SQL语句错误

Question

mysql> Select Emp_B AS Total 
    -> From (Select Sum(mines.NoOfWorkers) AS Emp_B from mines); 
ERROR 1248 (42000): Every derived table must have its own alias 

mysql> Select Emp_B AS Total 
    -> From (Select Sum(mines.NoOfWorkers) from mines) AS Emp_B; 
ERROR 1054 (42S22): Unknown column 'Emp_B' in 'field list' 

我遇到了这个SQL语句的问题。任何援助将是摩西赞赏

Answer 1

Select Emp_B AS Total 
From (Select Sum(mines.NoOfWorkers) AS Emp_B from mines) x; 

由于错误状态Every derived table must have its own alias 只要给它一个别名，像上面x。 OR AS x，但AS字是可选的。

或者为什么别名两次......

Select Total 
From (Select Sum(mines.NoOfWorkers) AS Total from mines) x; 

但由于SUM给你一个值，除非你已经简化了一些较大的查询，这给了完全相同的结果？

Select Sum(mines.NoOfWorkers) AS Total from mines; 

Answer 2

这应该适合你;然而，如果你只是做这一个子选择进入临时表，它是一种浪费，将其包装到另一个选择，但这只是恕我直言。

Select Emp_B.sum From (Select Sum(mines.NoOfWorkers) as sum from mines) AS Emp_B; 

Answer 3

Select temp.total 
From 
    (Select Sum(mines.NoOfWorkers) AS total 
    from mines 
) AS temp 
;