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Technical Articles

Database

Mysql Tutorial

How to use the CURDATE function in MySQL

CURDATE() returns the current date, and you need to pay attention to the data type and context when using it. 1. Basic usage: SELECTCURDATE() gets the current date. For example, 2025-04-05, you can also use the synonyms CURRENT_DATE or CURRENT_DATE(); 2. When filtering today's data in the WHERE clause, if the field is DATE type, you can directly compare it. If it is DATETIME type, you should use DATE(order_date)=CURDATE() or more efficient range query order_date>=CURDATE()ANDorder_date=CURDATE()-INTERVAL3

Aug 11, 2025 pm 11:54 PM

How to export a table to a CSV file in MySQL

The MySQL table exported as a CSV file can be implemented through the SELECT...INTOOUTFILE statement, which directly generates files on the server side without requiring additional tools; 1. Use the SELECT...INTOOUTFILE syntax to write the query results to the CSV file of the specified path; 2. Ensure that the export path is specified by the secure_file_priv variable and that the MySQL process has write permissions; 3. The target file cannot exist in advance, otherwise an error will be reported; 4. The executor must have FILE permissions; 5. Custom separators, quotes and newlines can be used through FIELDSTERMINATEDBY, ENCLOSEDBY and LINESTERMINATEDBY to customize the separators, quotes and newlines to

Aug 11, 2025 pm 11:24 PM

mysql csv file

How to perform a case-sensitive select in MySQL

By default, MySQL's SELECT query is case-insensitive when using case-insensitive sorting rules (such as utf8mb4_general_ci). To execute case-sensitive queries, you can use the following methods: 1. Use the BINARY keyword for binary comparison, such as SELECT*FROMusersWHEREBINARYusername='JohnDoe'; 2. Use the COLLATE clause to specify case-sensitive sorting rules, such as WHEREusernameCOLLATEutf8mb4_bin='JohnDoe'; 3. Define columns as case-sensitive sorting rules when creating or modifying tables, such as

Aug 11, 2025 pm 10:39 PM

mysql Case Sensitive

How to find the size of a table in MySQL

TofindthesizeofaspecifictableinMySQL,querytheinformation_schema.TABLESbyreplacing'your_database_name'and'your_table_name'intheprovidedSQLstatementtogetthetotalsizeinMB.2.Tolistalltablesinadatabaseorderedbysize,usethesameinformation_schema.TABLESwitha

Aug 11, 2025 pm 10:24 PM

How to insert data into a table in MySQL

Insert data into MySQL tables using the INSERTINTO statement. 1. The basic syntax is: INSERTINTO table name (column 1, column 2,...) VALUES (value 1, value 2,...); 2. Specify the corresponding column and value when inserting a single row, and the self-increment primary key can be omitted; 3. Insert multiple rows can list multiple sets of values after VALUES to improve efficiency; 4. The column name can be omitted to insert all columns, but it must be processed in the order of table definition and includes NULL to process the self-increment columns, which poses a risk of structural change; 5. Use INSERTIGNORE to avoid duplicate errors, or ONDUPLICATEKEYUPDATE to achieve update insertion; 6. Best practices include always specifying column names, ensuring data types match, and using preprocessing statements

Aug 11, 2025 pm 10:15 PM

How to resolve 'MySQL server has gone away' error

First, check and increase the values of wait_timeout and interactive_timeout to prevent shutdown due to excessive idle connection; 2. Increase the max_allowed_packet parameter to support large-capacity data transmission; 3. Check MySQL error logs and system resources to avoid service interruptions due to crashes or insufficient memory; 4. Implement connection health detection and automatic reconnection mechanisms at the application layer; 5. Troubleshoot external factors such as firewall, proxy or version bugs, and finally solve the problem by comprehensively adjusting the configuration and code.

Aug 11, 2025 pm 09:57 PM

How to find the size of a MySQL database and its tables?

Tocheckthesizeofaspecificdatabase,useaSELECTquerywithWHEREtable_schema='your_database_name'tofilterbydatabasename.2.Tolistalldatabaseswiththeirsizes,runaGROUPBYqueryontable_schemawithoutaWHEREclauseandorderbysizeindescendingorder.3.Tocheckindividualt

Aug 11, 2025 pm 09:51 PM

How to use the JOIN clause in MySQL?

ThemostcommonlyusedJOINtypesinMySQLareINNERJOIN,LEFTJOIN,RIGHTJOIN,andsimulatedFULLOUTERJOIN,eachdetermininghowrowsfromtwoormoretablesarecombinedbasedonmatchingcolumnvalues;INNERJOINreturnsonlymatchingrows,LEFTJOINincludesallrowsfromthelefttablewithN

Aug 11, 2025 pm 09:45 PM

mysql join

How to export a database using mysqldump

To correctly use mysqldump to export a database, you must first master its basic syntax and common options. 1. Export a single database using the command mysqldump-uusername-pdatabase_name>backup.sql, the system will prompt for inputting a password. The generated SQL file contains all the table structure and data required to rebuild the database; 2. When exporting multiple databases, add --databases options, such as mysqldump-uusername-p-databasesdb1db2>multiple_dbs_backup.sql, the output file will contain statements to create the database; 3. Use --

Aug 11, 2025 pm 09:44 PM

Database export

How to find tables without a primary key in a MySQL database?

To find tables without primary keys in MySQL, you can query information_schema; 1. Use subqueries to exclude tables with primary keys and filter basic tables in non-system databases; 2. Or use LEFTJOIN to connect TABLE_CONSTRAINTS to find tables with constraint name PRIMARYKEY but match result empty; 3. If it is for a specific database, you only need to specify TABLE_SCHEMA in the WHERE condition; this method is efficient and accurate, and can effectively identify tables with a lack of primary keys that may affect performance and data integrity.

Aug 11, 2025 pm 07:49 PM

How to use the NOT operator in MySQL

In MySQL, the NOT operator is used to invert the result of logical expressions. 1. The basic syntax is SELECTcolumn1, column2FROMtable_nameWHERENOTcondition, such as querying non-German customers; 2. Use NOTIN to exclude values in the list, but it should be noted that if the list contains NULL, the expected result may not be returned; 3. Use NOTBETWEEN to exclude values in a certain range; 4. Use NOTLIKE to find records that do not match the specified pattern; 5. When combining AND/OR, brackets should be used to clarify the logical priority to avoid errors caused by operator priority. At the same time, beware of the impact of NULL value on NOT operations. Finally, you must be vigilant about the impact of NULL value on NOT operations.

Aug 11, 2025 pm 07:16 PM

How to describe a table in MySQL

To describe the MySQL table structure, you can use the DESCRIBE or DESC commands; 1. Use DESCRIBEtable_name or DESCtable_name to view the field name, data type, whether NULL, key type, default value and extra attributes are allowed; 2. Use SHOWCREATETABLEtable_name to obtain complete table building statements containing primary keys, unique keys, storage engines and character sets; 3. Query the INFORMATION_SCHEMA.COLUMNS table to obtain detailed metadata such as column annotations, permissions, etc., and select the appropriate method according to your needs to complete a comprehensive understanding of the table structure.

Aug 11, 2025 pm 07:05 PM

How to revoke privileges from a user in MySQL

TorevokeprivilegesfromauserinMySQL,usetheREVOKEstatement;1.Specifytheprivilegetype(e.g.,SELECT,INSERT,ALLPRIVILEGES);2.Definethescopeusingdatabase_name.table_name(e.g.,mydb.usersormydb.*);3.Identifytheexact'username'@'host'account;4.OptionallyrevokeG

Aug 11, 2025 pm 06:34 PM

What is the character set and collation in MySQL?

Acharactersetdefineswhichcharacterscanbestored,whileacollationdetermineshowtheyarecomparedandsorted.1.Charactersetslikeutf8mb4supportfullUnicode,includingemojisandinternationalcharacters.2.Collationssuchasutf8mb4_0900_ai_ciproviderulesforcase-insensi

Aug 11, 2025 pm 05:22 PM

mysql character set