stata运算符

                                                         Relational
         Arithmetic              Logical            (numeric and string)
    --------------------     ------------------     ---------------------
     +   addition                &   and               >   greater than
     -   subtraction             |   or                <   less than
     *   multiplication          !   not               >=  > or equal
     /   division                ~   not               <=  < or equal
     ^   power                                         ==  equal
     -   negation                                      !=  not equal
     +   string concatenation                          ~=  not equal

    A double equal sign (==) is used for equality testing.

    The order of evaluation (from first to last) of all operators is ! (or ~), ^, - (negation), /,
    *, - (subtraction), +, != (or ~=), >, <, <=, >=, ==, &, and |.