be returned, so the format must be strictly corresponding; 4. Common formats include: %Y (four-digit year), %y (two-digit year), %m (month), %d (date), %H (hour), %i (minute), %s (seconds), etc.; 5. For months or week with English
    Mysql Tutorial . Database 365 2025-07-16 03:43:10
  • mysql concat string
    mysql concat string
    The most commonly used stitching strings in MySQL is the CONCAT() function, which can concatenate multiple fields or strings. If the NULL value is included, the result is NULL, which can be processed by IFNULL(); multi-field stitching can be separated with symbols to improve readability; if you need to splice multiple records, you need to use GROUP_CONCAT() and specify separators and adjust length limits. For example: SELECTCONCAT('Hello','','World'); output 'HelloWorld'; SELECTCONCAT(first_name,'',last_name)ASfull_nameFROMusers; names can be merged; SELECTGR
    Mysql Tutorial . Database 805 2025-07-16 03:38:02
  • how to handle null values in mysql
    how to handle null values in mysql
    The key to processing NULL values in MySQL is to understand their meaning and impact. 1. NULL represents unknown or missing values, which are different from 0 or empty strings; 2. When querying, you need to use ISNULL or ISNOTNULL to judge, and it is not available = or !=; 3. Use IFNULL or COALESCE functions to provide default values for NULL; 4. When inserting data, you can use NOTNULL constraints and DEFAULT to set the default values to avoid NULL; 5. The aggregate function will ignore NULL, and replace them if necessary before counting; 6. When grouping, all NULL values will be classified into a group. It is recommended that the avoidance field in the design stage be NULL.
    Mysql Tutorial . Database 736 2025-07-16 03:32:41
  • mysql add primary key to existing table
    mysql add primary key to existing table
    To add the MySQL table primary key, you must ensure that the fields are not empty and unique, and use the ALTERTABLE statement to operate. 1. Check whether the field is NOTNULL; 2. Confirm that the field value is unique and there is no duplicate data; 3. If the field does not meet the conditions, the data must be cleaned or modified first; 4. Use ALTERTABLEusersADDPRIMARYKEY(id) to add the primary key; 5. If there is already a primary key, use ALTERTABLEusersDROPPRIMARYKEY to delete the old primary key first, and then add the new primary key. Note that when the self-increment column is used as the primary key, the self-increment attribute becomes invalid after deletion and needs to be redefined.
    Mysql Tutorial . Database 246 2025-07-16 03:32:21
  • Methods for Data Import and Export in MySQL
    Methods for Data Import and Export in MySQL
    There are mainly the following methods for importing and exporting MySQL data: 1. Use SELECTINTOOUTFILE to export data to server files, and LOADDATAINFILE imports files into databases, which are suitable for large-scale local data operations; 2. Export databases or tables as SQL files through mysqldump tool, and imports them with mysql commands, which are suitable for cross-server migration and version control; 3. Use graphical tools such as phpMyAdmin for visual operations, supporting multiple formats of export and import, but may be limited in performance when processing big data; 4. Combined with programming languages such as Python to operate databases, it realizes flexible and automated import and export, which is suitable for integration into applications. Different methods are suitable for
    Mysql Tutorial . Database 788 2025-07-16 03:24:41
  • mysql lag function
    mysql lag function
    The LAG function is a window function in MySQL, which is used to obtain the value of a certain row before the current row. Its basic syntax is LAG(expression[,offset][,default])OVER([PARTITIONBYpartition_expression]ORDERBYsort_expression), where expression specifies a field or expression, offset defines the number of forward lines, default 1 line, default sets the default value when there is no previous line, default NULL, PARTITIONBY is used for group calculation, and ORDERBY determines sorting. Note when using it: it must be used with OVER().
    Mysql Tutorial . Database 391 2025-07-16 03:23:01
  • how to create a view in mysql
    how to create a view in mysql
    Views are virtual tables based on query results in MySQL, used to simplify complex queries, improve security and unified data access methods. Its creation syntax is a CREATEVIEWview_nameASSELECT statement, for example, you can create a completed_orders view to filter completed orders. To modify the view, you can use CREATEORREPLACEVIEW or ALTERVIEW, and to delete it, you can use DROPVIEW. When using it, you need to pay attention to: the view name is unique, does not store actual data, cannot contain TEMPORARY tables, updates are restricted, and performance depends on the original query efficiency.
    Mysql Tutorial . Database 338 2025-07-16 03:22:41

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