with sql, we just simply
select max(col1, col2) from t
with kdb, we will need to something like:
q)select [5] from t
time sym bid ask
-----------------------------
09:30:00.386 IBM 50.12 51.69
09:30:00.754 IBM 50.69 51.02
09:30:00.871 AAPL 50.2 51.4
09:30:01.548 AAPL 50.84 51.28
09:30:01.921 GOOG 50 51.82
q)select [5] from update greater:max each flip (bid; ask) from t
time sym bid ask greater
-------------------------------------
09:30:00.386 IBM 50.12 51.69 51.69
09:30:00.754 IBM 50.69 51.02 51.02
09:30:00.871 AAPL 50.2 51.4 51.4
09:30:01.548 AAPL 50.84 51.28 51.28
09:30:01.921 GOOG 50 51.82 51.82
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