高级 SQL 面试问题

原文:https://www . geesforgeks . org/advanced-SQL-面试-问题/

不同的公司有不同的面试方法。有些人会专注于工作经验和知识；其他人可能专注于个性，而其他人则介于两者之间。

因此，从不同的面试问题和答案中发现和学习，有助于从不同的角度了解在不同的市场中什么可能是最重要的。因此，这里有一个基于区块链的电子学习平台的高级 SQL 面试问题列表。

• Que-1: Explain the meaning of ‘index’. Explanation: Indexes help retrieve information from the database faster and with higher efficiency. In other words, it’s a method that enhances performance and there are 3 types of indexes:

  • 聚集–使用键值对表格进行重新排序并搜索信息
  • 非集群–保持表格的顺序
  • 唯一–禁止字段具有重复值

  而且，一个表可以有多个非聚类索引，但只能有 1 个单聚类索引。

• Que-2: You forgot your root password, what do you do ? Explanation:

  1. 用“跳过-授权-表的命令启动数据库。
  2. 设置新密码后，以正常模式重新启动数据库并输入新密码。
• Que-3: Are NULL values equal to a zero ? Explanation: No, because a “zero” has a numerical manner and NULL represent the absence of a character. This happens when the character is unknown or unavailable. Additionally, NULL shouldn’t be confused with blank space because data record without any value assigned is not the same as a plain blank space, with no data records attached.
• Que-4: Data disk gets overloaded, what do you do ? Explanation: You should apply a soft link: these links create a location where you are able to store your .frm and .idb files. This will resolve the overload problem.

• Que-5: Explain what‘auto increment’ is? Explanation: This command allows you to generate a unique number when a new record is written to a table. When you want to the primary key field value to be generated automatically each time you insert a new record, this is when this function comes in handy.

  另外值得注意的是，该命令可以在各种平台上使用。对于 SQL Servers 来说，“自动递增”命令是“标识”。

• Que-6: What are the most basic MySQL architecture components ? Explanation: There are three main components:

  1. 查询优化器；
  2. 连接管理器；
  3. 可插拔引擎。

• Que-7: Using an existing table, make an empty one. Explanation:

  sql Select * into employeecopy from employee where 1=2

• Que-8: How would you check your current SQL version ? Explanation: You can get the most current SQL version by issuing this command:

  sql SELECT VERSION()

• Que-9: Get alternative odd records from the table. Explanation: This can be achieved using the command:

  sql Select employeeId from (Select rowno, employeetId from employee) where mod(rowno, 2)=1

• Que-10: 什么命令会从表中选择唯一的记录？ 解说: 的“泾渭分明的命令。这里有一个例子:

Select DISTINCT employeeID from Employee 

• Que-11: What are variables of SQL ? Explanation: In SQL, there are two different variables:

  • 局部–这些变量只能存在于单个函数中
  • 全局–与局部相反，这意味着它们可以通过整个程序定位。 Que-12: What is a ‘datawarehouse’ and what it does ? Explanation: A “datawarehouse” is a system used for data analysis and reporting. Basically, it’s a warehouse of data. Data in DWs can be stored from various areas and sources and thus makes them central repositories of integrated data that is ready for usage. Que-13: For what ‘recursive stored procedure’ is mainly used ? Explanation: A recursive stored procedure is a procedure that will make the code calls itself until specific boundary condition is reached. This is a productivity type of thing, that allows programmers to use the same code a number of times. Que-14: Retrieve the first 3 characters from a character string. Explanation:* There are a few ways to do this. Nevertheless, the command presented below can be treated as a more popular and easier one:

  sql Select SUBSTRING(EmployeeSurname, 1, 5) as employeesurname from employee

  • Que-15: 如何从两个表中检索常用记录？ 解释: 通过执行以下任务:

  sql Select employeeID from employee. INTERSECT Select EmployeeID from WorkShift

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