The closest thing to STR() that MySQL has is FORMAT().
FORMAT(X,D)
Formats the number X to a format like '#,###,###.##', rounded to D decimals, and returns the result as a string. If D is 0, the result will have no decimal point or fractional part:
mysql> SELECT FORMAT(12332.123456, 4);
-> '12,332.1235'
mysql> SELECT FORMAT(12332.1,4);
-> '12,332.1000'
mysql> SELECT FORMAT(12332.2,0);
-> '12,332'
You may also want to look into CAST or CONVERT.
CAST() and CONVERT() are available as of MySQL 4.0.2.