面试题与答案

SQL (Structured Query Language) is a domain-specific language used for managing and manipulating relational databases.

SQL is a language, whereas MySQL is a relational database management system that uses SQL as its query language.

A primary key is a unique identifier for a record in a table. It must contain unique values and cannot be NULL.

An index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space.

A foreign key is a column or a set of columns in a table that refers to the primary key of another table, establishing a link between the two tables.

SELECT COUNT(*) FROM table;

A view is a virtual table based on the result of a SELECT query, and it does not store the data itself.

Example:

CREATE VIEW view_name AS SELECT column1, column2 FROM table WHERE condition;

UPDATE table SET column1 = value1, column2 = value2 WHERE condition;

The ORDER BY clause is used to sort the result set of a query in ascending or descending order based on one or more columns.

Example:

SELECT column1, column2 FROM table ORDER BY column1 DESC;

SELECT * FROM table ORDER BY timestamp_column DESC LIMIT 1;

CHAR is a fixed-length string data type, while VARCHAR is a variable-length string data type. CHAR pads spaces to the maximum length, while VARCHAR only stores the actual data without padding.

The DISTINCT keyword is used to return unique values in the result set of a SELECT query by eliminating duplicate rows.

Example:

SELECT DISTINCT column FROM table;

The UPDATE statement is used to modify the existing records in a table by changing the values of one or more columns.

Example:

UPDATE table SET column1 = value1, column2 = value2 WHERE condition;

The LIKE operator is used to search for a specified pattern in a column. It is often used with wildcard characters (% and _).

Example:

SELECT * FROM table WHERE column LIKE 'pattern%';

SELECT * FROM table ORDER BY column DESC LIMIT N;

The SELECT statement is used to query the database and retrieve data from one or more tables.

Example:

SELECT column1, column2 FROM table WHERE condition;

Normalization is the process of organizing data in a database to reduce redundancy and dependency.

INNER JOIN returns rows when there is a match in both tables. LEFT JOIN returns all rows from the left table and matching rows from the right table.

Example:

SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id;

SELECT department, AVG(salary) FROM Employee GROUP BY department;

UNION combines and returns unique rows from multiple SELECT statements, while UNION ALL returns all rows including duplicates.

Example:

SELECT column1 FROM table1 UNION SELECT column1 FROM table2;

DELETE is used to remove rows from a table based on a condition. TRUNCATE removes all rows from a table without logging individual row deletions.

Example:

DELETE FROM table WHERE condition;

ACID (Atomicity, Consistency, Isolation, Durability) properties ensure the reliability of database transactions.

The GROUP BY clause is used in a SELECT statement to arrange identical data into groups, typically used with aggregate functions.

Example:

SELECT column, COUNT(*) FROM table GROUP BY column;

A stored procedure is a set of SQL statements that can be stored in the database and executed by calling the procedure.

SELECT column, COUNT(*) FROM table GROUP BY column HAVING COUNT(*) > 1;

The HAVING clause is used in combination with the GROUP BY clause to filter the results of aggregate functions based on specified conditions.

Example:

SELECT column, COUNT(*) FROM table GROUP BY column HAVING COUNT(*) > 1;

A primary key is used to uniquely identify each record in a table and cannot contain NULL values. A unique key enforces uniqueness but can contain NULL values.

UNION is used to combine the results of two or more SELECT statements, while JOIN is used to retrieve data from multiple tables based on a related column between them.

A trigger is a set of instructions that are automatically executed (or 'triggered') in response to certain events on a particular table or view.

A self-join is a regular join, but the table is joined with itself. It is useful when you want to combine rows with related data in the same table.

Example:

SELECT employee1.name, employee2.name FROM Employee employee1, Employee employee2 WHERE employee1.manager_id = employee2.employee_id;

A composite key consists of more than one column and is used to uniquely identify a record in a table when a single column is not sufficient.

A stored procedure does not necessarily return a value, while a function must return a value. Functions can be used in SQL statements, whereas procedures cannot.

The CASE statement is used to perform conditional logic within a SQL query, similar to an IF-THEN-ELSE statement in other programming languages.

Example:

SELECT column1, CASE WHEN condition THEN 'Result1' ELSE 'Result2' END AS NewColumn FROM table;

A subquery is a query embedded within another query. It can be used to retrieve data that will be used in the main query. Example: SELECT column FROM table WHERE column IN (SELECT column FROM another_table);

SELECT table_name, COUNT(*) FROM information_schema.tables GROUP BY table_name;

The HAVING clause is used to filter the results of aggregate functions in a SELECT statement based on specified conditions.

Example:

SELECT column, COUNT(*) FROM table GROUP BY column HAVING COUNT(*) > 1;

UNION combines and returns unique rows from multiple SELECT statements, while UNION ALL returns all rows including duplicates.

Example:

SELECT column1 FROM table1 UNION SELECT column1 FROM table2;

An INDEX is a performance optimization feature that allows faster retrieval of records from a table. It is created on one or more columns of a table.

Example:

CREATE INDEX index_name ON table(column);

A self-referencing foreign key is a foreign key in a table that references the primary key of the same table, creating a relationship within the same table.

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

SELECT * FROM Employee WHERE department_id IS NULL;

INNER JOIN returns rows when there is a match in both tables, while OUTER JOIN returns all rows from one table and the matching rows from the other table, filling in the missing values with NULLs.

Example:

SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id;

A natural join is a type of JOIN that automatically matches columns with the same name in the joined tables. It eliminates duplicate columns in the result set.

Example:

SELECT * FROM table1 NATURAL JOIN table2;

A CTE is a temporary result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. It improves the readability and maintainability of complex queries.

Example:

WITH cte_name AS (SELECT * FROM table) SELECT * FROM cte_name;

The GROUP_CONCAT() function is used to concatenate values from multiple rows into a single string, grouped by a specified column.

Example:

SELECT department, GROUP_CONCAT(employee_name) FROM Employee GROUP BY department;

The COALESCE() function is used to return the first non-null expression among its arguments. A common use case is handling null values in a query.

Example:

SELECT COALESCE(column1, 'DefaultValue') FROM table;

SESSION_USER returns the current username, and SYSTEM_USER returns the login name of the current user.

Example:

SELECT SESSION_USER, SYSTEM_USER;

JSON functions in SQL are used to process JSON data. JSON_VALUE is used to extract a scalar value, and JSON_QUERY is used to extract an object or an array.

Example:

SELECT JSON_VALUE(json_column, '$.key') AS Value FROM table;

The STRING_AGG() function is used to concatenate values from multiple rows into a single string with a specified separator.

Example:

SELECT department, STRING_AGG(employee_name, ', ') AS EmployeeList FROM Employee GROUP BY department;

SELECT * FROM Employee WHERE JOIN_DATE >= DATEADD(MONTH, -N, GETDATE());

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;