Whis Sub Query

SubQuery:-
Query within a Query is called SubQuery.
Syntax for SubQuery is
SELECT column_name1 FROM Table_name1 WHERE column_name2 Comparison Operator
(SELECT column_name3 FROM Table_name2 WHERE [Condition])
Ex:-
SELECT * FROM emp WHERE sal = (SELECT MIN(sal) FROM employee)
CoRelated SubQuery:-
The main difference between SubQuery and co Related subquery is that in subquery child query excuted first and then parent query will excute. But in Co-Related subquery main query will excuted first and then Child query .
EX:
Select name from emp where exits (Select empid,salary from emp1 where emp1.name=emp.name)