我有4张桌子：费用，员工，访问，账单
成本
SpecialtyCode DiagnosisCode Charge
4001 1001 50
4001 1002 75
4001 1003 100
4001 1004 125
4001 1005 150
4002 1001 60
4002 1002 85
4002 1003 110
4002 1004 135
4002 1005 160
4003 1001 55
4003 1002 80
4003 1003 105
4003 1004 130
4003 1005 155
4004 1001 72
4004 1002 102
4004 1003 132
4004 1004 162
4004 1005 192
4005 1001 90
4005 1002 140
4005 1003 190
4005 1004 240
4005 1005 290
4006 1001 77
4006 1002 112
4006 1003 147
4006 1004 182
4006 1005 217
和员工
ENum EName Title
101 Zhivago Doctor
102 Welby Doctor
103 Jekyl Doctor
104 Caligari Doctor
105 Nightingale Nurse
106 Ratchet Nurse
和访问
PatientSSN,ENum,Date
302609074,101,20180215
302609074,102,20181010
302609074,105,20190505
302609074,105,20190506
997600970,103,20190115
997600970,105,20190415
997600970,104,20181212
997600970,104,20181213
874136439,102,20190808
874136439,102,20190910
874136439,102,20191001
874136439,102,20191002
874136439,102,20191003
831287780,101,20190404
831287780,102,20190505
831287780,103,20190606
831287780,104,20190707
882861510,106,20190202
882861510,105,20190303
882861510,106,20190404
882861510,105,20190505
882861510,106,20190606
796235486,104,20190808
445139565,102,20180707
445139565,102,20190929
524246868,103,20190909
524246868,103,20190910
416806352,102,20190818
946883650,103,20190717
946883650,106,20190817
946883650,103,20190917
946883650,106,20191002
129141378,103,20190909
950022926,101,20181111
950022926,105,20190323
950022926,101,20190623
799023031,106,20190930
719301054,103,20190110
719301054,103,20190210
719301054,103,20190310
316517393,102,20190626
148694321,101,20190102
148694321,101,20190203
148694321,101,20190304
332124842,104,20190405
332124842,104,20190506
332124842,104,20190607
结算
ssn,date,scode,dcode,ppay,ipay
302609074,20180215,4002,1001,25,35
302609074,20180215,4006,1003,40,100
302609074,20181010,4002,1001,25,35
302609074,20181010,4006,1003,40,100
302609074,20190505,4002,1001,25,35
302609074,20190506,4006,1003,40,100
997600970,20190115,4001,1002,25,50
997600970,20190115,4001,1003,40,30
997600970,20190115,4004,1004,40,50
997600970,20190415,4002,1004,40,95
997600970,20190415,4003,1005,40,100
997600970,20181212,4004,1001,25,47
997600970,20181213,4004,1001,25,47
874136439,20190808,4002,1002,25,50
874136439,20190808,4002,1003,40,60
874136439,20190910,4002,1004,40,95
874136439,20190910,4002,1005,60,90
874136439,20191001,4003,1002,25,50
874136439,20191002,4003,1002,25,50
874136439,20191003,4003,1002,25,55
831287780,20190404,4001,1001,25,0
831287780,20190505,4003,1002,25,40
831287780,20190606,4004,1003,25,100
831287780,20190707,4006,1004,60,100
882861510,20190202,4005,1002,60,60
882861510,20190303,4002,1004,25,100
882861510,20190404,4005,1002,40,100
882861510,20190505,4002,1004,50,100
882861510,20190606,4005,1003,50,0
796235486,20190808,4001,1001,25,25
445139565,20180707,4002,1001,40,35
445139565,20190929,4002,1003,40,70
445139565,20190929,4002,1004,40,95
445139565,20190929,4002,1005,40,120
524246868,20190909,4006,1005,100,100
524246868,20190910,4006,1005,100,100
416806352,20190818,4003,1001,25,30
416806352,20190818,4003,1005,50,100
946883650,20190717,4001,1004,25,100
946883650,20190817,4001,1004,25,100
946883650,20190917,4006,1004,50,132
946883650,20191002,4006,1004,50,132
129141378,20190909,4001,1002,25,50
129141378,20190909,4001,1003,0,100
129141378,20190909,4001,1004,0,125
129141378,20190909,4001,1005,0,150
950022926,20181111,4006,1004,182,0
950022926,20190323,4006,1004,100,82
950022926,20190623,4006,1004,0,182
799023031,20190930,4005,1003,50,100
719301054,20190110,4001,1003,25,75
719301054,20190210,4001,1003,25,75
719301054,20190310,4001,1003,25,75
316517393,20190626,4005,1005,0,0
148694321,20190102,4006,1002,40,72
148694321,20190203,4006,1002,40,72
148694321,20190304,4006,1002,40,72
332124842,20190405,4004,1001,25,47
332124842,20190506,4004,1002,25,80
332124842,20190607,4004,1003,25,107
我的问题是如何退还该员工（enum，ename，title）及其平均每次访问费用？
这是我想出的：
SELECT employee.eno, employee.ename, AVG(billing.charge)
FROM visits
JOIN billing
ON visits.ssn = outcome.ssn AND visits.date = outcome.date
JOIN costs
ON billing.scode = costs.scode AND billing.dcode = costs.dcode
JOIN employee
ON employee.eno = visits.eno
GROUP BY employee.eno, employee.ename;
哪个返回：
ENUM ENAME AVG(billing.charge)
---------- ------------------------------ ----------
103 Jekyl 130.666667
102 Welby 116.588235
104 Caligari 97.4285714
106 Ratchet 161.166667
105 Nightingale 135.571429
101 Zhivago 119.625
这是不正确的，因为对101名员工Zhivago来说，他进行了7次访问，但是由于他进行了8次计费，因为访问302609074,101,20180215产生了2次计费。因此，对于8个结算，AVG为119.625，而对于7次访问，则为136.71。