通过MAX挑战LEFT OUTER JOIN查询分组

另一种方式：

SELECt     def.id  AS id,     COALESCE(matrix.service_id, matrix2.service_id)     AS service_id,     COALESCE(matrix.provider_id, matrix2.provider_id)   AS provider_id,     COALESCE(matrix.amount, matrix2.amount, def.amount) AS amount FROM         billing_billingdefinition AS def     LEFT JOIN         billing_billingmatrix AS matrix  ON  matrix.definition_id = def.id  AND matrix.service_id = 25  AND matrix.provider_id = 24     LEFT JOIN         billing_billingmatrix AS matrix2 ON  matrix2.definition_id = def.id  AND matrix2.service_id IS NULL AND matrix2.provider_id = 24  ;