Creating a New Column Based on Conditions from Other Tables in SQL

In this article, we will explore how to add a new column based on the conditions from other tables in SQL. This is a common requirement in data analysis and reporting, where you need to create a new column that represents a calculated value or a derived attribute from one or more existing columns.

Understanding the Problem Statement

The problem statement provided by the user asks how to add a new column named “entry_page” to table B, where the values of the new column “entry_page” should be “page_location” with the earliest datetime value from table A by session ID. The user has tried to get the first value of “PAGE_LOCATION” by “EXT_GA_SESSION_ID” but encountered an error.

Background Information

To approach this problem, we need to understand some basic concepts in SQL, including:

• GROUP BY: This clause is used to group rows that have the same values for one or more columns.
• MIN(): This function returns the minimum value of a set of numbers.
• JOIN: This clause is used to combine rows from two or more tables based on a related column between them.

Step 1: Get the Oldest Event Time for Each Session ID

To get the oldest event time for each session ID, we can use the GROUP BY and MIN() functions. The query would look like this:

SELECT EXT_GA_SESSION_ID, min(EVENT_AT) as min_event_at
FROM A
GROUP BY EXT_GA_SESSION_ID;

This query groups the rows by session ID and returns the minimum event time for each group.

Step 2: Join with Table A to Get Related Page Location

Once we have the oldest event time for each session ID, we can join this dataset with table A to get the related page location. The query would look like this:

SELECT A.EXT_GA_SESSION_ID, A.PAGE_LOCATION
FROM A
JOIN (
  SELECT EXT_GA_SESSION_ID, min(EVENT_AT) as min_event_at
  FROM A
  GROUP BY EXT_GA_SESSION_ID
) AS S on S.EXT_GA_SESSION_ID = A.EXT_GA_SESSION_ID and S.min_event_at = A.EVENT_AT;

This query joins table A with the previous result on the session ID and event time, and returns the page location for each group.

Step 3: Join with Table B to Get the Expected Data

Finally, we join this query with table B to get the expected data. The query would look like this:

SELECT DISTINCT B.EXT_GA_SESSION_ID, T.PAGE_LOCATION as "entry_page"
FROM B
JOIN (
  SELECT A.EXT_GA_SESSION_ID, A.PAGE_LOCATION
  FROM A
  JOIN (
    SELECT EXT_GA_SESSION_ID, min(EVENT_AT) as min_event_at
    FROM A
    GROUP BY EXT_GA_SESSION_ID
  ) AS S on S.EXT_GA_SESSION_ID = A.EXT_GA_SESSION_ID and S.min_event_at = A.EVENT_AT
) as T on T.EXT_GA_SESSION_ID = B.EXT_GA_SESSION_ID;

This query joins table B with the previous result on the session ID, and returns the expected data.

Conclusion

In this article, we explored how to add a new column based on conditions from other tables in SQL. We used GROUP BY, MIN(), and JOIN clauses to achieve this goal. The final query joined the oldest event time for each session ID with table A, and then joined the result with table B to get the expected data.

Code Explanation

Here is the complete code that we have discussed:

SELECT 
  B.EXT_GA_SESSION_ID, 
  T.PAGE_LOCATION as "entry_page"
FROM B
JOIN (
  SELECT A.EXT_GA_SESSION_ID, A.PAGE_LOCATION
  FROM A
  JOIN (
    SELECT EXT_GA_SESSION_ID, min(EVENT_AT) as min_event_at
    FROM A
    GROUP BY EXT_GA_SESSION_ID
  ) AS S on S.EXT_GA_SESSION_ID = A.EXT_GA_SESSION_ID and S.min_event_at = A.EVENT_AT
) as T on T.EXT_GA_SESSION_ID = B.EXT_GA_SESSION_ID;

This query is the final solution to the problem presented in the question.

Example Use Case

Suppose we have two tables, table A and table B:

Table A:

Table B:

The query will return the expected data:

This is the final result after joining table A with table B.

Step-by-Step Process

Here is a step-by-step process to solve this problem:

1. Get the oldest event time for each session ID using GROUP BY and MIN().
2. Join with table A to get related page location.
3. Join with table B to get the expected data.

The final query is as follows:

SELECT 
  B.EXT_GA_SESSION_ID, 
  T.PAGE_LOCATION as "entry_page"
FROM B
JOIN (
  SELECT A.EXT_GA_SESSION_ID, A.PAGE_LOCATION
  FROM A
  JOIN (
    SELECT EXT_GA_SESSION_ID, min(EVENT_AT) as min_event_at
    FROM A
    GROUP BY EXT_GA_SESSION_ID
  ) AS S on S.EXT_GA_SESSION_ID = A.EXT_GA_SESSION_ID and S.min_event_at = A.EVENT_AT
) as T on T.EXT_GA_SESSION_ID = B.EXT_GA_SESSION_ID;

This is the complete solution to the problem presented in the question.

Conclusion

In this article, we explored how to add a new column based on conditions from other tables in SQL. We used GROUP BY, MIN(), and JOIN clauses to achieve this goal. The final query joined the oldest event time for each session ID with table A, and then joined the result with table B to get the expected data.

The final answer is the complete code that we have discussed:

SELECT 
  B.EXT_GA_SESSION_ID, 
  T.PAGE_LOCATION as "entry_page"
FROM B
JOIN (
  SELECT A.EXT_GA_SESSION_ID, A.PAGE_LOCATION
  FROM A
  JOIN (
    SELECT EXT_GA_SESSION_ID, min(EVENT_AT) as min_event_at
    FROM A
    GROUP BY EXT_GA_SESSION_ID
  ) AS S on S.EXT_GA_SESSION_ID = A.EXT_GA_SESSION_ID and S.min_event_at = A.EVENT_AT
) as T on T.EXT_GA_SESSION_ID = B.EXT_GA_SESSION_ID;

Last modified on 2023-07-07