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Home

Technical Articles

Database

Mysql Tutorial

What is the functional difference between the WHERE and HAVING clauses?

In SQL, the main difference between WHERE and HAVING is the execution timing and the type of data filtered. 1.WHERE filters a single row before grouping, and cannot use an aggregation function; 2.HAVING filters the aggregation results after grouping, allowing the use of an aggregation function. For example, when querying departments with more than 10 high-paying employees, WHERE first filters low-paying employees, then uses GROUPBY to group them, and finally uses HAVING to filter groups that meet the criteria. In terms of performance, WHERE should be used to reduce the amount of data, and HAVING should only be used when filtering the aggregate results.

Jun 20, 2025 am 12:55 AM

where having

Which is more efficient: COUNT(*), COUNT(1), or COUNT(column_name)?

InmodernSQLdatabases,COUNT(),COUNT(1),andCOUNT(column_name)havelittletonoperformancedifferenceinbasicqueries.1.COUNT()countsallrows,includingNULLs,andisbestfortotalrowcount.2.COUNT(1)behavesthesameasCOUNT(),withnoperformanceadvantage,andisusedmainlyb

Jun 20, 2025 am 12:53 AM

count(1) COUNT(*)

What are optimistic and pessimistic locks, and how to implement them in MySQL?

Pessimistic locks and optimistic locks are two strategies for handling concurrent database access. Pessimistic locking assumes conflicts and locks are immediately added when the data is modified, such as in MySQL using SELECT...FORUPDATE or SELECT...LOCKINSHAREMODE, which is suitable for high-competitive scenarios but may degrade performance. Optimistic locks assume fewer conflicts and do not lock immediately, but check version numbers or timestamps when updated. They are suitable for low-competitive scenarios and avoid lock overhead, but the application layer needs to handle conflicts. If you choose a pessimistic lock, if you write frequently and have high data consistency requirements; if you choose an optimistic lock, if you have fewer conflicts, you hope to improve concurrency and can handle it elegantly. In addition, pessimistic locks may lead to deadlocks, and optimistic locks require additional logic to handle conflicts.

Jun 20, 2025 am 12:51 AM

pessimistic lock optimistic locking

What is read-write splitting and how is it implemented?

Read-writesplittingimprovesdatabaseperformancebyseparatingreadandwriteoperationsacrossdifferentservers.Itworksbydirectingwritestotheprimarydatabaseandreadstoreplicas,reducingload,improvingresponsetime,andenhancingfaulttolerance.Commonimplementationme

Jun 20, 2025 am 12:37 AM

database Read and write separation

How to design a database sharding strategy?

The core of designing a database sharding strategy is "how to reasonably separate the data without affecting use". The key points include: 1. Select the right shard key, and fields such as user ID that are high-base, evenly distributed and commonly used as query conditions should be selected, such as user ID, to avoid using time or high-frequency query fields to prevent hot spots and cross-slicing problems; 2. Control the number of shards, set to 16 or 32 in the initial stage, and reasonably estimate the total data volume and node bearing capacity to avoid operation and maintenance or expansion problems caused by too much or too little; 3. Optimize query and transactions, reduce cross-slicing queries, use redundancy or intermediate layer aggregation to improve efficiency, pay attention to the limited transaction support in the shard environment, and it is necessary to cooperate with cache or secondary index to optimize query performance, report statistics and other operations, and it is recommended to process in parallel at the application layer to reduce database pressure.

Jun 20, 2025 am 12:35 AM

What is a subquery and does it affect performance?

Asubquerycanaffectperformancedependingonitsusage.1.Correlatedsubqueriesmayexecuterepeatedly,onceforeachrowintheouterquery.2.Poorlywrittensubqueriescanpreventefficientindexuse.3.Subqueriesaddcomplexity,makingqueryoptimizationharder.However,moderndatab

Jun 20, 2025 am 12:17 AM

performance Subquery

What is a typical process for MySQL master failover?

MySQL main library failover mainly includes four steps. 1. Fault detection: Regularly check the main library process, connection status and simple query to determine whether it is downtime, set up a retry mechanism to avoid misjudgment, and can use tools such as MHA, Orchestrator or Keepalived to assist in detection; 2. Select the new main library: select the most suitable slave library to replace it according to the data synchronization progress (Seconds_Behind_Master), binlog data integrity, network delay and load conditions, and perform data compensation or manual intervention if necessary; 3. Switch topology: Point other slave libraries to the new master library, execute RESETMASTER or enable GTID, update the VIP, DNS or proxy configuration to

Jun 19, 2025 am 01:06 AM

mysql failover

What is MySQL Group Replication (MGR)?

MySQLGroupReplication (MGR) is a plug-in high-availability clustering technology officially provided by MySQL, which is based on the Paxos protocol to achieve strong data consistency and automatic failover. 1. MGR synchronizes transaction logs and authenticates them among multiple nodes through the group communication system to ensure data consistency; 2. Its core features include automatic failover, multiple write/single-write mode optional, and built-in conflict detection mechanism; 3. Deployment requires at least three nodes, and configures key parameters such as GTID, row format log, and unique server_id; 4. Common processes include preparing servers, configuring parameters, initializing nodes, joining clusters and status checks. MGR is suitable for scenarios with high data consistency requirements, but is sensitive to network latency

Jun 19, 2025 am 01:06 AM

MGR

How to connect to a MySQL database using the command line?

The steps to connect to the MySQL database are as follows: 1. Use the basic command format mysql-u username-p-h host address to connect, enter the username and password to log in; 2. If you need to directly enter the specified database, you can add the database name after the command, such as mysql-uroot-pmyproject; 3. If the port is not the default 3306, you need to add the -P parameter to specify the port number, such as mysql-uroot-p-h192.168.1.100-P3307; In addition, if you encounter a password error, you can re-enter it. If the connection fails, check the network, firewall or permission settings. If the client is missing, you can install mysql-client on Linux through the package manager. Master these commands

Jun 19, 2025 am 01:05 AM

mysql Command Line

Why do indexes improve MySQL query speed?

IndexesinMySQLimprovequeryspeedbyenablingfasterdataretrieval.1.Theyreducedatascanned,allowingMySQLtoquicklylocaterelevantrowsinWHEREorORDERBYclauses,especiallyimportantforlargeorfrequentlyqueriedtables.2.Theyspeedupjoinsandsorting,makingJOINoperation

Jun 19, 2025 am 01:05 AM

mysql index

What is GTID (Global Transaction Identifier) and what are its advantages?

GTID (Global Transaction Identifier) ??solves the complexity of replication and failover in MySQL databases by assigning a unique identity to each transaction. 1. It simplifies replication management, automatically handles log files and locations, allowing slave servers to request transactions based on the last executed GTID. 2. Ensure consistency across servers, ensure that each transaction is applied only once on each server, and avoid data inconsistency. 3. Improve troubleshooting efficiency. GTID includes server UUID and serial number, which is convenient for tracking transaction flow and accurately locate problems. These three core advantages make MySQL replication more robust and easy to manage, significantly improving system reliability and data integrity.

Jun 19, 2025 am 01:03 AM

gtid Transaction identification

What is the difference between Percona XtraDB Cluster (PXC) and InnoDB Cluster?

PXC and InnoDBCluster are common high-availability clustering solutions in MySQL. The core differences are as follows: 1. Different synchronization mechanisms: PXC uses Galera multi-master replication, supports multi-node writing, and is suitable for high concurrent write scenarios; InnoDBCluster is based on MGR, and the default is single-master mode. Only one node can be written. Although it supports multi-master, the official recommends to use it with caution. 2. Different methods of data consistency guarantee: PXC authenticates before transaction submission to ensure consistency but may increase delays, and rolls back transactions in conflicts; copying after InnoDBCluster after submission, there is a short inconsistency window, and the final consistency is guaranteed through the Paxos protocol, and the network partition tends to maintain availability. 3. The complexity of deployment and operation and maintenance is different:

Jun 19, 2025 am 01:01 AM

How to safely purge old MySQL binlog files?

To clean MySQL binlog files, you should use the PURGEBINARYLOGS command or set the automatic expiration time, and files cannot be deleted directly. 1. Use the PURGE command to clean old logs by file name or time. Before execution, you need to confirm that the slave library no longer uses the relevant logs; 2. Check the current log status and slave library location through SHOWMASTERSTATUS and SHOWSLAVESTATUS to ensure the security of the cleaning range; 3. It is recommended to set the binlog_expire_logs_seconds parameter to achieve automatic cleaning, which is suitable for long-term operation environments; 4. Deleting files directly will cause serious problems such as master-slave synchronization failure and inconsistent log information, and must be avoided.

Jun 19, 2025 am 01:01 AM

mysql binlog

How to choose between DATETIME and TIMESTAMP in MySQL?

When selecting DATETIME and TIMESTAMP types in MySQL, they should be determined based on time zone processing, automatic update, time range, storage space and concurrency requirements. 1. If you need to automatically convert the time zone, you should select TIMESTAMP, which will automatically adjust the display time according to the connection time zone, and DATETIME will always remain the same; 2. If you need to automatically update the fields, TIMESTAMP supports ONUPDATE automatic refresh, and DATETIME only supports the default value; 3. If you need a larger time range (1000 to 9999), select DATETIME, and the TIMESTAMP range is smaller (1970 to 2038); 4. If you are sensitive to storage space, TIMESTAMP accounts for 4.

Jun 19, 2025 am 12:58 AM

datetime