BY Statement

Controls the operation of a SET, MERGE, MODIFY, or UPDATE statement in the DATA step and sets up special grouping variables

Valid: in a DATA step or a PROC step

Category: File-handling

Type: Declarative

Syntax

BY <DESCENDING> variable-1

< <DESCENDING> variable-n > <NOTSORTED><GROUPFORMAT>;

Arguments

DESCENDING

indicates that the data sets are sorted in descending order by the variable that is specified. DESCENDING means largest to smallest numerically , or reverse alphabetical for character variables.
Restriction: You cannot use the DESCENDING option with data sets that are indexed because indexes are always stored in ascending order.
Featured in: Example 2 on page 1115

GROUPFORMAT

uses the formatted values, instead of the internal values, of the BY variables to determine where BY-groups begin and end, and therefore how FIRST. variable and LAST. variable are assigned. Although the GROUPFORMAT option can appear anywhere in the BY statement, the option applys to all variables in the BY statement.
Restriction: You must sort the observations in a data set based on the value of the BY variables before using the GROUPFORMAT option in the BY statement.
Restriction: You can use the GROUPFORMAT option in a BY statement only in a DATA step.
Tip: Using the GROUPFORMAT option is useful when you define your own formats to display data that is grouped.
Tip: Using the GROUPFORMAT option in the DATA step ensures that BY groups that you use to create a data set match those in PROC steps that report grouped, formatted data.
Interaction: If you also use the NOTSORTED option, you can group the observations in a data set by the formatted value of the BY variables without requiring that the data set be sorted or indexed.
Comparison: BY-group processing in the DATA step using the GROUPFORMAT option is the same as BY-group processing with formatted values in SAS procedures.
See Also: By-Group Processing in the DATA Step in SAS Language Reference: Concepts
Featured in: Example 4 on page 1115

variable

names each variable by which the data set is sorted or indexed. These variables are referred to as BY variables for the current DATA or PROC step.
Tip: The data set can be sorted or indexed by more than one variable.
Featured in: Example 1 on page 1115, Example 2 on page 1115, Example 3 on page 1115, and Example 4 on page 1115

NOTSORTED

specifies that observations with the same BY value are grouped together but are not necessarily sorted in alphabetical or numeric order.
Restriction: You cannot use the NOTSORTED option with the MERGE and UPDATE statements.
Tip: The NOTSORTED option can appear anywhere in the BY statement.
Tip: Using the NOTSORTED option is useful if you have data that falls into other logical groupings such as chronological order or categories.
Featured in: Example 3 on page 1115

Details

How SAS Identifies the Beginning and End of a BY-Group SAS identifies the beginning and end of a BY group by creating two temporary variables for each BY variable: FIRST. variable and LAST. variable . The value of these variables are either 0 or 1. SAS sets the value of FIRST. variable to 1 when it reads the first observation in a BY group, and sets the value of LAST. variable to 1 when it reads the last observation in a BY group. These temporary variables are available for DATA step programming but are not added to the output data set.

For a complete explanation of how SAS processes grouped data and of how to prepare your data, see 'By-Group Processing in the DATA Step' in SAS Language Reference: Concepts .

In a DATA Step The BY statement applies only to the SET, MERGE, MODIFY, or UPDATE statement that precedes it in the DATA step, and only one BY statement can accompany each of these statements in a DATA step.

The data sets that are listed in the SET, MERGE, or UPDATE statements must be sorted by the values of the variables that are listed in the BY statement or have an appropriate index. As a default, SAS expects the data sets to be arranged in ascending numeric order or in alphabetical order. The observations can be arranged by

sorting the data set
creating an index for the variables
inputting the observations in order.

Note: MODIFY does not require sorted data, but sorting can improve performance.

In a PROC Step You can specify the BY statement with some SAS procedures to modify their action. Refer to the individual procedure in the Base SAS Procedures Guide for a discussion of how the BY statement affects processing for SAS procedures.

With SAS Data Views If you are using SAS data views, refer to the SAS documentation for your database management system before you use the BY statement.

Processing BY-Groups SAS assigns the following values to FIRST. variable and LAST. variable :

FIRST. variable has a value of 1 under the following conditions:
- when the current observation is the first observation that is read from the data set.
- when you do not use the GROUPFORMAT option and the internal value of the variable in the current observation differs from the internal value in the previous observation.
  
  If you use the GROUPFORMAT option, FIRST. variable has a value of 1 when the formatted value of the variable in the current observation differs from the formatted value in the previous observation.
- FIRST. variable has a value of 1 for any preceding variable in the BY statement.
  
  In all other cases, FIRST. variable has a value of 0.

LAST. variable has a value of 1 under the following conditions:
- when the current observation is the last observation that is read from the data set.
- when you use the GROUPFORMAT option and the internal value of the variable in the current observation differs from the internal value in the next observation.
  
  If you use the GROUPFORMAT option, LAST. variable has a value of 1 when the formatted value of the variable in the current observation differs from the formatted value in the previous observation.
- LAST. variable has a value of 1 for any preceding variable in the BY statement.
  
  In all other cases, LAST. variable has a value of 0.

Examples

Example 1: Specifying One or More BY Variables

Observations are in ascending order of the variable DEPT:
```
 by dept; 
```
Observations are in alphabetical (ascending) order by CITY and, within each value of CITY, in ascending order by ZIPCODE :
```
 by city zipcode; 
```

Example 2: Specifying Sort Order

Observations are in ascending order of SALESREP and, within each SALESREP value, in descending order of the values of JANSALES:
```
 by salesrep descending jansales; 
```
Observations are in descending order of BEDROOMS, and, within each value of BEDROOMS, in descending order of PRICE:
```
 by descending bedrooms descending price; 
```

Example 3: BY-Processing with Nonsorted Data

Observations are ordered by the name of the month in which the expenses were accrued:

 by month notsorted;

Example 4: Grouping Observations By Using Formatted Values

The following example illustrates the use of the GROUPFORMAT option.

 proc format;     value range        low   55 = 'Under 55'        55   60   = '55 to 60'        60   65   = '60 to 65'        65   70   = '65 to 70'       other   = 'Over 70';  run;  proc sort data=class out=sorted_class;     by height;  run;  data _null_;     format height range.;     set sorted_class;        by height groupformat;     if first.height then        put 'Shortest in ' height 'measures ' height:best12.;  run;

SAS writes the following output to the log:

 Shortest in Under 55 measures 51.3  Shortest in 55 to 60 measures 56.3  Shortest in 60 to 65 measures 62.5  Shortest in 65 to 70 measures 65.3  Shortest in Over 70 measures 72

Example 5: Combining Multiple Observations and Grouping Them Based on One BY Value

The following example shows how to use FIRST. variable and LAST. variable with BY group processing.

 options pageno=1 nodate ls=80 ps=64;  data Inventory;     length RecordID 8 Invoice $ 30 ItemLine $ 50;     infile datalines;     input RecordID Invoice ItemLine &;     drop RecordID;     datalines;  A74  A5296  Highlighters  A75  A5296  Lot # 7603  A76  A5296  Yellow Blue Green  A77  A5296  24 per box  A78  A5297  Paper Clips  A79  A5297  Lot # 7423  A80  A5297  Small Medium Large  A81  A5298  Gluestick  A82  A5298  Lot # 4422  A83  A5298  New item  A84  A5299  Rubber bands  A85  A5299  Lot # 7892  A86  A5299  Wide width, Narrow width  A87  A5299  1000 per box  ;  data combined;     array Line[4] $ 60 ;     retain Line1-Line4;     keep Invoice Line1-Line4;     set Inventory;     by Invoice;     if first.Invoice then do;        call missing(of Line1-Line4);        records = 0;     end;     records + 1;     Line[records]=ItemLine;     if last.Invoice then output;  run;  proc print data=combined;     title 'Office Supply Inventory';  run;

Output 7.2: Output from Combining Multiple Observations

 Office Supply Inventory                           1  Obs     Line1        Line2     Line3                       Line4     Invoice   1   Highlighters  Lot # 7603  Yellow Blue Green        24 per box    A5296   2   Paper Clips   Lot # 7423  Small Medium Large                     A5297   3   Gluestick     Lot # 4422  New item                               A5298   4   Rubber bands  Lot # 7892  Wide width, Narrow width 1000 per box  A5299