[Err] 1248 - Every derived table must have its own alias_综合

mysql执行多表查询时报错：

select * from (
SELECT EXTRACT(YEAR FROM NOW()) YEAR from DUAL 
UNION
SELECT EXTRACT(YEAR FROM NOW()) -1 YEAR from DUAL 
UNION
SELECT EXTRACT(YEAR FROM NOW()) -2 YEAR from DUAL 
UNION
SELECT EXTRACT(YEAR FROM NOW()) -3 YEAR from DUAL 
UNION
SELECT EXTRACT(YEAR FROM NOW()) -4 YEAR from DUAL 
)

[Err] 1248 - Every derived table must have its own alias

一般是在多表查询或者子查询的时候会出现这个错误，因为在嵌套查询中，子查询的结果是作为一个派生表给上一级进行查询，所以子查询的结果必须有一个别名。这句话的意思是每个派生出来的表必须有一个自己的别名

上面的例子中，把查询语句修改一下：

select * from (
SELECT EXTRACT(YEAR FROM NOW()) YEAR from DUAL 
UNION
SELECT EXTRACT(YEAR FROM NOW()) -1 YEAR from DUAL 
UNION
SELECT EXTRACT(YEAR FROM NOW()) -2 YEAR from DUAL 
UNION
SELECT EXTRACT(YEAR FROM NOW()) -3 YEAR from DUAL 
UNION
SELECT EXTRACT(YEAR FROM NOW()) -4 YEAR from DUAL 
) as total

如上所示，在子查询的后面增加一句 as total，相当于给子查询的结果集派生表取别名为total，问题就解决了。