Subquery returned more than one value

 The error "Subquery returned more than 1 value" occurs when a subquery that is expected to return a single value returns more than one. This usually happens when the subquery is used in a context where only a single value is allowed, such as in a column assignment, comparison, or in the WHERE clause.

To fix this, you need to ensure that the subquery returns only one value. Here are some common scenarios and solutions:

1. Using Subquery in WHERE Clause

If you are using a subquery in a WHERE clause, make sure it returns a single value. You can use aggregate functions or ensure that the subquery conditions are specific enough to return only one value.

Example Problematic Query:

SELECT * FROM Employees
WHERE EmployeeID = (SELECT EmployeeID FROM Departments WHERE DepartmentName = 'Sales');

Solution: Use TOP 1 or aggregate functions like MIN or MAX if it makes sense for your logic.

SELECT * FROM Employees
WHERE EmployeeID = (SELECT TOP 1 EmployeeID FROM Departments WHERE DepartmentName = 'Sales');

OR

SELECT * FROM Employees
WHERE EmployeeID = (SELECT MIN(EmployeeID) FROM Departments WHERE DepartmentName = 'Sales');

2. Using Subquery in SELECT Clause

If the subquery is in the SELECT clause, it should return a single value for each row in the outer query.

Example Problematic Query:

SELECT EmployeeID, (SELECT DepartmentName FROM Departments WHERE DepartmentID = Employees.DepartmentID) AS DepartmentName
FROM Employees;

Solution: Ensure the subquery returns a single value:

SELECT EmployeeID, 
       (SELECT TOP 1 DepartmentName FROM Departments WHERE DepartmentID = Employees.DepartmentID) AS DepartmentName
FROM Employees;

3. Using Subquery in SET Clause

If you are using a subquery in an UPDATE statement's SET clause, ensure it returns a single value.

Example Problematic Query:

UPDATE Employees
SET DepartmentID = (SELECT DepartmentID FROM Departments WHERE DepartmentName = 'Sales');

Solution: Make sure the subquery returns only one value:

UPDATE Employees
SET DepartmentID = (SELECT TOP 1 DepartmentID FROM Departments WHERE DepartmentName = 'Sales');
4. Using IN or EXISTS
If you need to check for multiple values, use IN or EXISTS instead of =.
Example Problematic Query:
SELECT * FROM Employees
WHERE EmployeeID = (SELECT EmployeeID FROM Departments WHERE DepartmentName = 'Sales');
Solution:
Use IN or EXISTS:
SELECT * FROM Employees
WHERE EmployeeID IN (SELECT EmployeeID FROM Departments WHERE DepartmentName = 'Sales');
OR
SELECT * FROM Employees
WHERE EXISTS (SELECT 1 FROM Departments WHERE DepartmentName = 'Sales' AND Departments.EmployeeID = Employees.EmployeeID);
Example to Illustrate the Issue and Solution
Problematic Query:
SELECT * FROM Orders
WHERE CustomerID = (SELECT CustomerID FROM Customers WHERE City = 'New York');
If there are multiple customers from 'New York', this query will fail.
Solution:
SELECT * FROM Orders
WHERE CustomerID IN (SELECT CustomerID FROM Customers WHERE City = 'New York');
By ensuring that your subqueries are appropriately constrained to return a single value
 or by using IN or EXISTS when expecting multiple values, you can resolve 
the "Subquery returned more than 1 value" error.