Select character variables that have all missing values - sql

Question

I have a SAS dataset with around 3,000 variables, and I would like to get rid of the character variables for which all values are missing. I know how to do this for numeric variables-- I'm wondering specifically about the character variables. I need to do the work using base SAS, but that could include proc SQL, which is why I've tagged this one 'SQL' also.
Thank you!
Edit:
Background info: This is a tall dataset, with survey data from 7 waves of interviews. Some, but not all, of the survey items (variables) were repeated across waves. I'm trying to create a list of items that were actually used in each wave by pulling all the records for that wave, getting rid of all the columns that have nothing but SAS's default missing values, and then running proc contents.

Answer 1

I created a macro that will check for empty character columns and either remove them from the original or create a new data set with the empty columns removed. It takes two optional arguments: The name of the data set (default is the most recently created data set), and a suffix to name the new copy (set suffix to nothing to edit the original).
It uses proc freq with the levels option and a custom format to determine the empty character columns. proc sql is then used to create a list of the columns to be removed and store them in a macro variable.
Here is the macro:
%macro delemptycol(ds=_last_, suffix=_noempty);
option nonotes;
proc format;
value $charmiss
' '= ' '
other='1';
run;
%if "&ds"="_last_" %then %let ds=&syslast.;
ods select nlevels;
ods output nlevels=nlev;
proc freq data=&ds.(keep=_character_) levels ;
format _character_ $charmiss.;
run;
ods output close;
/* create macro var with list of cols to remove */
%local emptycols;
proc sql noprint;
select tablevar into: emptycols separated by ' '
from nlev
where NNonMissLevels=0;
quit;
%if &emptycols.= %then %do;
%put DELEMPTYCOL: No empty character columns were found in data set &ds.;
%end;
%else %do;
%put DELEMPTYCOL: The following empty character columns were found in data set &ds. : &emptycols.;
%put DELEMPTYCOL: Data set &ds.&suffix created with empty columns removed;
data &ds.&suffix. ;
set &ds(drop=&emptycols);
run;
%end;
options notes;
%mend;
Examples usage:
/* create some fake data: Here char5 will be empty */
data chardata(drop= j randnum);
length char1-char5 $8.;
array chars(5) char1-char5;
do i=1 to 100;
call missing(of char:);
randnum=floor(10*ranuni(i));
do j=2 to 5;
if (j-1)<randnum<=(j+1) then chars(j-1)="FOO";
end;
output;
end;
run;
%delemptycol(); /* uses default _last_ for the data and "_noempty" as the suffix */
%delemptycol(ds=chardata, suffix=); /* removes the empty columns from the original */

Answer 2

There's probably a simpler way but this is what I came up with.
Cheers
Rob
EDIT: Note that this works for both character and numeric variables.
**
** TEST DATASET
*;
data x;
col1 = "a"; col2 = ""; col3 = "c"; output;
col1 = "" ; col2 = ""; col3 = "c"; output;
col1 = "a"; col2 = ""; col3 = "" ; output;
run;
**
** GET A LIST OF VARIABLE NAMES
*;
proc sql noprint;
select name into :varlist separated by " "
from sashelp.vcolumn
where upcase(libname) eq "WORK"
and upcase(memname) eq "X";
quit;
%put &varlist;
**
** USE A MACRO TO CREATE A DATASTEP. FOR EACH COLUMN THE
** THE DATASTEP WILL CREATE A NEW COLUMN WITH THE SAME NAME
** BUT PREFIXED WITH "DELETE_". IF THERE IS AT LEAST 1
** NON-MISSING VALUE FOR THE COLUMN THEN THE "DELETE" COLUMN
** WILL FINISH WITH A VALUE OF 0, ELSE 1. WE WILL ONLY
** KEEP THE COLUMNS CALLED "DELETE_" AND OUTPUT ONLY A SINGLE
** OBSERVATION TO THE FINAL DATASET.
*;
%macro find_unused_cols(iDs=);
%local cnt;
data vars_to_delete;
set &iDs end=eof;
%let cnt = 1;
%let varname = %scan(&varlist, &cnt);
%do %while ("&varname" ne "");
retain delete_&varname;
delete_&varname = min(delete_&varname, missing(&varname));
drop &varname;
%let cnt = %eval(&cnt + 1);
%let varname = %scan(&varlist, &cnt);
%end;
if eof then do;
output;
end;
run;
%mend;
%find_unused_cols(iDs=x);
**
** GET A LIST OF VARIABLE NAMES FROM THE NEW DATASET
** THAT WE WANT TO DELETE AND STORE TO A MACRO VAR.
*;
proc transpose data=vars_to_delete out=vars_to_delete;
run;
proc sql noprint;
select substr(_name_,8) into :vars_to_delete separated by " "
from vars_to_delete
where col1;
quit;
%put &vars_to_delete;
**
** CREATE A NEW DATASET CONTAINING JUST THOSE VARS
** THAT WE WANT TO KEEP
*;
data new_x;
set x;
drop &vars_to_delete;
run;

Answer 3

Rob and cmjohns, thank you SO MUCH for your help. Based on your solutions and an idea I had over the weekend, here is what I came up with:
%macro removeEmptyCols(origDset, outDset);
* get the number of obs in the original dset;
%let dsid = %sysfunc(open(&origDset));
%let origN = %sysfunc(attrn(&dsid, nlobs));
%let rc = %sysfunc(close(&dsid));
proc transpose data= &origDset out= transpDset;
var _all_;
run;
data transpDset;
set transpDset;
* proc transpose converted all old vars to character,
so the . from old numeric vars no longer means 'missing';
array oldVar_ _character_;
do over oldVar_;
if strip(oldVar_) = "." then oldVar_ = "";
end;
* each row from the old dset is now a column with varname starting with 'col';
numMiss = cmiss(of col:);
numCols = &origN;
run;
proc sql noprint;
select _NAME_ into: varsToKeep separated by ' '
from transpDset
where numMiss < numCols;
quit;
data &outDset;
set &origDset (keep = &varsToKeep);
run;
%mend removeEmptyCols;
I will try all 3 ways and report back on which one is fastest...
P.S. added 23 Dec 2010 for future reference: SGF Paper 048-2010: Dropping Automatically Variables with Only Missing Values

Answer 4

This is very simple method useful for all variables
proc freq data=class nlevels ;
ods output nlevels=levels(where=(nmisslevels>0 and nnonmisslevels=0));
run;
proc sql noprint;
select TABLEVAR into :_MISSINGVARS separated by ' ' from levels;
quit;
data want;
set class (keep=&_MISSINGVARS);
run;