hi
i want to change values of 2 fiels in mysql, is there any function for it?

Take a look at this:
http://www.w3schools.com/sql/sql_update.asp
Let us know how you get on!

Do you mean change the datatype for the column in the table? or do you mean change the data values themselves. i.e. for record 10 change field1 (column1) to 'alpha' and field2(column2) to 'beta' ?

Second one, suppose field1='a' & field2='b', i want to set field1='b' & field2='a'. & i cant add a third field for keeping value of one of them.
& some records values must change, so i cant rename column name.
in some programming language we have a function for change values of 2 variables, without making third variable, i just want to know is there such a function in mySQL?

use the same technique in sql

I can see 3 variables in use
As Rudy suggests, the same can be achieved using SQL with no problems whatso-ever.
Variables in SQL: http://www.databasejournal.com/featu...le.php/3087431

"change values of 2 variables, without making third variable"

you can do that with sql too

The problem can even be solved with a single variable
1) declare your variable
2) assign the value of Field1 to the variable
3a) update Field1 with value from Field2
3b) update Field2 with value in our variable