If you’re getting an error that reads “The function ‘LAG’ must have an OVER clause” in SQL Server, it’s probably because you’re calling the LAG() function without an OVER clause.

The LAG() function requires an OVER clause (and that clause must have an ORDER BY clause).

To fix this issue, be sure to include an OVER clause when calling the LAG() function.

Example of Error

Here’s an example of code that produces the error:

SELECT
    VendorId,
    ProductName,
    ProductPrice,
    LAG( ProductPrice )
FROM Products;

Result:

Msg 10753, Level 15, State 1, Line 5
The function 'LAG' must have an OVER clause.

Here I called the LAG() function without an OVER clause, which resulted in an error.

Solution

To fix this issue, add an OVER clause to the LAG() function:

SELECT
    VendorId,
    ProductName,
    ProductPrice,
    LAG( ProductPrice ) OVER ( 
        ORDER BY ProductPrice 
        ) AS LAG
FROM Products;

Result:

VendorId  ProductName                      ProductPrice  LAG  
--------  -------------------------------  ------------  -----
1004      Bottomless Coffee Mugs (4 Pack)  9.99          null 
1003      Hammock                          10            9.99 
1001      Long Weight (green)              11.99         10   
1004      Tea Pot                          12.45         11.99
1001      Long Weight (blue)               14.75         12.45
1001      Left handed screwdriver          25.99         14.75
1001      Right handed screwdriver         25.99         25.99
1002      Sledge Hammer                    33.49         25.99
1003      Straw Dog Box                    55.99         33.49
1003      Chainsaw                         245           55.99

This time we get the expected result.

It’s important to remember that the OVER clause must have an ORDER BY clause. Omitting this will cause another error.