Creating a New Column in SQL to Check for Subject Existence: Yes or No

Learn how to add a new column in SQL that checks if a student is enrolled in a specific subject and returns `Yes` or `No` accordingly.
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This video is based on the question https://stackoverflow.com/q/65838809/ asked by the user 'Yingdong Zhai' ( https://stackoverflow.com/u/14117742/ ) and on the answer https://stackoverflow.com/a/65846681/ 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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Creating a New Column in SQL to Check for Subject Existence: Yes or No

In the world of data management, it’s common to encounter situations where you want to enhance your datasets with additional insights. For database administrators or data analysts working with student records, a common requirement is to determine whether students are enrolled in particular subjects. For instance, how do you add a column that states if a student is taking 'Math'? This guide will guide you through this precise scenario in SQL.

The Problem Statement

Let’s go through it step-by-step using the following example.

You have two tables in your database:

Table 1 - Student: This contains the names of students.

Table 2 - Student Subjects: This includes the subjects enrolled by each student.

Table Structures

Table 1: Student

StudentabcdTable 2: Subjects

StudentSubjectaEnglishaMathbEnglishbSciencebMathcSciencecEnglishdMathDesired Outcome

You want to create a new column in Table 1 called HasMath, which indicates whether each student is enrolled in Math.

Expected outcome for Table 1:

StudentHasMathaYesbYescNodYesThe Solution

To achieve this, you can use the EXISTS clause in SQL, which checks for the presence of rows in a subquery. Here’s how you can implement the desired functionality:

SQL Query Implementation

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

Breaking Down the Query

SELECT t1.*: This part selects all columns from Table 1 (Student).

CASE Statement: This is the core logic that decides whether to return 'Yes' or 'No'.

EXISTS Subquery: This checks if there is any entry in Table 2 where:

The student in Table 2 matches the student in Table 1.

The subject is 'Math'.

If such a record exists, 'Yes' is returned; otherwise, 'No' is returned.

Overall Output: The final output will produce the original list of students including the new HasMath column that indicates if they are enrolled in Math.

Conclusion

This simple SQL query effectively enhances your data by allowing you to easily track students’ enrollment in specific subjects like Math. You can manipulate this method to check for other subjects by replacing the subject name in the query. Understanding how to sift through data using subqueries and conditional statements is essential for any aspiring SQL developer or data analyst. Happy querying!

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