[dotsch]

My take:

q)t1:([]col1:`a`b`c`d;col2:1 2 3 8;col3:11 22 33 88)
q)t2:([]col1:`a`a`b`c;col4:4 5 6 7;col5:44 55 66 77)
q)ej[`col1;t1;t2] uj select from t1 where not col1 in t2.col1
col1 col2 col3 col4 col5
------------------------
a    1    11   4    44
a    1    11   5    55
b    2    22   6    66
c    3    33   7    77
d    8    88

[dotsch]

It’s not only the disk space, performance will suffer. I guess this is why there is no trivial way to create such table (but it is possible).
Why do you want to spare the sym file?