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

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