Скачать или смотреть Solving the Cardinality Violation in MySQL: The Union Query Issue

Discover how to resolve the `Cardinality violation` issue in MySQL when using UNION with different columns. This guide simplifies the necessary steps and provides example queries for clarity.
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This video is based on the question https://stackoverflow.com/q/70829963/ asked by the user 'AZIS SOFYAN PRASETYO' ( https://stackoverflow.com/u/17474789/ ) and on the answer https://stackoverflow.com/a/70830165/ provided by the user 'Jens' ( https://stackoverflow.com/u/3636601/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Cardinality Violation in MySQL

If you've ever worked with MySQL, you may have encountered the frustrating Cardinality violation error. This typically occurs when you attempt to combine the results of two SELECT statements using a UNION operation, but the column counts do not match.

In this post, we'll analyze a specific case where two tables have different column names and different number of columns in their queries. Let's break down the scenario and walk through the solution step by step.

The Situation

You have two SELECT queries you want to combine using UNION:

First Query: Retrieves data from the academic_schedule_table.

Second Query: Pulls data from multiple tables, including appointment_table and dosen_schedule_table.

The issue arises because the number of columns in the first query is different from the number of columns in the second query, leading to the error message:

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

Solution: Harmonizing Your SELECT Statements

To resolve the Cardinality violation, you need to ensure that each SELECT statement returns the same number of columns and that they are of compatible types. Let’s refine the original SQL query.

Breakdown of the Original Query

The original query looks like this:

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

Adjusting the Second SELECT

In the second SELECT statement, the original number of columns was greater than the first. We can adjust it as follows:

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

Key Changes Made:

Consistent Column Count: Both SELECT statements now return exactly four columns: calendar_id, calendar_title, calendar_starttime, and calendar_endtime.

Corrected Syntax: Removed unnecessary columns from the second SELECT statement to ensure it aligns with the first.

Conclusion

By aligning the number of returned columns in both SELECT statements, the Cardinality violation error can be resolved. When utilizing the UNION operator in your SQL statements, always remember that consistency in column count and types across all SELECT queries is crucial.

If you're facing similar issues, follow the steps outlined above to fine-tune your queries and enjoy a smoother development experience with MySQL!