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Technical Articles

Database

How do you concatenate strings in a SQL query?

The method of connecting strings using SQL varies from database system. The most commonly used are the CONCAT function, the double vertical line operator (||) and the plus operator (); 1. Use the CONCAT() function: suitable for MySQL, PostgreSQL9.1, SQLServer2012, SQLite and Oracle12c, the syntax is SELECTCONCAT(first_name,'',last_name)ASfull_nameFROMusers, and in most cases NULL values will be regarded as empty strings; 2. Use the || operator: PostgreSQL, Oracle and SQLite support, the syntax is SE

Aug 11, 2025 pm 10:56 PM

sql String concatenation

How to update statistics for a table in SQL?

InSQLServer,useUPDATESTATISTICStable_nameorEXECsp_updatestatsforalltables;2.InPostgreSQL,useANALYZEtable_nametoupdatestatisticsmanually;3.InMySQL/MariaDB,useANALYZETABLEtable_nametorefreshindexandtablestatistics;4.InOracle,useEXECDBMS_STATS.GATHER_TA

Aug 11, 2025 pm 10:48 PM

sql 統(tǒng)計(jì)信息

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

What Are the Security Considerations When Installing Redis on a Linux System?

To safely install Redis on Linux systems, the following measures should be taken: 1) Bind Redis to the local host (127.0.0.1); 2) Set a strong password; 3) Run Redis as a non-root user; 4) Use VPN or SSH tunnel to protect network connections; 5) Configure the firewall to allow only trusted source connections; 6) Use SSL/TLS to encrypt data transmission; 7) Disable protection mode and update Redis regularly; 8) Monitor Redis logs and set alerts. These steps help ensure Redis's security and reliability on Linux, protecting data and system integrity.

Aug 11, 2025 pm 10:37 PM

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 rebuild or reorganize an index in SQL?

To reconstruct or reorganize the SQLServer index to reduce fragmentation and improve query performance, the method should be selected according to the degree of fragmentation: 1. No operation is required when fragmentation is less than 10%. 2. When fragmentation is between 10% and 30%, use ALTERINDEXindex_nameONtable_nameREORGANIZE for reorganization; 3. When fragmentation is more than 30%, use ALTERINDEXindex_nameONtable_nameREBUILD for reconstruction, which can be combined with WITH options such as FILLFACTOR, SORT_IN_TEMPDB and ONLINE to optimize the process; 4. You can query sys.dm_db_inde

Aug 11, 2025 pm 09:23 PM

How to use the LAG() and LEAD() functions?

LAG() and LEAD() are window functions in SQL that compare the current row with the previous row or the next row of data. 1. LAG(column,offset,default) is used to obtain the data of the previous row, offset specifies the number of traceback rows, default 1 line, default specifies the return value when there is no previous row; 2. LEAD(column,offset,default) is used to obtain the data of the next row, and the parameter meaning is the same as LAG(); 3. Use ORDERBY to define the sorting to ensure that the function can correctly identify the previous and next rows; 4. Use PARTITIONBY to compare the data in the group to avoid mixing data from different groups; 5. Set the default value to avoid NULL interference meter

Aug 11, 2025 pm 08:14 PM

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

What is the difference between SPOP and SRANDMEMBER?

The main difference between SPOP and SRANDMEMBER is whether the original collection is modified. 1. SPOP is used to remove and return one or more random elements, which will change the original collection, which is suitable for scenes where elements need to be randomly selected and deleted; 2. SRANDMEMBER only returns one or more random elements without removing them, which is a read-only operation, which is suitable for cases where random values need to be sampled or viewed without modifying the collection; 3. Both support multiple elements through the count parameter starting from Redis3.2 and 6.2, but only SPOP will modify the collection content.

Aug 11, 2025 pm 07:33 PM

redis gather