面试题与答案

SELECT MAX(salary) FROM Employee WHERE salary < (SELECT MAX(salary) FROM Employee);

SELECT MAX(salary) FROM Employee WHERE salary < (SELECT MAX(salary) FROM Employee WHERE salary < (SELECT MAX(salary) FROM Employee));

SELECT salary FROM Employee ORDER BY salary DESC LIMIT 1 OFFSET n-1;

SELECT department, AVG(salary) FROM Employee GROUP BY department HAVING COUNT(*) >= 5;

SQL injection is a security vulnerability where an attacker can insert malicious SQL code into a query, potentially gaining unauthorized access to a database.

A TRIGGER is a set of instructions that are automatically executed (or 'triggered') in response to specific events, such as INSERTs, UPDATEs, or DELETEs, on a particular table.

Example:

CREATE TRIGGER trigger_name BEFORE INSERT ON table FOR EACH ROW BEGIN -- trigger logic END;

The ROLLUP operator is used in conjunction with the GROUP BY clause to generate subtotals and grand totals for a set of columns in the result set.

Example:

SELECT department, city, SUM(salary) FROM Employee GROUP BY ROLLUP (department, city);

SELECT salary FROM Employee e1 WHERE n-1 = (SELECT COUNT(DISTINCT salary) FROM Employee e2 WHERE e1.salary <= e2.salary);

The LEAD() function is used to access the next row's data in the result set, while the LAG() function is used to access the previous row's data.

Example:

SELECT employee_name, salary, LEAD(salary) OVER (ORDER BY salary) AS NextSalary FROM Employee;

The CROSS APPLY operator is used to invoke a table-valued function for each row returned by the outer query. It is similar to the INNER JOIN clause.

Example:

SELECT * FROM table1 CROSS APPLY function(table1.column) AS alias;

SELECT department, employee_name, salary FROM (SELECT department, employee_name, salary, ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rnk FROM Employee) AS ranked WHERE rnk = 1;

The MERGE statement is used to perform insert, update, or delete operations on a target table based on the results of a join with a source table. It is also known as an 'upsert' operation.

Example:

MERGE INTO target_table USING source_table ON condition WHEN MATCHED THEN UPDATE SET column1 = value1 WHEN NOT MATCHED THEN INSERT (column1, column2) VALUES (value1, value2);

WINDOW functions perform a calculation across a set of table rows related to the current row. They are used with the OVER() clause to define a window or a subset of rows for the calculation.

Example:

SELECT employee_name, salary, AVG(salary) OVER (PARTITION BY department) AS AvgSalary FROM Employee;

SELECT department, employee_name, salary FROM (SELECT department, employee_name, salary, ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rnk FROM Employee) AS ranked WHERE rnk = 2;

XML functions in SQL are used to process XML data. XMLAGG is used to concatenate XML values, XMLPARSE is used to convert a string to XML, and XMLQUERY is used to extract data from XML.

The PIVOT operator is used to rotate rows into columns, while the UNPIVOT operator is used to rotate columns into rows.

Example:

SELECT * FROM (SELECT department, salary FROM Employee) AS SourceTable PIVOT (SUM(salary) FOR department IN ([Dept1], [Dept2], [Dept3])) AS PivotTable;

SELECT column, SUM(column) OVER (ORDER BY some_order_column) AS CumulativeSum FROM table;

ROW and RANGE are used in the OVER() clause of window functions to define the window or subset of rows for the calculation. ROW refers to a physical count, while RANGE considers values within a specified range.

SELECT department, employee_name, salary FROM (SELECT department, employee_name, salary, DENSE_RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS rnk FROM Employee) AS ranked WHERE rnk = 1;

The TRY...CATCH statement is used to handle errors in a SQL Server stored procedure. It allows for structured error handling and graceful error recovery.

Example:

BEGIN TRY -- SQL statements END TRY BEGIN CATCH -- Handling of errors END CATCH;