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Technical Articles

Database

How to find duplicate values in a MySQL table?

Tofindduplicatesinasinglecolumnlikeemail,useGROUPBYwithHAVINGCOUNT()>1toshowvaluesappearingmorethanonce.2.Forduplicatesacrossmultiplecolumnssuchasfirst_nameandlast_name,applyGROUPBYonthecombinedcolumnswithHAVINGCOUNT()>1.3.Todisplayallduplicate

Aug 14, 2025 pm 02:02 PM

What are the different storage engines in MySQL?

InnoDB is MySQL default storage engine, suitable for applications that require transactions, high concurrency and data integrity, and supports row-level locks, foreign keys and crash recovery; MyISAM is suitable for read-intensive non-transactional scenarios but is no longer recommended; Memory engine is used for temporary data storage in memory, fast but not durable; CSV is used for data exchange, Archive is suitable for appended log data; Blackhole is used for testing and replication, NDB is used for high availability cluster environments, Federated is used for accessing remote tables, and Merge is the deprecated MyISAM merge engine. The selection engine should be based on specific needs. InnoDB is suitable for most modern applications.

Aug 14, 2025 pm 01:47 PM

How to troubleshoot ORA-01034: ORACLE not available

First, confirm whether the Oracle instance is running. By checking the PMON process (Linux/Unix) or service status (Windows), if it is not running, the instance needs to be started; 2. Make sure that the ORACLE_SID and ORACLE_HOME environment variables are set correctly to avoid false positives caused by dependent variable errors; 3. Use sqlplus/assysdba to log in and execute the startup command to start the database, and if it fails, record specific errors; 4. Check the alert log file (located in $ORACLE_BASE/diag/rdbms///trace/alert_.log) to obtain detailed startup error information, such as missing control files and inaccessible data files

Aug 14, 2025 pm 01:12 PM

How to combine results from two tables using UNION in SQL?

To use UNION to merge the results of two tables, it is necessary to ensure that the number of columns in the SELECT statement is the same, the data types of the corresponding columns are compatible, and the columns correspond in order rather than names; 1. Use UNION to automatically remove duplicate rows. If all rows (including duplicates) are required, UNION should be used; 2. The column names of the result sets come from the first SELECT statement; 3. More than two query results can be merged, and the final result can only be sorted with ORDERBY at the end; 4. Common errors include mismatch in the number of columns, incompatible data types, and mistakenly thinking that the columns correspond to the name; therefore, UNION or UNIONALL should be selected according to whether they need to be deduplicated, and ensure that each query structure is consistent, and finally a consolidated unique result set is returned.

Aug 14, 2025 pm 12:28 PM

How to use a recursive CTE to query hierarchical data in SQL?

When querying hierarchical data using recursive CTE, first define the anchor member (such as the top-level node without a superior); 2. Then define the recursive member, and query layer by layer through self-reference connection; 3. Recursive automatically terminates when no new row returns; 4. UNIONALL must be used to avoid derecursive; 5. It is necessary to prevent circular references from causing infinite recursion, and some databases support CYCLE detection; 6. Path arrays can be constructed to display the complete link from the root to the current node; 7. PostgreSQL, SQLServer, SQLite, MySQL8.0 and Oracle all support recursive CTE; 8. Suitable for medium-depth tree structures, such as organizational structures or classification trees, for data with large depths, nested sets, etc.

Aug 14, 2025 pm 12:23 PM

sql query Recursive CTE

How to query for points within a certain distance in MySQL

To query points within a specific distance in MySQL, use the ST_Distance_Sphere() function to calculate the distance of geographic coordinates (latitude and longitude). 1. Create a table containing POINT type columns and specify SRID4326; 2. Use the ST_Distance_Sphere() function to filter points in the specified radius, with distance units in meters; 3. To improve performance, add spatial indexes and use bounding box filtering in combination with MBRWithin() to trigger the index. For example: SELECTid, name, ST_Distance_Sphere(coord,POINT(40.7128,-74.0060))ASdistanceFROMl

Aug 14, 2025 pm 12:22 PM

How to iterate through a result set in SQL

SQLdoesnotsupportdirectiterationlikeprocedurallanguages,butyoucanprocessrowsusingalternativemethods:1.Useset-basedoperationsforefficient,scalableupdates(e.g.,UPDATEemployeesSETsalary=salary*1.1WHEREdepartment='Engineering');2.Usecursorsforrow-by-rowp

Aug 14, 2025 pm 12:19 PM

sql 結(jié)果集遍歷

How to use subqueries in MySQL

A subquery is a SELECT statement nested in another SQL statement and can be used in WHERE, FROM, and SELECT clauses. 1. Using subqueries in WHERE can filter data based on another query result, such as finding employees with salary above average; they can also use IN, EXISTS and other operators to combine subquery filtering. 2. Use subqueries in the FROM clause to create temporary derived tables and must be given an alias for them. 3. The subquery in the SELECT clause must be a scalar subquery, which only returns a single row and single column value, but may affect performance. 4. Related subqueries rely on external queries and are executed once per line, which is less efficient and should be replaced by JOIN as much as possible. It is recommended to always wrap subqueries in brackets to avoid NOTIN and

Aug 14, 2025 am 11:54 AM

How to find the length of a comma-separated list in a column in SQL?

To calculate the number of elements in comma-separated columns in SQL, you can count the number of commas and add 1; specifically: 1. Use LENGTH or equivalent function to calculate the length of the original string; 2. Use REPLACE to remove the comma and calculate the length; 3. Subtract the two lengths to get the number of commas; 4. Add 1 to get the total number of elements; 5. Use CASE to process NULL or empty strings to return 0; this method is suitable for MySQL, PostgreSQL, SQLServer and SQLite. If the database supports split functions such as STRING_SPLIT or string_to_array, it is recommended to use split count to obtain more accurate results; the final query should include processing of the boundary situation to ensure the correct results.

Aug 14, 2025 am 11:53 AM

sql string length