Solution to the MySQL error "Every derived table must have its own alias"

MySQL reports an error when executing multi-table query:

[SQL] SELECT * from 
(
select e.account from employee e
UNION
SELECT u.account from `user` u
UNION
SELECT a.account from agent a
)
[Err] 1248 - Every derived table must have its own alias

This means that each derived table must have its own alias.

This error usually occurs when querying multiple tables or subqueries. In a nested query, the result of the subquery is used as a derived table for querying the upper level, so the result of the subquery must have an alias.

In the above example, modify the query statement:

SELECT * from 
(
select e.account from employee e
UNION
SELECT u.account from `user` u
UNION
SELECT a.account from agent a
)as total

As shown above, adding a sentence as total after the subquery is equivalent to giving the derived table of the subquery result set an alias of total, and the problem is solved.