mysql contracting 2 columns

How could I contract 2 columns, i.e. replace 2 columns with one?

EX:

| column a  | column b |     INTO     | column c |
------------------------              ------------
|     1     |     a    |              |    1-a   |
|     2     |     b    |       ?      |    2-b   |
|     3     |     c    |              |    3-b   |

With a single query?

I'm using 2 loops with :

for ($i=1; $i<=$x; $i++) {
for ($j=1; $j<=$y; $j++) {
sql ="SELECT * FROM table WHERE column 1 = '".$i."' AND column 2 = '".$j."'"
}
}

And would like to

sql ="SELECT * FR开发者_开发问答OM table WHERE column 3 = '".$i."-".$j."'"

Would this speed things up?

You can run a query like.

Select CONCAT(ColumnA,'-',ColumnB) AS ColumnC FROM MyTable;

Can you provide more information? Based off what you've asked so far, the following would do it:

SELECT CONCAT(a, '-', b) AS c
FROM theTable

CREATE TABLE temp (c VARCHAR(32)) SELECT CONCAT(a, "-", b) FROM foo;