hola,
no se si lo he entendido bien, pero te digo como yo lo haria, prueba si quieres y nos comentas
Código SQL
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select
(select count(*) from b_reviews where rating = 0) as Rating0,
(select count(*) from b_reviews where rating = 1) as Rating1,
(select count(*) from b_reviews where rating = 2) as Rating2,
(select count(*) from b_reviews where rating = 3) as Rating3,
(select count(*) from b_reviews where rating = 4) as Rating4,
(select count(*) from b_reviews where rating = 5) as Rating5
Creo que esto te daría lo que quieres, igual no es la consulta más eficiente del mundo pero te puede valer!!!
Espero haberte ayudado!