Convert DAX calculated column formula to SQL

Hi All,

I have two tables:
Orders table (Contains 1 row per orders placed with customer and revenue data as whole)

Order line items table (Contains brief Order data with line items bought within the order)

I used below formula to create a new column in 'Orders' table' which is referring values from 'Order Line Items' table and assigning values in new column. Now I need to create a new column in 'Orders' table itself.

What is the SQL equivalent of the following DAX (Power BI) Formula:

Order Table column = 
VAR Brand = CALCULATE ( SELECTEDVALUE ( 'Order Line Items'[Brand] ) )
RETURN
    IF ( 
        ISBLANK ( Brand ),
        "Mixed Order",
        Brand
    )