The FOR UPDATE clause is invalid for statements containing set operators

In SQL Server, the FOR UPDATE clause is not used, as this syntax is more commonly associated with other databases like Oracle. The error message "The FOR UPDATE clause is invalid for statements containing set operators" indicates that you're trying to use a feature that is not supported or incorrectly applied in SQL Server, especially in combination with set operations like UNION, INTERSECT, or EXCEPT.

To achieve similar functionality in SQL Server, you should use transaction control and locking hints to ensure the data integrity during updates. Below, I'll show you how to handle locking rows for updates, particularly when dealing with set operations.

Handling Locking and Updates in SQL Server

1. Using Transactions and Locking Hints

   Use transactions and locking hints like WITH (UPDLOCK) to ensure rows are locked for update.

2. Handling Set Operations Separately

   Perform set operations in separate steps, ensuring rows are locked before the operations.

Example Without Set Operations

For standard row locking and updating without set operations:

BEGIN TRANSACTION;

-- Acquire an update lock on the selected rows
SELECT *
FROM table_name
WITH (UPDLOCK)
WHERE condition;

-- Perform the update
UPDATE table_name
SET column_name = value
WHERE condition;

COMMIT TRANSACTION;

Example with Set Operations

When dealing with set operations, lock rows first, then perform the set operation:

BEGIN TRANSACTION;

-- Step 1: Lock rows in table1
SELECT *
FROM table1
WITH (UPDLOCK)
WHERE condition;

-- Step 2: Lock rows in table2
SELECT *
FROM table2
WITH (UPDLOCK)
WHERE condition;

-- Step 3: Perform the set operation (e.g., UNION)
SELECT column1, column2
FROM table1
WHERE condition
UNION
SELECT column1, column2
FROM table2
WHERE condition;

-- Step 4: Update the locked rows (example for table1)
UPDATE table1
SET column_name = value
WHERE condition;

COMMIT TRANSACTION;

Explanation

• BEGIN TRANSACTION: Starts a new transaction.
• WITH (UPDLOCK): Acquires an update lock on the selected rows.
• SELECT: Retrieves the rows to be locked.
• UNION: Performs the set operation.
• UPDATE: Updates the locked rows.
• COMMIT TRANSACTION: Commits the transaction, releasing the locks.

Example in Detail

Consider two tables, Employees and Departments, and you need to lock rows and perform a union operation before updating.

BEGIN TRANSACTION;

-- Step 1: Lock rows in Employees
SELECT *
FROM Employees
WITH (UPDLOCK)
WHERE DepartmentID = 1;

-- Step 2: Lock rows in Departments
SELECT *
FROM Departments
WITH (UPDLOCK)
WHERE DepartmentID = 1;

-- Step 3: Perform the UNION operation
SELECT EmployeeID, EmployeeName
FROM Employees
WHERE DepartmentID = 1
UNION
SELECT DepartmentID, DepartmentName
FROM Departments
WHERE DepartmentID = 1;

-- Step 4: Update the locked rows in Employees
UPDATE Employees
SET Salary = Salary * 1.1
WHERE DepartmentID = 1;

COMMIT TRANSACTION;

This approach ensures that rows in both Employees and Departments are locked for the duration of the transaction, the union operation is performed, and the rows are updated safely.