Can I Restrict WHERE Clauses to Specific Fields in MySQL?

Can WHERE Clauses Be Restricted to a Specific Field in MySQL?

In MySQL, the WHERE clause filters rows based on specified conditions. However, it's not possible to limit the application of a WHERE clause to a specific field within a query.

Understanding the Problem

In the context of the question, the goal is to display a list of available courses and the associated user's enrollment status. Each row represents a course and shows the enrollment status (userHabilitado) for a particular user. However, the user's userID must be taken from the URL and used to filter the results for that specific user.

Solution: Exploiting Non-Full Group By

Prior to MySQL 5.7, MySQL allowed non-full group by, which meant that aggregate functions like group_concat could be used with non-aggregated columns (NON AGGS). However, starting with MySQL 5.7, the ONLY_FULL_GROUP_BY setting is enabled by default, disallowing such queries.

In this case, the query attempts to add userHabilitado to the list of columns being displayed, which are already being grouped by cursos.cursoID. However, the userHabilitado value being displayed is not filtered by a WHERE clause and thus displays the first value found for that user, regardless of the course.

Workaround: Disabling Non-Full Group By

One workaround is to disable ONLY_FULL_GROUP_BY for the specific query by using the SQL_CALC_FOUND_ROWS modifier:

<code class="sql">SET SQL_CALC_FOUND_ROWS = 0;</code>

However, this is not recommended as it can introduce potential discrepancies in results and should be used with caution.

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