How to create a hash index in mysql

When dealing with large tables, MySQL performance and maintainability face challenges, and it is necessary to start from structural design, index optimization, table sub-table strategy, etc. 1. Reasonably design primary keys and indexes: It is recommended to use self-increment integers as primary keys to reduce page splits; use overlay indexes to improve query efficiency; regularly analyze slow query logs and delete invalid indexes. 2. Rational use of partition tables: partition according to time range and other strategies to improve query and maintenance efficiency, but attention should be paid to partitioning and cutting issues. 3. Consider reading and writing separation and library separation: Read and writing separation alleviates the pressure on the main library. The library separation and table separation are suitable for scenarios with a large amount of data. It is recommended to use middleware and evaluate transaction and cross-store query problems. Early planning and continuous optimization are the key.

MySQL supports CHECK constraints to force domain integrity, effective from version 8.0.16; 1. Add constraints when creating a table: Use CREATETABLE to define CHECK conditions, such as age ≥18, salary > 0, department limit values; 2. Modify the table to add constraints: Use ALTERTABLEADDCONSTRAINT to limit field values, such as name non-empty; 3. Use complex conditions: support multi-column logic and expressions, such as end date ≥start date and completion status must have an end date; 4. Delete constraints: use ALTERTABLEDROPCONSTRAINT to specify the name to delete; 5. Notes: MySQL8.0.16, InnoDB or MyISAM needs to be quoted

Useamany-to-manyrelationshipwithajunctiontabletolinkitemsandtagsviathreetables:items,tags,anditem_tags.2.Whenaddingtags,checkforexistingtagsinthetagstable,insertifnecessary,thencreatemappingsinitem_tagsusingtransactionsforconsistency.3.Queryitemsbyta

To display all databases in MySQL, you need to use the SHOWDATABASES command; 1. After logging into the MySQL server, you can execute the SHOWDATABASES; command to list all databases that the current user has permission to access; 2. System databases such as information_schema, mysql, performance_schema and sys exist by default, but users with insufficient permissions may not be able to see it; 3. You can also query and filter the database through SELECTSCHEMA_NAMEFROMinformation_schema.SCHEMATA; for example, excluding the system database to only display the database created by users; make sure to use

Check whether the MySQL service is running, use sudosystemctlstatusmysql to confirm and start; 2. Make sure that bind-address is set to 0.0.0.0 to allow remote connections and restart the service; 3. Verify whether the 3306 port is open, check and configure the firewall rules to allow the port; 4. For the "Accessdenied" error, you need to check the user name, password and host name, and then log in to MySQL and query the mysql.user table to confirm permissions. If necessary, create or update the user and authorize it, such as using 'your_user'@'%'; 5. If authentication is lost due to caching_sha2_password

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

DELETEremovesspecificorallrows,keepstablestructure,allowsrollbackandtriggers,anddoesnotresetauto-increment;2.TRUNCATEquicklyremovesallrows,resetsauto-increment,cannotberolledbackinmostcases,doesnotfiretriggers,andkeepstablestructure;3.DROPremovesthee

Using mysqldump is the most common and effective way to back up MySQL databases. It can generate SQL scripts containing table structure and data. 1. The basic syntax is: mysqldump-u[user name]-p[database name]>backup_file.sql. After execution, enter the password to generate a backup file. 2. Back up multiple databases with --databases option: mysqldump-uroot-p--databasesdb1db2>multiple_dbs_backup.sql. 3. Back up all databases with --all-databases: mysqldump-uroot-p