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Technical Articles

Database

Auditing Data Changes with SQL Triggers

SQL triggers are suitable for scenarios such as unified management of change logs at the database level, and not wanting to change application code but needing to record changes in key data. It is suitable for systems with low concurrency and will not cause performance bottlenecks, and is not suitable for high-frequency writing, complex business logic or frequent correlation and updates of multiple tables. When using it, you need to pay attention to transaction control, permission allocation, naming specifications and avoid loop triggering. The trigger automatically performs predefined actions by listening to INSERT, UPDATE or DELETE operations. It is often used for auditing. For example, the AFTERUPDATE trigger records the change information to the log table, and combines OLD and NEW keywords to obtain before and after update data to achieve efficient tracking.

Aug 12, 2025 pm 05:14 PM

sql trigger 數(shù)據(jù)審計

How to use the OFFSET and FETCH clauses for pagination in SQL?

OFFSETandFETCHclausesareusedforpaginationinSQL,requiringanORDERBYclause,whereOFFSETspecifiesrowstoskipandFETCHNEXTspecifiesrowstoretrieve;2.ForpagePwithNrowsperpage,useOFFSET(P-1)*NROWSFETCHNEXTNROWSONLY;3.Performancedecreaseswithlargeoffsets,sokeyse

Aug 12, 2025 pm 05:04 PM

How to implement a 'try-catch' block in SQL?

SQL itself does not have a try-catch structure like JavaScript or C#, but mainstream databases provide an error handling mechanism in their respective procedural extensions: 1. SQLServer uses BEGINTRY...BEGINCATCH, which can catch runtime errors and combine transactions to ensure data integrity; 2. PostgreSQL supports BEGIN...EXCEPTION in DO blocks or functions of PL/pgSQL, which can handle specific exceptions such as unique conflicts; 3. MySQL does not support try-catch, but error response behavior can be defined through DECLAREHANDLER in stored procedures; 4.Ora

Aug 12, 2025 pm 04:39 PM

How to check if a table exists before creating it in SQL

To check whether the table exists and then creates it, you should give priority to using the "CREATETABLEIFNOTEXISTS" syntax supported by the database, which is applicable to MySQL, PostgreSQL and SQLite, to avoid repeated creation errors; for SQLServer, you need to query the sys.tables system view to determine whether the table exists before creating it; while IFNOTEXISTS is not supported in Oracle, and you must query USER_TABLES or use an exception handling mechanism to capture the existing errors in the table (such as ORA-00955). This method combines metadata query or exception capture to ensure compatibility and security across database platforms.

Aug 12, 2025 pm 04:16 PM

How do you comment your SQL code?

IcommentSQLcodetomakeitclear,maintainable,andunderstandable,especiallyinteamenvironmentsorwhenrevisitingquerieslater.1.Useinlinecommentswith--forshortexplanations,suchasclarifyingconditionsorworkarounds,placingthemontheirownlineorattheendofalineifitd

Aug 12, 2025 pm 04:05 PM

How to use the ON DUPLICATE KEY UPDATE statement in MySQL

ONDUPLICATEKEYUPDATE is used in MySQL to handle unique key conflicts. 1. When there is a primary key or unique constraint conflict in the insert row, an update is performed instead of an error is reported; 2. Use VALUES (column) to refer to the insert value, and can update specific fields in combination with expressions; 3. Applicable to counters, upsert operations and batch data import; 4. Only trigger unique key conflicts, and updates each row at most once; 5. Different from INSERTIGNORE (ignore) and REPLACE (delete and insert), it is more efficient and safe; 6. It is necessary to ensure that the table has a correct unique index, and pay attention to the self-increment field behavior; this mechanism reduces database round trips and improves performance.

Aug 12, 2025 pm 04:04 PM

How to use aggregate functions in MySQL?

The aggregation function in MySQL is used to calculate the data and return a single value, often used in conjunction with the GROUPBY clause to summarize the data. 1. The COUNT() function can count row counts. COUNT(*) contains NULL values. COUNT(column name) only counts non-NULL values; 2. SUM() and AVG() respectively calculate the sum and average of the numeric columns, and can control the decimal places in combination with the ROUND() function; 3. MAX() and MIN() return the maximum and minimum values, which are suitable for numerical, date and other types; 4. Use GROUPBY to group by specified columns, and apply aggregate functions to each group to implement classification statistics; 5. Use the HAVING clause to filter the results after grouping, and WHERE is used for row filtering before grouping

Aug 12, 2025 pm 04:01 PM

mysql aggregate function

How to use the IN operator in MySQL?

TheINoperatorinMySQLchecksifavaluematchesanyinaspecifiedlist,simplifyingmultipleORconditions;itworkswithliterals,strings,dates,andsubqueries,improvesqueryreadability,performswellonindexedcolumns,supportsNOTIN(withcautionforNULLs),andcanbecombinedwith

Aug 12, 2025 pm 03:46 PM

mysql IN操作符

What are the comparison operators available in SQL?

The comparison operator in SQL is used to compare values in a query. It mainly filters data in the WHERE clause and returns a boolean result (true, false or unknown). 1.= means equal to, such as SELECTFROMemployeesWHEREage=30; 2. or != means not equal to, is standard SQL, != is supported by most databases; 3.> means greater than, such as SELECTFROMproductsWHEREprice>100; 4.=85; 6.

Aug 12, 2025 pm 03:27 PM

How to get the last inserted ID in SQL

Getting the last inserted ID depends on the database system. MySQL uses LAST_INSERT_ID(), SQLServer recommends SCOPE_IDENTITY(), PostgreSQL and newer SQLite versions support RETURNING clauses, SQLite also uses last_insert_rowid(), and the ID should be obtained immediately after insertion to ensure correctness.

Aug 12, 2025 pm 03:01 PM

sql

Designing MySQL Databases for Billing and Subscription Services

When designing MySQL databases for billing and subscription services, the core goal is to ensure data accuracy, scalability, and query efficiency. 1. Use the intermediate table user_subscriptions to manage many-to-many relationships between users and subscription plans, and support history; 2. Bills record each deduction information, and indexes are established according to user_id and due_date for easy query; 3. Payment records separate table payments, supporting multiple payment methods and refund processing; 4. Automatically update subscription status through timed tasks, generate bills and trigger notifications; 5. Reasonably design index and table structure to improve performance and maintenance. Good database design helps the system stay stable and efficient when user growth and function expansion

Aug 12, 2025 pm 03:00 PM

How to use MySQL partitioning to manage large tables?

MySQL partitions can improve the performance and management of large tables. The key is to select the appropriate type and match the query mode: 1. Select RANGE (by time range), LIST (discrete value grouping), HASH (even distribution) or KEY (support non-integer columns) partitions according to the data characteristics; 2. Design partitions need to be around query mode to ensure that WHERE conditions include partition columns to achieve partition cropping; 3. Verify the cropping effect through EXPLAINPARTITIONS to avoid unnecessary sub-partitions; 4. Regularly manage the life cycle, such as adding new partitions in advance, quickly deleting old partitions, reorganizing future partitions, and automating with events or scripts; 5. Comply with restrictions and best practices, such as the unique key must contain partition columns, and InnoDB supports partitions

Aug 12, 2025 pm 02:57 PM

How to select random rows from a table in MySQL

To select random rows from MySQL table, the most common method is to use ORDERBYRAND(); the specific steps are: 1. Use SELECTFROMtable_nameORDERBYRAND()LIMITN to obtain N random records; 2. Note that this method has poor performance on large tables, because the full table needs to be scanned and sorted; 3. For large tables, you can use random offset SELECTFROMtable_nameLIMIT1OFFSETFLOOR(RAND()(SELECTCOUNT()FROMtable_name)); 4. If there is an ID gap, you can use SELECTFROMtable_nameWHEREid

Aug 12, 2025 pm 02:52 PM

How to handle time-series data effectively in MySQL?

To effectively process time series data in MySQL, schema design, indexing and query need to be optimized. The specific methods are: 1. Use DATETIME(6) or TIMESTAMP(6) to store timestamps to support microsecond accuracy and facilitate date calculation, avoid using integer timestamps; 2. Create composite indexes (such as sensor_id, timestamp) for time range and entity queries, and partition by time (such as one partition per month) to improve query efficiency and deletion speed; 3. Use batch insertion and transactions to reduce overhead when writing, and only select necessary fields when reading, use indexes and pre-aggregate data (such as maintaining hourly summary tables) to accelerate analysis; 4. Use efficient data retention through partition deletion or batch deletion to avoid large-scale DE

Aug 12, 2025 pm 02:49 PM