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 FROM que
1. Correlated:In a SQL database query, a correlated subquery is a subquery that uses values from the outer query in order to complete. Because a correlated subquery requires the outer query to be executed first, the correlated subquery must run once for every row in the outer query. It is...
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...
A subquery in SQL is like a query within a query. It helps you ask specific questions within a larger question. This nested query is used to filter or calculate data in a structured way. What is an SQL server? An SQL server is a software application that manages and stores databases. I...
We break the query into a subquery that finds sum of credits and sum of credit-grade-points, taking null grades into account The outer query divides the above to get the average, taking care of divide by 0. create view student grades(ID, GPA) as select ID, credit points / decode(...
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...
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 ...
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...
explainselectqpd.*fromquestion_practice_detail qpdwhereqpd.device_idin(selectup.agefromuser_profile up) 由于Mysql会对select进行优化,基本无法出现type为unique_subquery的场景,如果你能重现,请一定要告诉虚竹哥 index_subquery 与unique_subquery类似,但是IN语句中的查询字段为数据表中的非唯一索引字段。一样无法...