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  • How to count the total number of rows in a table?
    How to count the total number of rows in a table?
    The clear answer to counting the total number of rows in the table is to use the database counting function. The most direct method is to execute the SQL COUNT() function, for example: SELECTCOUNT()AStotal_rowsFROMyour_table_name; secondly, for big data tables, you can view the system table or information schema to get the estimated value, such as PostgreSQL uses SELECTreltuplesFROMpg_classWHERErelname='your_table_name'; MySQL uses SELECTTABLE_ROWSFROMinformation_schema.TABLESWHERETA
    Mysql Tutorial . Database 939 2025-06-13 00:30:01
  • How to use a JOIN in an UPDATE statement?
    How to use a JOIN in an UPDATE statement?
    The key to updating data with JOIN is the syntax differences between different databases. 1. SQLServer needs to connect tables in the FROM clause, such as: UPDATEt1SETt1.column=t2.valueFROMTable1t1INNERJOINTable2t2ONt1.id=t2.ref_id; 2.MySQL needs to JOIN directly after UPDATE, such as: UPDATETable1t1JOINTable2t2ONt1.id=t2.ref_idSETt1.column=t2.value; 3.PostgreSQL combines FROM and WHERE, such as: UPDA
    Mysql Tutorial . Database 669 2025-06-13 00:27:11
  • In which MySQL version did the CHECK constraint actually start working?
    In which MySQL version did the CHECK constraint actually start working?
    MySQL has only truly supported and enforced CHECK constraints since version 8.0.16, and was previously parsed but not actually executed. 1. Before 8.0.16, although CHECK constraints were syntactically supported, storage engines such as MyISAM and InnoDB did not implement their data verification functions; 2. Developers cannot rely on this function to ensure data integrity, and insertion or update operations will not trigger verification; 3. Since 8.0.16, CHECK constraints have been enforced by the server, supporting column-level and table-level constraints, complex expressions, and are applicable to all storage engines; 4. Users can use ENFORCED or NOTENFORCED keywords to control their enabled status; 5. After upgrading to this version, please note that the old data may not meet the requirements.
    Mysql Tutorial . Database 463 2025-06-13 00:24:50
  • When do I need to run the FLUSH PRIVILEGES command?
    When do I need to run the FLUSH PRIVILEGES command?
    In MySQL or MariaDB, you need to run the FLUSHPRIVILEGES command after manually modifying the permission table. 1. When you directly execute INSERT, UPDATE or DELETE operations on permission tables such as mysql.user, mysql.db, etc., you must run this command to make the changes effective immediately; 2. When using standard permission management commands such as GRANT, REVOKE or CREATEUSER, you do not need to execute FLUSHPRIVILEGES, because these commands will automatically reload permissions; 3. After modifying the permission table through scripts or external tools, the command should be manually executed, otherwise the changes will not take effect; 4. It is not recommended to directly edit the system permission table, and it is recommended to use standard SQL missions.
    Mysql Tutorial . Database 407 2025-06-13 00:23:21
  • How to restore a master from a slave's data?
    How to restore a master from a slave's data?
    Recovering the master database usually does not obtain data from the slave database, but when the master database goes down and no backup is available, you can follow the following steps: 1. Check whether the slave database has the latest data, run SHOWSLAVESTATUS\G to confirm that Seconds_Behind_Master is 0 and Last_Error is empty; 2. Stop the copy thread of the slave database and reset the copy information, use the STOPSLAVE and RESETSLAVEALL commands; 3. After configuring the original slave database as a new master database, update the application connection settings and reconfigure the new slave database to point to the master database, use CHANGEMASTERTO to specify the correct binary log file and location; 4. Create a new copy user and grant corresponding permissions; 5. Avoid self-directed
    Mysql Tutorial . Database 787 2025-06-13 00:22:40
  • How to perform a wildcard search, and what is the difference between % and _?
    How to perform a wildcard search, and what is the difference between % and _?
    % matches any number of characters suitable for broad searches, and \_ matches a single character suitable for precise positioning. For example: Li% matches all contents starting with Li, Li\_ only matches three letter names such as Liu or Lia; use LIKE to trigger wildcard characters, which contain special characters and need to be escaped; there are differences in the rules of wildcard characters in different environments.
    Mysql Tutorial . Database 790 2025-06-13 00:20:50
  • What is a Phantom Read and how can it be solved?
    What is a Phantom Read and how can it be solved?
    Phantom reading refers to the phenomenon of executing the same query twice in a transaction but obtaining different row sets, which are usually caused by inserting or deleting data by another transaction. 1. Use serialized isolation levels to lock the entire data range to prevent phantom reading but may affect performance; 2. Use range locks or key range locks to avoid full table locks and prevent new rows from inserting; 3. Use optimistic concurrency control to detect and deal with phantom reading problems during submission. This problem is particularly important when multiple queries are required to maintain consistency, such as financial reporting, inventory management and other scenarios.
    Mysql Tutorial . Database 637 2025-06-12 10:40:30
  • How to calculate the difference between two dates in days or seconds?
    How to calculate the difference between two dates in days or seconds?
    To calculate the number of days or seconds between two dates, the core method is to subtract the time unit into a unified unit. The specific methods include: 1. Use programming languages ??(such as Python's datetime module to create a date object, and then subtract the result through days and total_seconds()); 2. Use subtraction formulas to calculate the difference in the day and multiply by 86400 to obtain the difference in the second; 3. Use online tools or manual conversion to calculate the difference in the day first and then multiply by 86400 seconds per day. Different methods are suitable for different scenarios, logically consistent and simple and easy to implement.
    Mysql Tutorial . Database 555 2025-06-12 10:38:30
  • How to enable and view the MySQL slow query log?
    How to enable and view the MySQL slow query log?
    To enable MySQL slow query logs, you need to modify the configuration file and set relevant parameters. 1. Add slow_query_log=1 to enable logging in the [mysqld] part of my.cnf or my.ini; 2. Set slow_query_log_file to specify the log path, such as /var/log/mysql/mysql-slow.log; 3. Define the slow query threshold through long_query_time, the default unit is seconds. If set to 1, it means to record queries that exceed 1 second; 4. After modification, restart MySQL or use the SETGLOBAL command to take effect dynamically; 5. The log is in text format by default, and you can use tail, cat and other commands to view it.
    Mysql Tutorial . Database 430 2025-06-12 10:38:11
  • What is the leftmost prefix rule for composite indexes?
    What is the leftmost prefix rule for composite indexes?
    Theleftmostprefixruleincompositeindexesmeansqueriesmustreferencetheleftmostcolumnstousetheindexeffectively.Forexample,anindexon(last_name,first_name,email)helpsqueriesfilteringonlast_name,last_nameandfirst_name,orallthreecolumns.However,queriesfilter
    Mysql Tutorial . Database 561 2025-06-12 10:36:12
  • What is the purpose of the InnoDB Buffer Pool?
    What is the purpose of the InnoDB Buffer Pool?
    The function of InnoDBBufferPool is to improve MySQL read and write performance. It reduces disk I/O operations by cacheing frequently accessed data and indexes into memory, thereby speeding up query speed and optimizing write operations; 1. The larger the BufferPool, the more data is cached, and the higher the hit rate, which directly affects database performance; 2. It not only caches data pages, but also caches index structures such as B-tree nodes to speed up searches; 3. Supports cache "dirty pages", delays writing to disk, reduces I/O and improves write performance; 4. It is recommended to set it to 50%~80% of physical memory during configuration to avoid triggering swap; 5. It can be dynamically resized through innodb_buffer_pool_size, without restarting the instance.
    Mysql Tutorial . Database 765 2025-06-12 10:28:20
  • What is the MySQL binary log (binlog) and what is it used for?
    What is the MySQL binary log (binlog) and what is it used for?
    MySQL's binary log (binlog) is a binary log that records database change operations, and is used in scenarios such as data recovery, master-slave replication and auditing. 1. Binlog is a logical log file that records all operation events that modify data, such as INSERT, UPDATE, DELETE, etc., but does not include SELECT or SHOW query statements; 2. Its main uses include: data recovery through replay logs, supporting master-slave copying to achieve data synchronization, and used to analyze operation records to meet audit requirements; 3. Enable binlog requires setting log-bin, server-id, binlog_format and expire_logs_day in the configuration file.
    Mysql Tutorial . Database 997 2025-06-11 15:41:11
  • What is the purpose of SELECT ... FOR UPDATE?
    What is the purpose of SELECT ... FOR UPDATE?
    ThemainpurposeofSELECT...FORUPDATEistolockselectedrowsduringatransactiontopreventothersessionsfrommodifyingthemuntilthetransactioncompleteswhichensuresdataconsistencyinconcurrentenvironmentssuchasbankingandinventorysystems1Itplacesrow-levellocksallow
    Mysql Tutorial . Database 840 2025-06-11 15:37:11
  • What problems can a long-running transaction cause?
    What problems can a long-running transaction cause?
    Long transactions can cause multiple problems in the database environment. 1. Locking and blocking: Long transactions hold locks for a long time, preventing other transactions from accessing data, resulting in delay or timeout; 2. Increased risk of deadlock: Cross-waiting of multiple transactions is prone to cause deadlocks, and the database needs to interrupt transaction processing, which may lead to data inconsistency; 3. High resource consumption: Transaction logs and rollback segments occupy more memory and disk space, affecting backup and recovery and system performance; 4. Data consistency and recovery challenges: Uncommitted transactions extend the failure recovery time, and data delay or inconsistency may be caused in the replication environment; therefore transactions should be submitted or rolled back as soon as possible to avoid the above problems.
    Mysql Tutorial . Database 715 2025-06-11 15:33:10

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