Agrupar y contar

[gnzsoloyo]

Probemos así:

Código MySQL:

SELECT tabla_a.id_a, SUM(IF(tabla_b.a_id IS NOT NULL, 1, 0)) total_a_en_b 
FROM tabla_a LEFT JOIN tabla_b ON tabla_a.id_a = tabla_b.a_id
GROUP BY tabla_a.id_a;