1. Correlated Sub Query :
Correlated sub query are those which work on row by row basis. As for every row it takes the values from the main Query into the sub-query to return a value. Thus it is dependent on the main query i.e Outer Query.
Suppose we have a table like this below :
And we want the output as
To get the result column in the above image we will make use of a correlated query to get the desired output.
Query:
Correlated sub query are those which work on row by row basis. As for every row it takes the values from the main Query into the sub-query to return a value. Thus it is dependent on the main query i.e Outer Query.
Suppose we have a table like this below :
And we want the output as
To get the result column in the above image we will make use of a correlated query to get the desired output.
Query:
SELECT *, ( SELECT Sum(m.Name1)
FROM MATHTBL M
WHERE m.id <= m2.id
) AS Result
FROM MATHTBL m2
The Inner Query is called as Correlated Query.
2. Uncorrelated Subquery :
The uncorrelated query is not dependent on the main query. It Just provides the values
required to the main query.
Example:
SELECT empname
FROM Employee
WHERE empid IN (
SELECT empid
FROM empsalary
WHERE empsal > 1000
)
