The MERGE statement takes two or more identically sequenced files and
  combines them according to the key values you specify.  During the
  process, it makes records available, in merged order, to routines in
  OUTPUT PROCEDURE or to an output file.

  Format -

   MERGE mergefile

      { ON { ASCENDING  } KEY { mergekey } ... } ...
      {    { DESCENDING }                      }

      [ COLLATING SEQUENCE IS alpha ]

       USING infile { infile } ...

      { OUTPUT PROCEDURE IS first-proc [ { THRU    } end-proc ] }
      {                                [ { THROUGH }          ] }
      { GIVING { outfile } ...                                  }

1  –  mergefile

  is a file-name described in a sort-merge file description (SD) entry in
  the Data Division.

2  –  mergekey

  is the data-name of a data item in a record associated with mergefile.

3  –  alpha

  is an alphabet-name defined in the SPECIAL-NAMES paragraph of the
  Environment Division.

4  –  infile

  is the file-name of an input file.  It must be described in a file
  description (FD) entry in the Data Division.

5  –  first-proc

  is the section-name or paragraph-name of the output procedure's first
  (or only) section or paragraph.

6  –  end-proc

  is the section-name or paragraph-name of the output procedure's last section
  or paragraph.

7  –  outfile

  is the file-name of an output file.  It must be described in a file
  description (FD) entry in the Data Division.