无法排序查询，SQL数据库表

Question

I'm trying to understand the dynamics of mysql, And I'm not able to understand this error statement:

Query:-

SELECT employee_id, First_name, Salary AS Pay FROM employee
ORDER BY Salary DESC
WHERE Salary > 70000
LIMIT 3;

I want to retrieve data from the employee table and then sort it up & only take out only the ones which are greater than 7000 . But instead i Get this error,

ER_PARSE_ERROR: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE Salary > 70000 LIMIT 3' at line 1

But , If the code is written this way,

SELECT employee_id, First_name, Salary AS Pay FROM employee
WHERE Salary > 70000
ORDER BY Salary DESC
LIMIT 3;

It runs, perfectly. But the query has a complete different meaning.

I hope you can understand what i mean, Why can't we query after sorting the table?

Answer 1

SQL is a descriptive language, not a procedural language.

A SQL query describes the result set. Whether you remove rows with salary > 7000 and then sort, or sort and then remove them, you actually get the same answer.

You don't get to specify the order of operations; that is the job of the SQL compiler and optimizer.

Answer 2

you can sort before you select by a trick of sub selecting ie:

SELECT * FROM (SELECT employee_id, First_name, Salary AS Pay FROM employee
ORDER BY Salary DESC) a
WHERE Pay > 70000
LIMIT 3;

Mysql will first create a temporary memory table with ordered content and then your where will be applied to that table as will the limit.