SELECT employeeId, AVG(bonus_2017), AVG(bonus_2018) FROM SalaryBonus WHERE bonus_2018 <> 0 AND bonus_2017 <> 0 GROUP BY employeeId

SELECT employeeId, AVG(CASE WHEN bonus_2017 <> 0 THEN bonus_2017 ELSE 0 END) as bonus_2017, AVG(CASE WHEN bonus_2018 <> 0 THEN bonus_2018 ELSE 0 END) as bonus_2018 FROM SalaryBonus GROUP BY employeeId LIMIT 6

SELECT  sal1.employeeId, bb, aa FROM SalaryBonus sal1 LEFT JOIN(SELECT  employeeId, AVG(bonus_2018) as aa FROM SalaryBonus WHERE bonus_2018 <> 0 GROUP BY  employeeId LIMIT 6) as sal2 USING (employeeId) LEFT JOIN (SELECT  employeeId, AVG(bonus_2017) as bb FROM SalaryBonus WHERE bonus_2017 <> 0 GROUP BY  employeeId LIMIT 6 ) as sal3 USING (employeeId) GROUP BY sal1.employeeId

SELECT  employeeId e, (SELECT employeeId , SUM(bonus_2017) FROM 
                      (SELECT employeeId,bonus_2017 FROM SalaryBonus WHERE bonus_2017 <>0  LIMIT 6) as aaa) as b2017
                      FROM  SalaryBonus as asd GROUP BY employeeId

(SELECT employeeId,bonus_2017 FROM SalaryBonus WHERE bonus_2017 <>0  LIMIT 6) 

(SELECT employeeId,bonus_2017 FROM SalaryBonus WHERE bonus_2017 <>0 AND e=employeeId  LIMIT 6) 

SELECT @a := employeeId,  (SELECT AVG(bonus_2017) FROM (SELECT bonus_2017, month FROM SalaryBonus 
                                 WHERE employeeId = @a LIMIT 6 ) as bbb
                          WHERE  employeeId = @a) FROM SalaryBonus GROUP BY employeeId

@a  Подзапрос AVG
1     1650.0000
2     1650.0000
3     1650.0000