.Q.ff[x;y]

Where

• x is table to modify
• y is a table of columns to add to x and set to null

returns x, with all new columns in y, with values in new columns set to null of the appropriate type.

If there is a common column in x and y, the column from x is kept (i.e. it will not null any columns that exist in x).

q)src:0N!flip`sym`time`price`size!10?'(`3;.z.t;1000f;10000)
 sym time         price    size
 ------------------------------
 mil 10:30:32.148 470.7883 6360
 igf 00:28:17.727 634.6716 7885
 kao 06:52:34.397 967.2398 4503
 baf 10:07:47.382 230.6385 4204
 kfh 00:45:40.134 949.975  6210
 jec 05:12:49.761 439.081  8740
 kfm 16:31:50.104 575.9051 8732
 lkk 04:54:11.685 591.9004 4756
 kfi 13:01:04.698 848.1567 3998
 fgl 05:18:45.828 389.056  9342

q).Q.ff[src] enlist `sym`ratioA`ratioB!3#1
 sym time         price    size ratioA ratioB
 --------------------------------------------
 mil 10:30:32.148 470.7883 6360
 igf 00:28:17.727 634.6716 7885
 kao 06:52:34.397 967.2398 4503
 baf 10:07:47.382 230.6385 4204
 kfh 00:45:40.134 949.975  6210
 jec 05:12:49.761 439.081  8740
 kfm 16:31:50.104 575.9051 8732
 lkk 04:54:11.685 591.9004 4756
 kfi 13:01:04.698 848.1567 3998
 fgl 05:18:45.828 389.056  9342