This is how to update a column of a table to be the group-by-count of rows, in another table.
User table has a field called postcount which represents the number of posts that a user has made. And the Tip table has all the posts. So we are going to get the number of posts that a user has made from the Tip table and set that value to the User table.
Here's how to do it:
with UserCounts as ( select count(*) as counts, t.userId as usid from tip t group by t.userId ) update [user] set postcount = UserCounts.counts from [user] inner join UserCounts on UserCounts.usid = [user].id