asof join

[Jerome]

I was reading http://code.kx.com/wiki/Reference/aj but cannot
understand why left join is used? Why not do t1 lj xkey `date`sym
instead?

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q)a:aj[c1...cn;t1;t2]


The result is a table with records from the left join of t1 and t2.

For each record in t1, the result has one record with the items in t1,
and
if there are matching records in t2, the items of the last (in row
order) matching record are appended to those of t1
if there is no matching record in t2, the remaining columns are null

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