Column Equals Two Columns

Databases
1

i need to get the data from two columns (client, stknum) into a column named clistknum.

how do I modify this query (UPDATE plan SET clistknum = client) to get the client AND the stknum data into the clistknum column?

2

this query (UPDATE plan SET clistknum = CONCAT(client,stknum)) does the job , but how do i get an underscore between the data?

3

CONCAT_WS ! What will they think of next?