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Learn how to efficiently handle the `Ambiguous column name` error in SQL when using subqueries, particularly for facilities inspection data.
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How to Resolve the Ambiguous Column Name Error in SQL Subqueries

SQL can sometimes be tricky, particularly when it comes to subqueries and aggregate functions. If you've recently encountered the Ambiguous column name error while working on your SQL queries, you're not alone. This error typically arises when the SQL engine is confused about which column you're referencing, especially when multiple tables are involved. In this guide, we will explore a practical solution to an SQL problem related to facility inspections: identifying facilities that didn't meet inspection requirements.

The Problem at Hand

The challenge involves a dataset containing facility inspections and the requirement that certain facilities must have at least three types of inspections each year. Specifically, we need to identify facilities that:

Are classified under Risk Level 1.

Failed to have three inspections in 2013.

Had at least one failed Canvass-type inspection during that year.

You may have attempted to write a query, but encountered this error:

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

Understanding the Error

The error message indicates that the SQL engine cannot determine which Risk column you're trying to reference. This typically happens when the same column name appears in more than one table that you're joining or referencing in your query.

A Clear Solution: Using Aggregation

To create a solution for our problem without triggering the Ambiguous column name error, we can restructure the query. The aim is to use aggregation to count the number of failed Canvass inspections for each facility, while ensuring the query filters out only the relevant inspections.

Step-by-Step Breakdown

Table Structure:
Assume the FoodInspectionOriginal table has the following columns:

DBAName – Name of the facility

InspectionDate – Date of the inspection

InspectionType – Type of inspection (e.g., "Canvass")

InspectionStatus – Status of the inspection (e.g., "successful", "failed")

Risk – Risk classification of the facility

Revised SQL Query:
Here’s how the SQL query should look to effectively find the facilities that meet the conditions:

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

Explanation of the Query Components

Aggregate Functions:
The SUM(CASE WHEN...) constructs are key for counting the number of failed inspections and grouping this by DBAName.

Date Filter:
The date filter ensures that only inspections from the year 2013 are included, using a range from January 1, 2013, to December 31, 2013.

HAVING Clause:
This clause is used to enforce the conditions that eliminate facilities that don't meet the inspection frequency or failed requirements.

Conclusion

By restructuring your SQL query and using proper aggregation techniques, you can effectively resolve the Ambiguous column name error and achieve the desired results. This solution not only clarifies the criteria needed to identify the facilities in question but also helps you become more adept at handling similar challenges in SQL.

Feel free to try this adjusted query and see how it simplifies your data retrieval process!