How to Apply a WHERE Clause to a Specific Field in MySQL When Using GROUP BY?

Can a WHERE Clause Be Applied to a Specific Field in MySQL?

When creating a query that includes a WHERE clause, it's possible to restrict the condition to a specific field. In your case, you're attempting to display the userHabilitado field for each course, filtered by a specific user. However, the provided query doesn't include the desired WHERE clause.

MySQL's Behavior Before Version 5.7

Prior to MySQL 5.7, a setting known as ONLY_FULL_GROUP_BY was disabled by default. This allowed a group by clause to be combined with non-aggregated columns (NON AGGS) that were not part of the group by. As a result, the query might return incorrect or ambiguous data.

MySQL 5.7 and Beyond

In MySQL 5.7 and later, ONLY_FULL_GROUP_BY is enabled by default. This requires all NON AGGS to be included in the group by clause, preventing data inaccuracies.

Example

Consider the following example query:

<code class="sql">SELECT
  cursos.cursoID AS idcurso,
  cursos.estadoCurso,
  GROUP_CONCAT(cursosUsuarios.userID SEPARATOR ',') AS usuarios,
  MAX(IF(usuarios.userID = 70, cursosUsuarios.userHabilitado, NULL)) AS userHabilitado
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>

This query uses the MAX(IF()) function to conditionalize the aggregation of the userHabilitado field. It checks if the usuarios.userID matches the provided user ID (70) and returns the corresponding userHabilitado value, otherwise it returns NULL. By including MAX() in the GROUP BY clause, we're able to retain the conditional aggregated result for each cursoID.

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