how to use subquery in select statement mysql code example

Example: variables in select statement in sql server

USE AdventureWorks2014;
GO
DECLARE @MyVariable int;
SET @MyVariable = 1;
-- Terminate the batch by using the GO keyword.
GO 
-- @MyVariable has gone out of scope and no longer exists.

-- This SELECT statement generates a syntax error because it is
-- no longer legal to reference @MyVariable.
SELECT BusinessEntityID, NationalIDNumber, JobTitle
FROM HumanResources.Employee
WHERE BusinessEntityID = @MyVariable;