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Home

Technical Articles

Database

Mysql Tutorial

How to get the list of databases on a MySQL server?

To get the database list on the MySQL server, the most common method is to use the SHOWDATABASES command, and after logging in to MySQL, execute SHOWDATABASES; it can display all databases that the current user has permission to access, and system databases such as information_schema, mysql, performance_schema and sys are usually listed; in addition, the same results can be obtained by querying the INFORMATION_SCHEMA.SCHEMATA table, which is suitable for scenarios where filtering or scripting operations are required, such as using SELECTSCHEMA_NAMEFROMINFORMATION_SCHEMA_SCHEMA_

Aug 26, 2025 am 04:17 AM

What is the role of my.cnf or my.ini file in MySQL?

Themy.cnformy.inifileisessentialforcustomizingMySQLbehavior,asitallowsadministratorstocontrolserversettings,optimizeperformance,managelogs,andapplyconfigurationstospecificcomponents;withoutit,MySQLrunsondefaults,limitingperformance,security,andmonito

Aug 26, 2025 am 03:49 AM

What is a foreign key in MySQL?

AforeignkeyinMySQLisacolumnorsetofcolumnsthatreferencesaprimaryoruniquekeyinanothertabletoenforcereferentialintegrity;itensuresdataconsistencybyallowingonlyvalidvaluesfromthereferencedtable,preventsaccidentaldeletionofrelatedrecordsdependingonconstra

Aug 26, 2025 am 02:49 AM

What is the difference between COUNT(*) and COUNT(column) in MySQL?

COUNT()countsallrowsincludingthosewithNULLvalues,whileCOUNT(column)onlycountsrowswherethespecifiedcolumnisnotNULL.1.COUNT()includeseveryrowregardlessofNULLsandisusedtogetthetotalnumberofrows.2.COUNT(column)excludesNULLsinthespecifiedcolumnandisusedto

Aug 26, 2025 am 12:56 AM

How to use IF statements in MySQL

MySQL supports two conditional logic processing methods: 1. Use the IF() function in the query, with the syntax of IF(condition, value_if_true, value_if_false), which is suitable for SELECT, UPDATE and other statements, such as returning "Pass" or "Fail" based on student scores; 2. Use IF statements in stored procedures, with the syntax of IFTHENELSEIFELSEENDIF, which is used for complex control processes, such as judging the inventory quantity and returning different messages; the two cannot be mixed, IF() is recommended for simple conditions, IF statements are used for complex logic, and CASE is recommended for multi-layer conditions to improve readability.

Aug 25, 2025 pm 02:55 PM

mysql if statement

How to partition a large table in MySQL

To effectively partition MySQL large tables, first select the appropriate partition type, such as RANGE for time series data; secondly select the partition key that matches the query pattern and make sure it is included in the primary key; then regularly manage partitions, such as adding new partitions and deleting old partitions; finally ensure performance through query optimization and monitoring. 1. Select RANGE, LIST, HASH or KEY partitions, and the time data is recommended RANGE or RANGECOLUMNS; 2. The partition key should match high-frequency query conditions and avoid frequent update columns; 3. Regularly use ALTERTABLE to add, delete or reorganize partitions; 4. Ensure that the query contains partition keys to enable partition pruning, and verify the execution plan through EXPLAIN; pay attention to Inn

Aug 25, 2025 pm 02:17 PM

Troubleshooting MySQL High Availability Failover

MySQL failover problems are usually caused by master-slave replication exceptions, inaccurate detection mechanisms, VIP configuration errors, or inconsistent data. 1. The abnormal master-slave replication status will cause no available switching nodes. You need to check the Slave_IO/SQL_Running status, replication delay and errors; 2. The inaccurate fault detection mechanism may cause misjudgment, and multi-dimensional detection should be used and reasonable timeout and retry should be set; 3. The VIP does not drift correctly or the application connection configuration will cause the new master library to be still unable to access after the switch. You need to confirm the VIP binding, DNS update and connection pool reconnection mechanism; 4. The risk of data inconsistency after the switch can be alleviated through semi-synchronous replication, GTID checking and relaylog compensation mechanisms to ensure that the data before and after the switch is complete and consistent.

Aug 25, 2025 pm 02:10 PM

How to use the ANY and ALL operators in MySQL

ANY returns the result that at least one value in the subquery meets the condition. 2. ALL requires that the condition holds all return values ??of the subquery; ANY is equivalent to SOME, =ANY is equivalent to IN, !=ALL is equivalent to NOTIN; use ANY to judge "at least one match", ALL judges "must match all values", and pay attention to the impact of NULL values ??on ALL. Both only support single-column subqueries.

Aug 25, 2025 pm 02:09 PM

Designing Scalable Database Schemas with MySQL

Designing a scalable MySQLschema requires following the following key points: 1. Clarify core business needs, avoid over-design, first implement the main process, and then expand on demand; 2. Use paradigms and anti-paradigms reasonably, standardize core data, moderate redundancy in common query fields and maintain consistency through triggers or application layers; 3. It is recommended to use self-incremental integers for the primary key, and the index is created based on query conditions, pay attention to the leftmost matching principle of combined indexes and field length control; 4. Plan the sub-table strategy in advance to reserve the possibility of splitting, but wait until the data volume is large before implementing horizontal or vertical sub-tables.

Aug 25, 2025 pm 02:00 PM

how to use explain in mysql

MySQL's EXPLAIN is an important tool used to analyze SQL query execution plans. 1. It uses EXPLAIN before the SELECT statement to show how the query is executed; 2. Key fields include id (operation unique identifier), type (connection type, common values ??from best to worse are system to ALL), key (actual index), rows (number of scan lines), Extra (extra information such as Usingfilesort); 3. Common optimization suggestions include checking whether the type is ALL (full table scan), confirming whether the key is used correctly, avoiding Usingfilesort or Usingtemporary, reducing complex JOINs and

Aug 25, 2025 pm 01:10 PM

How to use the OR operator in MySQL

In MySQL, the OR operator is used for record filtering in a WHERE clause that satisfies at least one condition. 1. The basic syntax of OR is SELECTcolumn1, column2FROMtable_nameWHEREcondition1ORcondition2. As long as any condition is true, the line will be returned. 2. For example, when querying dept='Sales'ORsalary>58000, Alice, Bob and Charlie are included because one of the conditions is met, while Diana does not meet any conditions are excluded. 3. When mixing OR and AND, AND has higher priority and must use brackets to clarify the logical relationship.

Aug 25, 2025 am 10:54 AM

mysql

How to write a correlated subquery in MySQL

AcorrelatedsubqueryinMySQLdependsontheouterqueryandexecutesonceperrowfromtheouterquery.2.ItiswrittenintheSELECT,WHERE,orFROMclauseandreferencesacolumnfromtheouterqueryusingaliaseslikee1ande2.3.Exampleusecasesincludefindingemployeesearningmorethanthei

Aug 25, 2025 am 08:45 AM

mysql Subquery

Optimizing MySQL for IoT and Time-Series Data

Using MySQL to process IoT and time series data requires optimization. First, design time and device-based primary keys such as (device_id, created_at), avoid increasing the ID, and partitioning according to time range to improve query efficiency; second, enable InnoDB row or table compression, set innodb_compression_level, and separate old data to save storage space; third, create (device_id, created_at) composite index, avoid over-index, use over-index and pre-aggregation to reduce query load; finally, adjust innodb_write_io_threads, increase innodb_log_file_size

Aug 25, 2025 am 07:50 AM

How to use the HAVING clause in MySQL

HAVING is used to filter the aggregation results after grouping. It must be used after GROUPBY and is suitable for conditions containing aggregation functions such as COUNT and SUM; 1. Use HAVING to filter the aggregated data, such as SUM (amount)>2500; 2. It can be shared with WHERE. WHERE filters rows first, HAVING and then filters the group; 3. If you use the aggregation conditions in WHERE, you will get an error, and you should use HAVING instead; 4. HAVING can refer to alias in SELECT and support multi-condition combinations, such as HAVINGavg_amount>900ANDnum_sales>=2; 5. HAVIN is not available without GROUPBY.

Aug 25, 2025 am 07:48 AM