Operand should contain 1 column(S);

select t.Tname,c.Cname,a.Avg(Score) from Teacher t left join Course c on t.T = c.T left join SC a on c.C = a.C group by (Tname,Cname)

出现Operand should contain 1 column(S);

解决办法删除点group by 后面的一项结果就显示出来了

select t.Tname,c.Cname,a.Avg(Score) from Teacher t left join Course c on t.T = c.T left join SC a on c.C = a.C group by (Tname)