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Technical Articles

Database

How do you use the HAVING clause to filter groups in SQL?

Use the HAVING clause to filter grouped data after GROUPBY, especially when the conditions involve aggregation functions such as COUNT(), SUM(), AVG(); 2. Unlike the WHERE clause, WHERE is used to filter a single row before grouping, while HAVING is used to filter groups based on the aggregation result after grouping; 3. HAVING must be placed after GROUPBY, and column alias in SELECT cannot be used, and the aggregate expression must be repeated; 4. WHERE and HAVING can be used at the same time, the former filter the original row and the latter filter the grouping results; 5. Common application scenarios include finding customers whose order number exceeds the specified value, departments whose average salary is higher than a certain value, or excluding groups with NULL values; 6. Total

Aug 04, 2025 pm 12:12 PM

sql HAVING子句

What is the difference between LEFT JOIN and INNER JOIN in MySQL?

INNERJOINreturnsonlyrowswithmatchesinbothtables,excludingunmatchedrows;2.LEFTJOINreturnsallrowsfromthelefttableandmatchingrowsfromtherighttable,withNULLsforunmatchedrighttablecolumns;useINNERJOINtogetonlyrelateddataandLEFTJOINtoincludealllefttablerow

Aug 04, 2025 am 11:42 AM

mysql join

how to use window functions in mysql

MySQL supports window functions since version 8.0, which greatly improves the processing efficiency of complex queries. 1. Window functions can calculate a set of rows without merging and paralleling, such as ranking, cumulative statistics, etc.; 2. Common functions include ROW_NUMBER(), RANK(), DENSE_RANK(), SUM()OVER(), AVG()OVER(), LAG(), LEAD(); 3. When using it, you must match the OVER() clause, and PARTITIONBY and ORDERBY can be specified; 4. Typical application scenarios include ranking query (such as TopNperGroup), cumulative statistics (such as RunningTotal), and context comparison (such as month-on-month analysis

Aug 04, 2025 am 11:32 AM

What is an execution plan in Oracle and how to read it?

AnexecutionplaninOracleisaroadmapshowinghowthedatabaseexecutesaSQLquery,revealingoperationsliketablescans,indexusage,joins,andsorts;itmattersbecauseithelpsidentifyinefficientindexuse,unnecessaryfulltablescans,andcostlyoperations,enablingSQLperformanc

Aug 04, 2025 am 11:26 AM

Optimizing SQL ORDER BY for Query Performance

To optimize the performance of ORDERBY in SQL, you must first understand its execution mechanism and make rational use of index and query structure. When the sorting field has no index, the database will trigger "filesort", consuming a lot of resources; therefore, direct sorting of large tables should be avoided and the amount of sorted data should be reduced through WHERE conditions. Secondly, establishing a matching index for sorting fields can greatly speed up queries, such as creating reverse order indexes in MySQL 8.0 to improve efficiency. In addition, deep paging (such as LIMIT1000, 10) should be used instead with index-based cursor paging (such as WHEREid>12345) to skip invalid scans. Finally, combining caching, asynchronous aggregation and other means can also further optimize the sorting performance in large data set scenarios.

Aug 04, 2025 am 11:19 AM

sql order by

What is the OFFSET-FETCH clause in SQL for pagination?

OFFSET-FETCHisusedforpaginationinSQL,requiringORDERBYandsupportedinSQLServer,Oracle12c ,andPostgreSQL(notinMySQL).1.OFFSETNROWSskipsNrows.2.FETCHNEXTMROWSONLYretrievesthenextMrows.Itenablesprecisepagecontrolbutslowswithlargeoffsets;keysetpaginationis

Aug 04, 2025 am 11:14 AM

A Complete Guide to SQL: From Basic Syntax to Advanced Query Optimization

SQLmasterybeginswithunderstandingcorecommandsandprogressestoadvancedoptimization.1.Startwithbasicsyntax:useSELECTtoretrievedata,WHEREtofilterrows,INSERTtoaddrecords,UPDATEtomodifydata,DELETEtoremoverecords,ORDERBYtosortresults,andLIMIT/OFFSETforpagin

Aug 04, 2025 am 11:12 AM

sql Query optimization

How do you use a subquery within a SELECT statement in SQL?

AsubqueryintheSELECTclausereturnsasinglevaluetocomputeafieldforeachrow,suchasretrievingadepartmentnameforeachemployeebylookinguprelateddatainanothertable.2.AsubqueryintheWHEREclausefiltersresultsbasedonaconditionderivedfromanotherquery,suchasfindinge

Aug 04, 2025 am 11:08 AM

Implementing SQL Snapshots for Point-in-Time Recovery

SQL snapshots are an efficient means to achieve point-in-time recovery, especially for rapid rollback to a specific point-in-time when misoperation or data is damaged. 1. SQL snapshots are read-only and static database views that record the status of a certain point in time, record data page changes through the sparse file mechanism, which is fast creation speed and low resource utilization. 2. The advantages of using snapshot recovery include fast speed, few resources, no impact on the primary database, and suitable for misoperation of recovery, but cannot replace complete backups. 3. The syntax for creating snapshots is simple, and it is implemented through the RESTORE command when restoring. 4. Practical suggestions include creating before key operations, creating automatically regularly, paying attention to snapshot file growth, and multiple snapshots support multi-point-in-time recovery. 5. Notes include affecting the performance of write operations, snapshot files should be stored and restored separately

Aug 04, 2025 am 11:07 AM