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  • mysql table lock vs row lock
    mysql table lock vs row lock
    Table locks are suitable for low-concurrency, batch operation or maintenance scenarios. For example, when using the MyISAM engine, performing DDL operations or full-table scans, their overhead is small but their concurrency is poor. Row locks are suitable for high-concurrency write scenarios. They are supported by the InnoDB engine. Fine-grained locks are realized through index hits to improve concurrency but may cause deadlocks. When choosing, according to business needs, use InnoDB row locks with more writes and reads, and use MyISAM table locks with less data or mainly reads, and ensure that querying and indexing can avoid lock upgrades.
    Mysql Tutorial . Database 278 2025-07-12 01:48:41
  • mysql composite index example
    mysql composite index example
    MySQL composite index follows the principle of leftmost prefix, and the query condition must include the leftmost column of the index before it can hit the index. 1. The index structure is organized in the order of definition, such as (name, age, city) first sorted by name, and then subdivided in sequence; 2. The hit condition includes continuous combinations starting with the leftmost column, such as WHEREname=... or WHEREname=...ANDage=...; 3. If the leftmost column is not included, it cannot be hit if only age or city is used; 4. When creating, the high-divided and commonly used query fields should be placed in front of it, and redundancy and excessive indexing should be avoided; 5. The use of functions, OR without index support, and fuzzy matching at the beginning of % will cause the index to fail.
    Mysql Tutorial . Database 611 2025-07-12 01:36:01
  • mysql create table syntax
    mysql create table syntax
    The key to creating a MySQL table is to master the basic syntax and common options of CREATETABLE statements. 1. The basic syntax requires specifying field names, data types and constraints, such as NOTNULL, PRIMARYKEY, AUTO_INCREMENT; 2. Common field types include INT, VARCHAR(n), TEXT, DATE, DATETIME, TIMESTAMP and DECIMAL(m,d), which should be selected according to actual needs to optimize storage and performance; 3. Constraints include NOTNULL, UNIQUE, DEFAULT, PRIMARYKEY and FOREIGNKEY. When using foreign keys, the two tables must be engines that support foreign keys (such as
    Mysql Tutorial . Database 633 2025-07-12 01:27:51
  • mysql delete from table where
    mysql delete from table where
    When deleting data using the DELETEFROMtableWHERE statement in MySQL, you must pay attention to accuracy and security. 1. This statement is used to delete records according to the specified conditions. If the WHERE condition is omitted, the entire table will be cleared; 2. Common problems include field name errors, string not quoted, improper LIKE match, etc. It is recommended to use SELECT to confirm the target data before deletion; 3. In actual operations, the principles of checking first and then deleting, adding LIMIT tests, using transaction processing, and early backup should be followed; 4. When deleting a large amount of data, it is necessary to execute it in batches to avoid table locking and performance problems. If necessary, TRUNCATE or DROP tables should be considered.
    Mysql Tutorial . Database 739 2025-07-12 01:24:51
  • mysql ifnull function
    mysql ifnull function
    IFNULL is a function in MySQL used to process NULL values. Its function is to return the first parameter when it is not NULL, otherwise it returns the second parameter. 1. Common usages include replacing the NULL value in the query results, such as "not filled in" when the phone is empty; 2. Prevent NULL from causing the entire result to be NULL in the operation, such as replacing the price of NULL with 0 and participating in multiplication calculation; 3. Use it in combination with the aggregate function to ensure that the results such as SUM are not NULL; 4. Notes include trying to keep the parameter types consistent, not be able to judge multiple NULLs, and avoiding covering up the business logic meaning; 5. It is very practical when using practical applications such as displaying user addresses or calculating employee income. Overall, IFNULL is a simple and effective
    Mysql Tutorial . Database 672 2025-07-12 01:15:31
  • how to get table size in mysql
    how to get table size in mysql
    To see how much space a table occupies in MySQL, you can achieve it in the following ways: 1. Query the size of a single table, use SQL statements to obtain the data and index size from information_schema.tables; 2. Check the size of all tables in the entire database, and list all tables through information_schema.tables and sort them by total size; 3. Use the SHOWTABLESTATUS command to quickly view the size information of the table, and pay attention to unit conversion. In addition, you should ensure access when performing these operations, and consider storage engine differences and statistical accuracy issues.
    Mysql Tutorial . Database 147 2025-07-12 00:55:10
  • mysql event scheduler example
    mysql event scheduler example
    To enable MySQL event scheduler, 1. Use SHOWVARIABLESLIKE'event_scheduler'; view the status; 2. If OFF, add event_scheduler=ON in the configuration file or run SETGLOBALevent_scheduler=ON; Create Event example: Update the "to be paid" order status for more than 30 days at 2 am every day to "cancel". The statement includes the specified name, execution frequency, start time and SQL to be executed; manage Events include SHOWEVENTS, modify ALTEREVENT, and delete DROPEVENT; Notes include permissions
    Mysql Tutorial . Database 530 2025-07-11 03:06:00
  • mysql update statement with join
    mysql update statement with join
    In MySQL, using UPDATE combined with JOIN can update data according to the association table. The syntax is: UPDATE table 1JOIN table 2ON condition SET field = value WHERE condition; common uses include: 1. Use commas to separate multiple assignment expressions when updating multiple fields; 2. Use LEFTJOIN to avoid missing main table records, and use IFNULL to process null values; 3. Use alias and add qualifiers to prevent field conflicts; precautions include: WHERE condition position affects logic, be sure to limit the update range, and it is recommended to use SELECT to verify the matching result before execution.
    Mysql Tutorial . Database 995 2025-07-11 03:02:20
  • how to check mysql version
    how to check mysql version
    To view the version of MySQL, there are several ways to do it. 1. Use the command line: Enter mysql--version or mysql-V in the terminal or CMD; 2. Execute SQL query in the MySQL client: SELECTVERSION(); or SHOWVARIABLESLIKE'version'; 3. Windows users can use command prompt or PowerShell to enter the bin folder of the MySQL installation directory to run commands; 4. Graphical tools such as MySQLWorkbench, phpMyAdmin, etc. also support viewing version information, which is suitable for users who are not familiar with the command line. Different methods apply to different scenarios:
    Mysql Tutorial . Database 815 2025-07-11 02:58:21
  • how to generate uuid in mysql
    how to generate uuid in mysql
    InMySQL,UUIDsaregeneratedusingtheUUID()functionandfollowversion1standards,combiningaserver'sMACaddresswithatimestamp.TogenerateaUUID,useSELECTUUID();orinsertitintoatablewithINSERTINTOusers(id,name)VALUES(UUID(),'Alice');.EnsurethecolumnisCHAR(36)fore
    Mysql Tutorial . Database 810 2025-07-11 02:55:01
  • Understanding Character Sets and Collations in MySQL
    Understanding Character Sets and Collations in MySQL
    The character set determines which characters are stored in the database, and it is recommended to use utf8mb4; the sorting rules affect comparison and sorting behavior. Commonly used character sets include latin1, utf8, and utf8mb4, among which utf8mb4 supports emoji. Common sorting rules include utf8mb4_unicode_ci (case insensitive), utf8mb4_bin (case sensitive), and utf8mb4_0900_ci (modern language habits). Set the level from high to low to: Connection layer > Table level > Database level > Server global. The configuration methods are: the server level is set in my.cnf or my.ini, specified when the database level is created, and defined when the table level is created.
    Mysql Tutorial . Database 380 2025-07-11 02:50:01
  • Using SHOW PROCESSLIST to Monitor Active Queries in MySQL
    Using SHOW PROCESSLIST to Monitor Active Queries in MySQL
    To view the query MySQL is currently executing, you can use the SHOWPROCESSLIST command; 1. This command displays all current connection thread information, including Id, User, Host, db, Command, Time, State and Info; 2. Focus on threads with large Time value, State is in Waiting or Locked state, and complex SQL in Info; 3. After discovering the problem thread, you can use KILL [thread_id] to terminate its execution; 4. You can combine SHOWFULLPROCESSLIST, logging, performance mode and third-party tools to improve the inspection efficiency.
    Mysql Tutorial . Database 768 2025-07-11 02:44:10
  • mysql max_connections
    mysql max_connections
    The settings of MySQL's max_connections parameter must be reasonably adjusted according to server performance and business needs. To view the current maximum number of connections, use SHOWVARIABLESLIKE'max_connections'; to view the used number of connections. If the connection has been used, it is often close to the maximum value, consider increasing the parameter. There are two ways to adjust: temporary modification is through SETGLOBALmax_connections=1000; and permanent modification requires max_connections=1000 in my.cnf or my.ini and restart MyS
    Mysql Tutorial . Database 739 2025-07-11 02:42:30
  • Storing and Querying JSON Data in MySQL
    Storing and Querying JSON Data in MySQL
    MySQL supports JSON data types and is suitable for processing dynamic or semi-structured data. 1. Selecting JSON data type can provide verification and built-in function support; 2. Use JSON_EXTRACT() or -> symbol query fields, note that the string needs to be quoted; 3. You can index fields in JSON by generating columns to improve performance; 4. Suitable for frequent structure changes and sparse field scenarios, but not for strong type constraints or high-performance nested query scenarios. When using it, you need to weigh flexibility and query complexity.
    Mysql Tutorial . Database 730 2025-07-11 02:39:50

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