Trouble with a UPDATE

Hi!
I'm trying to replace a field  of a table with the value of a field in another table, and I'm writed that:

SQLExecute('UPDATE tableA, tableB SET field_tableA = field_tableB WHERE field_tableA = tableB.id');

tableA                 tableB
field_tableA       tableB.id
                             field_tableB

... and it returns "Error message: near "," : syntax error" ....

could any one help me please??

Thanks!!!