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help for keyby                                                   (Roger Newson)
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Key the dataset by a variable list

        keyby varlist [ , noorder missing fast ]


Description

    keyby sorts the dataset currently in memory by the variables in a 
    varlist, checking that the variables in the varlist uniquely identify the
    observations.  This makes the variables in the varlist a primary key for
    the dataset in memory.  If the user does not specify otherwise, then
    keyby also reorders the variables in the varlist to the start of the
    variable order in the dataset, and checks that all values of these
    variables are nonmissing.  keyby can be useful if the user combines
    multiple datasets using merge, which may cause a dataset in memory to
    become unsorted.


Options

    noorder specifies that the variables in the varlist are not reordered to
        the beginning of the variable order of the dataset in memory.  If
        noorder is nor specified, then the variables in varlist are reordered
        to the beginning of the variable order (see order).

    missing specifies that missing values in the variables in the varlist are
        allowed.  If missing is not specified, then missing values in the
        variables in the varlist cause keyby to fail.

    fast is an option for programmers.  It specifies that keyby will take no
        action to restore the existing dataset in memory in the event of
        failure.  If fast is not specified, then keyby will take this action,
        which uses an amount of time depending on the size of the dataset in
        memory.


Examples

        . keyby foreign make

        . keyby foreign make, noorder

        . keyby rep78 make, missing


Author

    Roger Newson, National Heart and Lung Institute, Imperial College London,
    UK.
    Email: r.newson@imperial.ac.uk


Also see

     Manual: [D] sort, [D] gsort, [D] merge, [D] order, [U] 12.2.1 Missing
             values
    On-line: help for sort, gsort, merge, order, missing