Creating Multiple New Rows Using Data from Other Rows in Database Querying

Understanding the Problem: Creating Multiple New Rows with Data from Other Rows

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In this article, we’ll delve into a common problem in database querying where you need to create multiple new rows using data from other rows. We’ll explore the concept of joins and subqueries, and how they can be used to achieve this goal.

Background: The Problem Statement

The question arises when you have two tables with overlapping columns that contain related data. You want to use data from one table to populate fields in another table, creating new rows as needed. For example, imagine a TeachingSets table containing information about teachers and classes, and a desired output where each class is associated with all the teachers who teach it.

The Initial Query: A Subquery-Based Approach

The initial query provided attempts to use a subquery within the SELECT statement to achieve this goal. However, this approach results in an error when dealing with multiple matching values from the subquery, as we’ll discuss later.

-- The original query that fails due to multiple matching values
SELECT 
    class
    teacher
    subject
FROM TeachingSets

A Deeper Look at Joins and Subqueries

To solve this problem, we need to understand the basics of joins and subqueries.

Joins

A join is used to combine data from two or more tables based on a related column. There are several types of joins:

INNER JOIN: Returns only the rows that have matching values in both tables.
LEFT JOIN (or LEFT OUTER JOIN): Returns all the rows from the left table and matching rows from the right table, if any.
RIGHT JOIN (or RIGHT OUTER JOIN): Similar to a LEFT JOIN but with the opposite order of tables.

Subqueries

A subquery is a query nested inside another query. It’s often used in conjunction with joins to retrieve specific data.

The Correct Approach: Joining and Using `CASE` Statements

To create multiple new rows using data from other rows, we can use a combination of joins and the CASE statement.

Creating a Temporary Table

First, let’s create a temporary table (@t) that represents our desired output structure:

-- Create a temporary table with columns for Class, Teacher, and Subject
declare @t table(Class int,Teacher varchar(10),Subject varchar(10));

Next, we’ll populate this table with the initial data using INSERT INTO statements:

-- Insert initial data into the temporary table
insert into @t values
 (1,'Adam','English')
,(2,'Bill','Maths')
,(3,'Carl','Maths')
,(4,'Dave','Science')
,(5,'Evan','Maths');

The Main Query

Now, we can write our main query that joins the original TeachingSets table with itself using an INNER JOIN on the Subject column. We’ll use the CASE statement to create new rows for each class:

-- Join the TeachingSets table with itself and use CASE statements to create new rows
select c.Class
      ,s.Teacher
      ,s.Subject
from @t as c
    join @t as s
        on c.Subject = s.Subject
order by c.Class
         ,s.Teacher;

This query will produce the desired output where each class is associated with all the teachers who teach it.

Conclusion

In this article, we explored a common problem in database querying where multiple new rows need to be created using data from other rows. We delved into the world of joins and subqueries, understanding their roles in combining data from related tables. By using CASE statements within an INNER JOIN, we were able to create a temporary table with the desired output structure and populate it with initial data. This approach can be applied to various database querying scenarios, making your code more efficient and effective.

Last modified on 2023-11-08