I am novice in Informix though I have worked in Microsoft SQL Server.
I want to update a table from the other table in informix. How do I do this?
For example, I can execute a query in SQL Server as

update TableA set orderqty = b.totQty
from TableA as a, TableB as b
where a.shiftdate = b.shiftdate;

Appreciate your valuable input.

This is ANSI syntax for your query:

UPDATE TableA
SET orderqty = (
SELECT totQty
FROM TableB
WHERE TableA.shiftdate = TableB.shiftdate)
;

HTH