Key column/s of a table

keys x    keys[x]

Where x is a table (by value or reference), returns as a symbol vector the primary key column/s of x – empty if none.

q)\l trade.q        / no keys
q)keys trade
`symbol$()
q)keys`trade
`symbol$()
q)`sym xkey`trade   / define a key
q)keys`trade
,`sym