Скачать или смотреть How to Effectively Query Students Enrolled in Multiple Courses in SQL

Discover a concise method for retrieving student IDs and names enrolled in specific courses using SQL. Learn tips to enhance your query efficiency!
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This video is based on the question https://stackoverflow.com/q/66406397/ asked by the user 'emiclo007' ( https://stackoverflow.com/u/7822900/ ) and on the answer https://stackoverflow.com/a/66409287/ provided by the user 'Gordon Linoff' ( https://stackoverflow.com/u/1144035/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction: The Problem with SQL Queries

When working with SQL, it's common to encounter scenarios where you need to filter data based on multiple criteria. A typical challenge arises when trying to find students who are enrolled in a specific course and at least one other course. For example, consider a situation where we need to find the student IDs and names of all students enrolled in CS348 and either ECE264 or CS503. If your SQL query is only returning the students from CS348 and not checking the conditions for the other two courses, there's definitely something to fix.

Let’s delve into a structured solution to effectively accomplish this task.

Understanding the Database Structure

Imagine we have two tables: STUDENT and ENROLMENT. Below is a brief overview of their structures:

STUDENT Table

s_ids_name234Anglo Anebal235Abram Ace236Adelbert Antti237William Walker238Emila Wdyth239Judith Elba240Benjamin Bratt241Tawny KitaenENROLMENT Table

c_names_idCS110240CS110241CS348235CS348239CS348237CS250236CS250241ECE264236ECE264237ECE264238MA525236CS503238CS503239CS448240CS250240MA511240The Original Query

Your initial query might look something like this:

[[See Video to Reveal this Text or Code Snippet]]

The Solution: Refining the Query

Your approach is on the right track, but the use of = with the subquery is incorrect in capturing the desired result. Instead, you should utilize IN to effectively combine conditions. Here’s a refined version of the query:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Solution:

Table Aliases: Implementing table aliases (s for STUDENT and e for ENROLMENT) improves readability and simplifies your SQL statement.

Using IN: Instead of using =, switch to IN for subqueries when you need to check membership against multiple values.

Removing DISTINCT: It's likely unnecessary for both the outer and subqueries as a student cannot enroll in the same class more than once.

Alternative Approach: Aggregation

Another efficient way to achieve this is through the use of aggregation, which checks conditions on groups of records. Here’s how this improved query looks:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Aggregation Query:

Grouping Records: We group results by student ID and name.

Using HAVING: The HAVING clause allows us to filter groups based on calculated conditions. Here, it checks for enrollment in CS348 and either ECE264 or CS503.

Conclusion: Best Practices in SQL Queries

When constructing SQL queries that involve multiple conditions, it is crucial to correctly understand the logic you wish to apply. By utilizing the right clauses and structures, you can efficiently retrieve the data you need. Applying these techniques will not only improve your SQL skills but also enhance the performance of your database queries. Happy querying!