Скачать или смотреть How to Use CASE in PostgreSQL to Compare Values Between Columns

Learn how to effectively use the `CASE` statement in PostgreSQL to compare values between columns. This guide will walk you through a common scenario where you may need to set a column value to null based on comparisons with another column.
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Using CASE to Compare Values in PostgreSQL

In the world of databases, you often find yourself needing to manipulate and compare data across different columns. One common scenario arises when you want to assess the relationship between two columns and adjust one of them based on that relationship. In this guide, we will tackle a specific use case with PostgreSQL involving a comparison between two columns, Col A and Col B.

The Problem

Imagine you have a dataset in PostgreSQL with two columns, Col A and Col B. Your objective is to modify the values in Col A based on their equality with Col B. Specifically:

If Col A and Col B have the same value, you want to set Col A to null.

If Col A and Col B do not have the same value, then Col A should retain its original value.

You may have attempted to solve this using a CASE statement, but perhaps it didn't yield the expected results. Let's dissect the original attempt and then refine it.

Understanding the Original Attempt

Below is a simplified version of what you may have used:

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

Issues with the Original Query

String Literals: In the original query, 'Col A' and 'Col B' are treated as string literals rather than column names. You should reference the columns directly without any quotes.

Assignment in CASE: The way to set a column to null or pass the original value is incorrect; you don't assign values with = inside a CASE statement.

Misleading Results: The current approach would lead to incorrect outputs, as it would always evaluate to true for entries where the string 'Col A' equals 'Col B', which is not the intended operation.

The Solution

To properly achieve the desired outcome, you should modify your SQL query using the CASE statement correctly. Here’s how you can implement it:

Correct SQL Query

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

Explanation of the Correct Query

CASE Statement:

when col_a = col_b then null checks if the values in Col A and Col B are equal. If they are, it sets the result to null.

else col_a retains the value of Col A if it does not equal Col B.

Alias: The result of this CASE statement is given the alias col_a_corrected, which allows you to easily identify the modified values.

Selecting from my_table: This query operates on the dataset named my_table, analyzing and producing a new column without altering the existing data.

Conclusion

In this guide, you learned how to effectively use the CASE statement in PostgreSQL to compare values between two columns and perform conditional modifications. This kind of operation is essential in data manipulation and integrity checks. By making simple adjustments to your SQL code, you can obtain the correct results and enhance your database querying skills.

Now, give it a try in your own PostgreSQL setup and see the difference!