How to Display User-Specific Course Availability with Conditional WHERE Clauses in MySQL?

Using Conditional WHERE Clauses for Specific Fields in MySQL

Problem Description

In a MySQL database with multiple tables for students, courses, and enrollments, the goal is to display a list of available courses where students can enroll. The requirement is to show the userHabilitado field only for a specific user, ensuring the values are correctly associated with the corresponding courses. However, the current query returns incorrect userHabilitado values.

MySQL Limitations and Solutions

Prior to MySQL 5.7, the default behavior allowed non-FULL group by queries, where some non-aggregated (NON AGGS) columns could be included outside the group by clause. However, in MySQL 5.7 and later, the ONLY_FULL_GROUP_BY setting is enabled by default. This requires all NON AGGS to be part of the group by clause, or an error will occur.

In the given query, the userHabilitado field is included in the group by, but the WHERE clause filters based on the entire cursos table instead of a specific user. To rectify this, MySQL does not allow conditional WHERE clauses within the group by fields.

Proposed Solution

To resolve this limitation, it is recommended to use a subquery to filter the cursos table based on the specific user and then join the result with the main query. This ensures that the userHabilitado field is evaluated only for the selected user:

<code class="sql">SELECT 
cursos.cursoID AS idcurso, 
cursos.estadoCurso,
(
    SELECT userHabilitado 
    FROM cursosUsuarios 
    WHERE cursosUsuarios.userID = 70 AND cursosUsuarios.cursoID = cursos.cursoID
) AS 'ok',                              
GROUP_CONCAT(cursosUsuarios.userID SEPARATOR ',') AS 'usuarios'
FROM cursos 
LEFT JOIN cursosUsuarios
ON cursos.cursoID = cursosUsuarios.cursoID
LEFT JOIN usuarios
ON cursosUsuarios.userID = usuarios.userID
WHERE cursos.estadoCurso='abierto'
GROUP BY cursos.cursoID;</code>

In the subquery, the cursosUsuarios table is filtered to retrieve the userHabilitado value for the specific user (in this case, 70) and the matching cursoID. This value is then joined with the main query, ensuring that the correct userHabilitado field is associated with the corresponding course.

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