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Technical Articles

Database

Mysql Tutorial

What is the maximum number of columns in a MySQL table?

MySQL 8.0.19 and above support the InnoDB table up to 4,096 columns, but the actual number of available columns is limited by row size (about 8,000 bytes), and requires the use of Dynamic or Compressed row format; the upper limit of earlier versions was 1,017 columns; the MyISAM engine supports 4,096 columns but is limited by 65,534 byte row size; despite this, more than tens of columns should be avoided during design, and it is recommended to optimize the structure through normalization, association tables or JSON columns to ensure maintainability and performance.

Sep 01, 2025 am 08:00 AM

How to join tables in MySQL

Table joins in MySQL are implemented through SELECT statement combined with JOIN clause. The main types include: 1.INNERJOIN: Only the matching rows in the two tables are returned; 2.LEFTJOIN: Return all rows in the left table and the right table match rows, if there is no match, the right table field is NULL; 3.RIGHTJOIN: Return all rows in the right table and the left table match rows, if there is no match, the left table field is NULL; 4.FULLOUTERJOIN: MySQL does not directly support it, but can be simulated by LEFTJOIN and RIGHTJOIN combined with UNION; use ON to specify the connection conditions, and it is recommended to use table alias to simplify query. Multi-table connections need to be linked step by step, and it should be ensured that the connection column has been indexed to improve performance.

Sep 01, 2025 am 07:57 AM

mysql table join

What is the difference between ENUM and SET data types in MySQL?

The ENUM type only allows the selection of a single value from a predefined list, which is suitable for single-select scenarios such as state or gender; the SET type allows the selection of zero or more values, which is suitable for multiple-select scenarios such as permissions or tags. ENUM supports up to 65,535 members, and is stored internally with indexes starting at 1; SET supports up to 64 members, and is stored internally in bitmap form, with each value corresponding to a binary bit. Inserting an invalid value in ENUM will report an error or be saved as an empty string, and SET will automatically ignore the invalid value or process it according to SQL mode. For example, ENUM('active','inactive') can only store one state, while SET('read','write') can store the 'read, write' combination. because

Sep 01, 2025 am 07:03 AM

set enum

Implementing MySQL Data Archiving with Partitioning

MySQL data archive can be implemented through partitioning to improve performance and maintenance efficiency. 1. Select the appropriate partitioning strategy: Priority is given to using RANGE partitions to archive by time, such as dividing order data by month; or use LIST partitions to archive by classification. 2. Notes should be paid attention to when designing table structure: Partition fields must be included in primary keys or unique constraints, and queries should be equipped with partition fields to enable partition cropping. 3. Automatic archives can regularly perform deletion of old partitions through scripts, and record logs and check the existence of partitions to avoid mistaken deletion. 4. Limited applicable scenarios: small tables, query without partition fields, cloud database restriction partitioning function, etc., other archive solutions should be considered, such as timed migration of archive tables. Under reasonable design, partition archives can efficiently manage historical data, otherwise it is easy to induce

Sep 01, 2025 am 04:12 AM

How to check the status of a MySQL server

Usemysqladmin-uroot-pstatustogetkeymetricslikeuptime,threads,andqueries,ormysqladminpingtocheckiftheserverisalive;2.Loginwithmysql-uroot-pandrunSHOWSTATUSLIKE'variable_name'toviewspecificserverstatusvariablessuchasUptime,Threads_connected,Queries,and

Sep 01, 2025 am 04:10 AM

How to export a MySQL table to a CSV file?

Use SELECTINTOOUTFILE to export tables as CSV files on MySQL server. It requires FILE permissions and the target path is writable. For example: SELECT*FROMusersINTOOUTFILE'/tmp/users.csv'FIELDSTERMINATEDBY','ENCLOSEDBY'"'LINESTERMINATEDBY'\n'; If there is no server access permission, you can export them through client commands combined with shell redirection or using scripting languages ??such as Python. The scripting method can better handle special characters, encodings and references to ensure data integrity. Therefore, it is recommended to replication

Sep 01, 2025 am 04:08 AM

How to use the REPLACE statement in MySQL?

REPLACE is used in MySQL to insert new rows. If a unique key or primary key conflict occurs, the old row will be deleted first and then inserted new rows; 2. Use scenarios include ensuring that the record exists and can be deleted and reinserted; 3. The syntax supports VALUES, SET and SELECT forms; 4. The example shows that the replacement operation is triggered through the primary key or unique key; 5. Notes: The automatic incrementing ID may change, the trigger will be deleted and then inserted, the performance is low, and data will be lost if the column is not specified; 6. A safer alternative is to use INSERT...ONDUPLICATEKEYUPDATE for updates rather than full row replacement.

Sep 01, 2025 am 01:09 AM

mysql replace

How to use the BETWEEN operator in MySQL

BETWEEN is an operator in MySQL used to filter data within a specified range and contains boundary values; 1. When used in numbers, such as salaryBETWEEN30000AND50000, equivalent to >= and

Aug 31, 2025 am 07:15 AM

mysql between

How to drop a temporary table in MySQL

To delete temporary tables in MySQL, you must use the DROPTEMPORARYTABLE statement; 1. You must include the TEMPORARY keyword to explicitly specify that the deleted temporary table is specified, otherwise MySQL will try to find a permanent table with the same name, which may cause an error; 2. Use DROPTEMPORARYTABLEtable_name; syntax to delete existing temporary tables, such as DROPTEMPORARYTABLEtemp_users; 3. To avoid errors when the table does not exist, you should add IFEXISTS clauses, such as DROPTEMPORARYTABLEIFEXISTStemp_users; 4. Temporary tables are only in the current session

Aug 31, 2025 am 06:43 AM

How to calculate the difference between two dates in MySQL

To calculate the difference between two dates in MySQL, the function should be selected according to the required units: 1. Use DATEDIFF() to obtain the difference in the number of days, such as DATEDIFF('2025-04-05','2025-03-01') to return 35, the function only compares the date part and the result symbol depends on the order; 2. Use TIMESTAMPDIFF() to obtain the difference in other units, such as TIMESTAMPDIFF(MONTH,'2025-03-01','2025-04-05') to return 1, supporting units such as YEAR, QUARTER, MONTH, DAY, HOUR, MINUTE, SECOND, etc.; 3. It can be calculated by direct subtraction but

Aug 31, 2025 am 06:08 AM

How to restore a database from a backup in MySQL

Torestorefromamysqldumpfile,firstcreatethedatabaseifitdoesn’texistusingCREATEDATABASEIFNOTEXISTSyour_database_name,thenrunmysql-uusername-pyour_database_name

Aug 31, 2025 am 05:52 AM

What is the innodb_file_per_table option in MySQL?

Theinnodb_file_per_tableoptionshouldbeenabledtoalloweachInnoDBtabletohaveitsown.ibdfile,improvingspacereclamation,manageability,andstorageoptimization,withbenefitsoutweighingminorfilesystemoverhead,anditisenabledbydefaultinMySQL5.6andlaterversions.

Aug 31, 2025 am 04:30 AM

mysql innodb

How to compare and synchronize two MySQL databases?

Compareschemasusingmysqldumpwith--no-dataanddiffortoolslikept-schema-sync;2.CompareandsyncdatausingCHECKSUMTABLE,SELECTCRC32withGROUP_CONCAT,orpreferablypt-table-checksumandpt-table-syncforaccuracy;3.AutomatewithMySQLreplication,customscriptsusingPyt

Aug 31, 2025 am 03:30 AM

How to grant permissions on a specific database only in MySQL

TograntpermissionsonaspecificdatabaseonlyinMySQL,usetheGRANTstatementwiththedatabasenameexplicitlyspecified.1.SpecifythedatabaseintheGRANTstatementusingtheformat:GRANTpermission_typeONdatabase_name.TO'username'@'host';forexample,GRANTALLPRIVILEGESONs

Aug 31, 2025 am 02:20 AM