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Technical Articles

Database

Mysql Tutorial

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

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 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

How to monitor query cache performance in MySQL

CheckQcachestatusvariablesusingSHOWSTATUSLIKE'Qcache%'tomonitorhits,inserts,prunes,andmemoryusage;2.Calculatehitratewith(Qcache_hits/(Qcache_hits Qcache_inserts))*100,aimingfor70–80%orhigher;3.AssessmemoryefficiencybyanalyzingQcache_free_memoryandfra

Aug 12, 2025 am 05:55 AM

How to list all users in MySQL

TolistallusersinMySQL,executeSELECTUser,HostFROMmysql.user;afterlogginginwithsufficientprivileges.2.Fordetailedinformationsuchasauthenticationpluginandaccountstatus,useSELECTUser,Host,authentication_string,plugin,account_locked,password_expiredFROMmy

Aug 12, 2025 am 05:52 AM

How to find the length of a string in MySQL

TofindthelengthofastringinMySQL,useLENGTH()forbytesorCHAR_LENGTH()forcharacters.1.LENGTH()returnsthenumberofbytes,soSELECTLENGTH('Hello')returns5,butSELECTLENGTH('café')returns5inUTF8mb4because'é'uses2bytes.2.CHAR_LENGTH()returnsthenumberofcharacters

Aug 12, 2025 am 05:01 AM

How to add a primary key to an existing table in MySQL?

To add a primary key to an existing table, use the ALTERTABLE statement with the ADDPRIMARYKEY clause. 1. Ensure that the target column has no NULL value, no duplication and is defined as NOTNULL; 2. The single-column primary key syntax is ALTERTABLE table name ADDPRIMARYKEY (column name); 3. The multi-column combination primary key syntax is ALTERTABLE table name ADDPRIMARYKEY (column 1, column 2); 4. If the column allows NULL, you must first execute MODIFY to set NOTNULL; 5. Each table can only have one primary key, and the old primary key must be deleted before adding; 6. If you need to increase it yourself, you can use MODIFY to set AUTO_INCREMENT. Ensure data before operation

Aug 12, 2025 am 04:11 AM

mysql primary key

How to use the SET data type in MySQL

TheSETdatatypeinMySQLstoreszeroormorevaluesfromapredefinedlist,idealformultiplechoiceslikepreferencesortags.2.DefineaSETcolumnbyspecifyingallowedstringvaluesinparenthesesduringtablecreation,suchasSET('email','sms','push','mail').3.Insertvaluesascomma

Aug 12, 2025 am 04:08 AM

How to use the ALTER TABLE statement in MySQL?

ALTERTABLEinMySQLisusedtomodifyanexistingtable’sstructure,andthemostcommonoperationsinclude:1.AddingacolumnusingADDCOLUMN,optionallyspecifyingpositionwithAFTERorFIRST;2.DroppingacolumnusingDROPCOLUMN,whichpermanentlyremovesthecolumnanditsdata;3.Modif

Aug 12, 2025 am 03:52 AM

mysql

How to use the LIMIT clause in MySQL?

Use LIMIT to limit the number of rows returned by the query, such as SELECTFROMemployeesLIMIT5 returns up to 5 rows; 2. Combining ORDERBY can ensure the order, such as SELECTFROMemployeesORDERBYsalaryDESCLIMIT3 to obtain the highest salary of 3 employees; 3. Use OFFSET to implement paging, such as LIMIT5OFFSET10 means skipping the first 10 rows and returning 5 rows, which is equivalent to LIMIT10,5; 4. It is often used to paging, obtain the first N records or data sampling; 5. Note that the order should always be guaranteed with ORDERBY. LIMIT is MySQL-specific syntax, and high offset may affect performance. It is recommended that

Aug 12, 2025 am 03:32 AM

mysql limit

How to analyze the slow query log in MySQL

First, make sure to enable and correctly configure the slow query log, set slow_query_log=ON, long_query_time=1.0, and log_queries_not_using_indexes=ON, and restart MySQL or dynamic application configuration; 2. Use mysqldumpslow tool to analyze the log, view the most frequent slow query through mysqldumpslow-sc-t10, or filter specific tables with the -g parameter; 3. Use pt-query-digest in PerconaToolkit for in-depth analysis, generate detailed statistics and optimization suggestions, such as identifying that Rows_examined is much larger than Ro

Aug 12, 2025 am 02:13 AM

How to add a column to a table in MySQL

To add columns to an existing MySQL table, you need to use the ALTERTABLE statement to match the ADDCOLUMN clause; the basic syntax is ALTERTABLEtable_nameADDCOLUMNcolumn_namedata_type[constraints], where the COLUMN keyword can be omitted; for example, add VARCHAR column: ALTERTABLEusersADDCOLUMNemailVARCHAR(100); add columns with NOTNULL constraints: ALTERTABLEusersADDCOLUMN created_atDATETIMENOTNULL; add a lever to a VARCHAR column; add a lever to a VARCHAR column: ALTERTABLEusersADDCOLUMN created_atDATETIMENOTNULL; add a lever to a VARCHAR column: ALTERTABLEusersADDCOLUMNcreated_atDATETIMENOTNULL; add a lever to a VARCHAR column: ALTERTABLEusersADDCOLUMNcreated_atDATETIMENOTNULL; add a lever to a VARCHAR column: ALTERTABLEusersADDCOLUMNcreated_atDATETIMENOTNULL; add a lever to a VARCHAR column: ALTERTABLEusersADDCOLUMNcreated_atDATETIMENOTNULL; add a lever to a VARCHAR column: ALTERTABLEusersADDCOLUMNcreated_atDATETIMENOTNULL;

Aug 12, 2025 am 01:56 AM