What does this SQL query do? Please explain

Question

What means this query?

@numberx = @numberx -1
UPDATE  th
SET     @numberX= numberY= @numberX + 1
FROM    Table1 th
INNER JOIN Table2 td ON th.Id = td.idth
WHERE   td.anything = @anything

At line 3, what is this "double equality"? And what is this "from" and "inner" on an UPDATE?

Answer 1

It's a multiple table update for assigning consecutive numbers to each row in Table1 where the corresponding row in Table2 has a specific value for the column anything.

For each matching row it sets the column numberY to the value of @numberX + 1. It also reassigns that value back to @numberX, which causes @numberX to be incremented for each row.

This is known as a "quirky update". It is an undocumented and not guaranteed approach to generate running totals. In 2012 SUM() OVER (ORDER BY ...) should be used instead.