Imagine a sample table
CREATE TABLE mytable (myid integer, myval double precision);
I want to update
myval to the previous value, ordered by
myid, if a condition is met (that myval is equal to 0).
For that to work properly, the query has to start updating from the lowest
myid and end with the highest one.
I really don't know from where to start this time. The following says that
window functions are not allowed in UPDATE:
UPDATE mytable SET myval = LAG(myval) OVER (ORDER BY myid) WHERE myval = 0 RETURNING *;
And more complex alternatives with FROM subqueries have ended in syntax errors or dumb outputs because the subquery is evaluated once instead of once per row. This last phrase makes me think about the
SELECT ... LEFT JOIN LATERAL ... structure, but I haven't been able to make it work with the update statement.