Oracle deletes duplicate data and keeps the first record

The SQL learning curve is steep, but it can be mastered through practice and understanding the core concepts. 1. Basic operations include SELECT, INSERT, UPDATE, DELETE. 2. Query execution is divided into three steps: analysis, optimization and execution. 3. Basic usage is such as querying employee information, and advanced usage is such as using JOIN connection table. 4. Common errors include not using alias and SQL injection, and parameterized query is required to prevent it. 5. Performance optimization is achieved by selecting necessary columns and maintaining code readability.

SQL commands are divided into five categories in MySQL: DQL, DDL, DML, DCL and TCL, and are used to define, operate and control database data. MySQL processes SQL commands through lexical analysis, syntax analysis, optimization and execution, and uses index and query optimizers to improve performance. Examples of usage include SELECT for data queries and JOIN for multi-table operations. Common errors include syntax, logic, and performance issues, and optimization strategies include using indexes, optimizing queries, and choosing the right storage engine.

Advanced query skills in SQL include subqueries, window functions, CTEs and complex JOINs, which can handle complex data analysis requirements. 1) Subquery is used to find the employees with the highest salary in each department. 2) Window functions and CTE are used to analyze employee salary growth trends. 3) Performance optimization strategies include index optimization, query rewriting and using partition tables.

MySQL is an open source relational database management system that provides standard SQL functions and extensions. 1) MySQL supports standard SQL operations such as CREATE, INSERT, UPDATE, DELETE, and extends the LIMIT clause. 2) It uses storage engines such as InnoDB and MyISAM, which are suitable for different scenarios. 3) Users can efficiently use MySQL through advanced functions such as creating tables, inserting data, and using stored procedures.

SQLmakesdatamanagementaccessibletoallbyprovidingasimpleyetpowerfultoolsetforqueryingandmanagingdatabases.1)Itworkswithrelationaldatabases,allowinguserstospecifywhattheywanttodowiththedata.2)SQL'sstrengthliesinfiltering,sorting,andjoiningdataacrosstab

SQL indexes can significantly improve query performance through clever design. 1. Select the appropriate index type, such as B-tree, hash or full text index. 2. Use composite index to optimize multi-field query. 3. Avoid over-index to reduce data maintenance overhead. 4. Maintain indexes regularly, including rebuilding and removing unnecessary indexes.

To delete a constraint in SQL, perform the following steps: Identify the constraint name to be deleted; use the ALTER TABLE statement: ALTER TABLE table name DROP CONSTRAINT constraint name; confirm deletion.

A SQL trigger is a database object that automatically performs specific actions when a specific event is executed on a specified table. To set up SQL triggers, you can use the CREATE TRIGGER statement, which includes the trigger name, table name, event type, and trigger code. The trigger code is defined using the AS keyword and contains SQL or PL/SQL statements or blocks. By specifying trigger conditions, you can use the WHERE clause to limit the execution scope of a trigger. Trigger operations can be performed in the trigger code using the INSERT INTO, UPDATE, or DELETE statement. NEW and OLD keywords can be used to reference the affected keyword in the trigger code.