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Technical Articles

Database

How to optimize queries in MySQL

UseproperindexingbyaddingindexesoncolumnsinWHERE,JOIN,ORDERBY,andGROUPBYclauses,usingcompositeindexesformultiple-columnfilters,avoidingover-indexing,andremovingunusedindexes.2.WriteefficientqueriesbyselectingonlyneededcolumnsinsteadofusingSELECT*,avo

Aug 12, 2025 pm 01:58 PM

mysql Query optimization

How to use the JSON_ARRAY function in MySQL

The JSON_ARRAY() function is used to create JSON arrays. 1. You can pass in strings, numbers, NULL or JSON values to generate an array, such as JSON_ARRAY('apple','banana') returns ["apple","banana"]; 2. You can combine table columns to generate JSON arrays for each row, such as SELECTJSON_ARRAY(id, name, price)FROMproducts; 3. Support nested structures and can contain other JSON arrays or objects; 4. When any parameter is NULL, the result is displayed as null, and when there is no parameter, the empty array is returned []; this function is applicable

Aug 12, 2025 pm 01:54 PM

How to find the number of months between two dates in SQL?

MySQL uses TIMESTAMPDIFF(MONTH, start_date, end_date) to calculate the number of months; 2. SQLServer uses DATEDIFF(MONTH, start_date, end_date); 3. PostgreSQL uses EXTRACT(YEARFROMAGE(end_date, start_date))*12 EXTRACT(MONTHFROMAGE(end_date, start_date)); 4. Oracle uses MONTHS_BETWEEN(end_date, start_date))

Aug 12, 2025 pm 01:34 PM

How to convert a string to a date in SQL

To convert a string to a date, you need to select the appropriate function and format according to the database system; SQLServer uses CAST or CONVERT, MySQL uses CAST or STR_TO_DATE, PostgreSQL uses CAST or TO_DATE, Oracle uses TO_DATE, SQLite uses DATE function; it is necessary to ensure that the string format is consistent with the function requirements, it is recommended to use ISO format (YYYY-MM-DD) to improve compatibility, and invalid input is processed through TRY_CAST and other methods, and finally the string is successfully converted to the date type ends.

Aug 12, 2025 pm 01:11 PM

How to backup and restore a database in SQL

InSQLServer,useBACKUPDATABASEandRESTOREDATABASEcommandswithproperfilepathsandoptionslikeWITHREPLACE;2.InMySQL,usemysqldumptocreatebackupsandmysqlcommandorSOURCEtorestore;3.Followbestpracticessuchasschedulingregularbackups,testingrestores,storingbacku

Aug 12, 2025 pm 01:00 PM

sql database backup

How to read a query execution plan in SQL?

ToreadaSQLexecutionplaneffectively,startbyunderstandingitisadatabase-generatedroadmapshowinghowaqueryisexecuted;useestimatedplanstopreviewandactualplanstoanalyzeruntimeperformance.Readtheplanfromrighttoleftandtoptobottom,wheredataflowsfromsources(tab

Aug 12, 2025 pm 12:56 PM

How to use the UNION ALL operator in SQL

UNIONALLcombinesmultipleSELECTstatementsintoasingleresultsetwhileretainingallrows,includingduplicates,makingitfasterthanUNION.1.EachSELECTmusthavethesamenumberofcolumnswithcompatibledatatypes,andcolumnnamesaredeterminedbythefirstSELECT.2.UseUNIONALLw

Aug 12, 2025 am 11:54 AM

sql

How do you use COMMIT and ROLLBACK to manage transactions in SQL?

COMMIT is used to permanently save changes in transactions, and ROLLBACK is used to undo changes; 1. A transaction is a set of SQL statements executed as a single logical unit; 2. Use COMMIT to make all changes effective and irreversible; 3. Use ROLLBACK to roll back to the pre-transaction state when an error or cancellation of an operation; 4. Automatic commit mode must be turned off to use both; 5. Partial rollback can be achieved through save points; 6. Transactions ensure ACID characteristics; 7. It is recommended to use transactions for associated operations, keep them short and test rollback behavior. In short, COMMIT and ROLLBACK jointly ensure the consistency of SQL data.

Aug 12, 2025 am 11:49 AM

MySQL Database Performance Tuning for Complex Queries

Optimizing complex queries requires many aspects to start. 1. Use EXPLAIN to analyze the execution plan and see if the full table is scanned or temporary tables are used; 2. Design the index reasonably, combine indexes are better than single column indexes to avoid leading column mismatch, over-index and low-distinguishing fields; 3. Reduce data processing, avoid SELECT*, split large queries, and control the number of JOINs; 4. Adjust configuration parameters such as sort_buffer_size, join_buffer_size, and innodb_buffer_pool_size, but be cautiously verified.

Aug 12, 2025 am 10:56 AM

How to configure MySQL for UTF-8 support completely

To implement the full UTF-8 support of MySQL, you must use utf8mb4 and ensure that all levels of configuration are consistent. 1. Use utf8mb4 instead of utf8 because it supports 4-byte Unicode characters (such as emoji); 2. Configure the default character set of client, mysql and mysqld segments in my.cnf to utf8mb4, and enable the innodb_large_prefix and Barracuda file formats; 3. Convert existing databases, tables and columns to utf8mb4 through the ALTERDATABASE, ALTERTABLE and MODIFY commands; 4. Set the character set when applying connections, such as using PD in PHP;

Aug 12, 2025 am 09:48 AM

What is the difference between DELETE, TRUNCATE, and DROP in MySQL?

DELETE is used to delete specific or all rows in a table, supporting WHERE conditions, transaction rollback and triggers, but the speed is slow and the self-increment counter does not reset; TRUNCATE quickly deletes all rows by recreating the table, which is fast and the self-increment counter is reset, but it cannot rollback and does not trigger the trigger; DROP deletes the entire table structure and data, which cannot be rolled back, which is suitable for completely removing tables.

Aug 12, 2025 am 09:26 AM

mysql Data deletion

How to select a row with the maximum value in a column in SQL?

Use the subquery method to obtain all rows corresponding to the maximum value of a specific column, which is suitable for all SQL databases and can handle parallelism; 2. Use ORDERBY and LIMIT (or TOP, FETCHFIRST) to quickly return a single row, suitable for scenarios where only one result is needed and performance-oriented; 3. Use window functions such as ROW_NUMBER() and RANK() to achieve more complex logical control, such as partitioning or explicit processing of parallelism; 4. If explicit processing of parallelism is required, it is recommended to use a combination of subquery and MAX() to ensure that all maximum rows are returned. When selecting a method, it should be decided based on whether all parallel rows need to be returned and the database system used. The subquery method is the most common, and ORDERBY plus LIMI

Aug 12, 2025 am 09:04 AM

sql maximum value

What is a sequence in Oracle?

Oracle sequence is a database object used to generate unique numbers, mainly used for primary key value creation; its core mechanism is to obtain the next value through NEXTVAL and CURRVAL get the current value; common uses include primary key generation, audit trail and batch processing; when creating, use CREATESEQUENCE statements and set parameters such as STARTWITH, INCREMENTBY, MAXVALUE, MINVALUE, CYCLE and CACHE; the advantage is that it has high performance, supports concurrent access, and complies with SQL standards; it should be noted that the sequence does not guarantee gap-free, because rollback, cache or failure may cause number jumps. Each NEXTVAL call consumes the sequence value, and cache can improve performance but increase

Aug 12, 2025 am 08:30 AM

How to rename a table in MySQL

To rename MySQL table, you should use the RENAMETABLE statement; its basic syntax is RENAMETABLEold_table_nameTOnew_table_name; for example, RENAMETABLEusers_oldTOusers; 1. Single table renaming: one or more tables can be renamed at a time; 2. Atomic operation: During the renaming period, the table is locked and unavailable until it is completed; 3. Permission requirements: ALTER and DROP permissions are required for the original table, and CREATE and INSERT permissions are required for the database; 4. Storage engine compatibility: supports mainstream engines such as InnoDB and MyISAM; multiple tables can be renamed at once using RENAMETABLE.

Aug 12, 2025 am 08:06 AM