How do I UPDATE from a SELECT in SQL Server?

tl;dr

In order to update a table from a select statement in SQL Server, you must first join the two tables together. This is done by using an INNER JOIN. This will join the two tables together based on a common id.

Once the tables are joined, you can then use the UPDATE statement to set the values of the columns in the first table to the values of the columns in the second table. You can also add a WHERE clause to the statement to specify which rows should be updated.

For example, if you wanted to update the columns col1 and col2 in Table_A to the values of col1 and col2 in Table_B, you would use the following statement:

UPDATE 
    Table_A
SET 
    Table_A.col1 = Table_B.col1,
    Table_A.col2 = Table_B.col2
FROM 
    Some_Table AS Table_A
    INNER JOIN Other_Table AS Table_B
        ON Table_A.id = Table_B.id
WHERE 
    Table_A.col3 = 'fun';