四、多表查询:子查询(Subquery) 1. 什么是子查询? 子查询是嵌套在另一个查询中的查询,用于帮助主查询筛选数据。常见用法是通过子查询获取特定条件的数据,再用于主查询中。 --- 2. 子查询示例 - 嵌套查询 比如,我们想查询“北京大学”的用户设备相关数据,可以这样写: SELECT device_id, question_id, result ...
SQL Practice #2 – Combining Subquery & Aggregate Function Write a query that returns customer id and name and the number of calls related to that customer. Return only customers that have more than the average number of calls of all customers. 1 2 3 4 5 6 7 8 9 10 11 12 SELECT...
The use of comparison operators like <,> or = can be done in a subquery. These kinds ofSQL Interview Questionsare a bit difficult to understand and answer accordingly. There are a lot ofSQL Interview Questionsapart from this, which the candidates must practice qualifying for the interview. Fo...
18. What is a Subquery? What are its types? 19. What is a Primary Key? 20. What are Constraints in SQL? 21. What are Tables and Fields? 22. What is the difference between SQL and MySQL? 23. What is SQL? 24. What is RDBMS? How is it different from DBMS? 25. What is DBMS?
Subqueries: Know what a subquery is and how to apply it. Nested Queries: A discussion of exactly what nested queries in SQL are all about. Correlated Subqueries: Learn how correlated subqueries work and when to use them. Common Use Cases: Think of some real-world examples of using subqueries...
The types of subquery available and how to use them Convert rows-to-columns with PIVOT The basics of using window functions to calculate running totals Register athttps://devgym.oracle.com/pls/apex/dg/class/databases-for-developers-next-level.html ...
SQL允许子查询出现在返回单个值的表达式能够出现的任何地方,只要该子查询只返回包含单个属性的单个元组;这样的子查询称为标量子查询(scalar subquery) 举例:列出所有的系以及他们拥有的教师数 ··· select dept_name, (select count() from instructor where department.dept_name = instructor.dept_name ) as num...
Using EXISTS with a Correlated Subquery We have already used the EXISTS operator to check the existence of a result of a subquery. EXISTS operator can be used in correlated subqueries also. Using EXISTS the following query display the employee_id, manager_id, first_name and last_name of those...
To get the middle, first combined the results from question 1 and 2. I then use a subquery to exclude these students from my result. What remained was the middle. The combined results are colored codedgreen. Thesubqueryto exclude them isblue. ...
SQL CTE vs. Subquery: This Debate Ain’t Over Yet!- Apr 16, 2025. CTEs and subqueries are often seen as the same thing, written slightly differently. That’s far from the truth. Let’s examine their differences, find a winner, and end this debate about which one is better. ...