为什么这个查询不起作用，而这个却可以呢？

Question

In MySQL Shell, two nearly identical queries. One works, one doesn't. The difference is a key word. Why is that?

The working query just gets a salary of your choice depending on what limit number you use in the subquery. So "Limit 4" would give you the fourth highest salary.

SELECT SALARY
FROM (
      SELECT DISTINCT SALARY
      FROM TBLEMPLOYEE
      ORDER BY SALARY DESC LIMIT 3
      ) RESULT
ORDER BY SALARY LIMIT 1

This successfully gives the third highest salary.

If I remove the word "result" it no longer works. What function does result serve? Is that an alias? I get this: ERROR 1248 (42000): Every derived table must have its own alias

Answer 1

Every derived table (AKA sub-query) must indeed have an alias. Ie each query in brackets must be given an alias (AS whatever), which can the be used to refer to it in the rest of the outer query.Yes result is an alias.it could be any name in the place of result like myalias or something else.

SELECT SALARY
FROM (
      SELECT DISTINCT SALARY
      FROM TBLEMPLOYEE
      ORDER BY SALARY DESC LIMIT 3
      ) AS RESULT
ORDER BY SALARY LIMIT 1

more example

SELECT ID
FROM (SELECT ID,
             msisdn 
      FROM (SELECT * FROM TT2) as myalias
     ) as anotheralias;