# wj, wj1

Window join

wj [w; c; t; (q; (f0;c0); (f1;c1))]
wj1[w; c; t; (q; (f0;c0); (f1;c1))]


Where

• t and q are simple tables to be joined (q should be sorted symtime with p# on sym)
• w is a pair of lists of times/timestamps, begin and end
• c are the names of the common columns, syms and times, which must have integral types
• f0, f1 are aggregation functions applied to values in q columns c0,c1 over the intervals

returns for each record in t, a record with additional columns c0 and c1, which are the results of the aggregation functions applied to values over the matching intervals in w.

Typically this might be:

wj[w;symtime;trade;(quote;(max;ask);(min;bid))]


A quote is understood to be in existence until the next quote.

To see all the values in each window, pass the identity function :: in place of the aggregates

E.g. wj[w;c;t;(q;(::;c0);(::;c1))]

## Multi-column arguments

Since 3.6 2018.12.24, wj and wj1 support multi-col args, forming the resulting column name from the last argument e.g.

wj[w; f; t; (q; (wavg;asize;ask); (wavg;bsize;bid))]


## Interval behaviour

wj and wj1 are both [] interval, i.e. they consider quotes ≥beginning and ≤end of the interval.

For wj, the prevailing quote on entry to the window is considered valid as quotes are a step function.

wj1 considers quotes on or after entry to the window. If the join is to consider quotes that arrive from the beginning of the interval, use wj1.

Behavior prior to V3.0

Prior to V3.0, wj1 considered only quotes in the window except for the window end (i.e. quotes ≥start and <end of the interval).

version wj1 wj
3.0+ [] prevailing + []
2.7/2.8 [) prevailing + []
q)t:([]sym:3#ibm;time:10:01:01 10:01:04 10:01:08;price:100 101 105)
q)t
sym time     price
------------------
ibm 10:01:01 100
ibm 10:01:04 101
ibm 10:01:08 105

q)a:101 103 103 104 104 107 108 107 108
q)b:98 99 102 103 103 104 106 106 107
q)q:([]sym:ibm; time:10:01:01+til 9; ask:a; bid:b)
q)q
--------------------
ibm 10:01:01 101 98
ibm 10:01:02 103 99
ibm 10:01:03 103 102
ibm 10:01:04 104 103
ibm 10:01:05 104 103
ibm 10:01:06 107 104
ibm 10:01:07 108 106
ibm 10:01:08 107 106
ibm 10:01:09 108 107

q)f:symtime
q)w:-2 1+\:t.time

q)wj[w;f;t;(q;(max;ask);(min;bid))]
--------------------------
ibm 10:01:01 100   103 98
ibm 10:01:04 101   104 99
ibm 10:01:08 105   108 104


The interval values may be seen as:

q)wj[w;f;t;(q;(::;ask);(::;bid))]
--------------------------------------------------
ibm 10:01:01 100   101 103         98 99
ibm 10:01:04 101   103 103 104 104 99 102 103 103
ibm 10:01:08 105   107 108 107 108 104 106 106 107


Window joins with multiple symbols should be used only with a p#sym like schema.

Typical RTD-like g# gives undefined results.

Window join is a generalization of as-of join

An as-of join takes a snapshot of the current state, while a window join aggregates all values of specified columns within intervals.

aj, asof`
Joins
Q for Mortals §9.9.9 Window Joins