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194 lines
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<meta name="DC.Title" content="Select and omit records using logical files" />
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<meta name="abstract" content="The system can select and omit records when using a logical file. This can help you exclude records in a file for processing convenience or for security." />
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<title>Select and omit records using logical files</title>
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<body id="rbafosorec"><a name="rbafosorec"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Select and omit records using logical files</h1>
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<div><p>The system can select and omit records when using a logical file.
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This can help you exclude records in a file for processing convenience or
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for security.</p>
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<p>The process of selecting and omitting records is based on comparisons identified
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in position 17 of the DDS form for the logical file, and is similar to a series
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of comparisons coded in a high-level language program. For example, in a logical
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file that contains order detail records, you can specify that the only records
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you want to use are those in which the quantity ordered is greater than the
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quantity shipped. All other records are omitted from the access path. The
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omitted records remain in the physical file but are not retrieved for the
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logical file. If you are adding records to the physical file, all records
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are added, but only selected records that match the select/omit criteria can
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be retrieved using the select/omit access path.</p>
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<div class="p">In DDS, to specify select or omit, you specify an S (select) or O (omit)
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in position 17 of the DDS form. You then name the field (in positions
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19 through 28) that will be used in the selection or omission process. In
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positions 45 through 80 you specify the comparison. <div class="note"><span class="notetitle">Note:</span> Select/omit specifications
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appear after key specifications (if keys are specified).</div>
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</div>
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<div class="p">Records can be selected and omitted by several
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types of comparisons: <ul><li>VALUES. The contents of the field are compared to a list of not more than
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100 values. If a match is found, the record is selected or omitted. In the
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following example, a record is selected if one of the values specified in
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the VALUES keyword is found in the <em>Itmnbr</em> field. <pre>|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
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A S ITMNBR VALUES(301542 306902 382101 422109 +
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A 431652 486592 502356 556608 590307)
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A</pre>
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</li>
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<li>RANGE. The contents of the field are compared
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to lower and upper limits. If the contents are greater than or equal to the
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lower limit and less than or equal to the upper limit, the record is selected
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or omitted. In the following example, all records with a range 301000 through
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599999 in the <em>Itmnbr</em> field are selected.<pre>|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
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A S ITMNBR RANGE(301000 599999)
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A</pre>
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</li>
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<li>CMP.
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The contents of a field are compared to a value or the contents of another
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field. Valid comparison codes are EQ, NE, LT, NL, GT, NG, LE, and GE. If the
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comparison is met, the record is selected or omitted. In the following example,
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a record is selected if its <em>Itmnbr</em> field is less than or equal to 599999:</li>
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</ul>
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</div>
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<pre>|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
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A S ITMNBR CMP(LE 599999)
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A</pre>
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<p>The value for a numeric field for which the CMP, VALUES, or RANGE keyword
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is specified is aligned based on the decimal positions specified for the field
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and filled with zeros where necessary. If decimal positions were not specified
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for the field, the decimal point is placed to the right of the farthest right
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digit in the value. For example, for a numeric field with length 5 and decimal
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position 2, the value 1.2 is interpreted as 001.20 and the value 100 is interpreted
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as 100.00.</p>
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<p>The status of a record is determined by evaluating select/omit statements
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in the sequence you specify them. If a record qualifies for selection or omission,
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subsequent statements are ignored.</p>
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<p>Normally the select and omit comparisons are treated independently from
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one another; the comparisons are ORed together. That is, if the select or
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omit comparison is met, the record is either selected or omitted. If the condition
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is not met, the system proceeds to the next comparison. To connect comparisons
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together, you leave a space in position 17 of the DDS form. Then, all the
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comparisons that were connected in this fashion must be met before the record
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is selected or omitted. That is, the comparisons are ANDed together.</p>
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<p>The fewer comparisons, the more efficient the task is. So, when you have
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several select/omit comparisons, try to specify the one that selects or omits
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the most records first.</p>
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<p>The following examples show ways to code select/omit functions. In these
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examples, few records exist for which the <em>Rep</em> field is JSMITH. The
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examples show how to use DDS to select all the records before 1988 for a sales
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representative named JSMITH in the state of New York. All give the same results
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with different efficiency. <strong>3</strong> shows the most efficient way.</p>
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<pre>|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
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A S ST CMP(EQ 'NY') <strong>1</strong>
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A REP CMP(EQ 'JSMITH')
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A YEAR CMP(LT 88)
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A
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|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
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A O YEAR CMP(GE 88) <strong>2</strong>
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A S ST CMP(EQ 'NY')
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A REP CMP(EQ 'JSMITH')
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A
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|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
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A O REP CMP(NE 'JSMITH') <strong>3</strong>
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A O ST CMP(NE 'NY')
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A S YEAR CMP(LT 88)
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A</pre>
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<dl><dt class="dlterm">1</dt>
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<dd>All records must be compared with all of the select fields <em>St</em>, <em>Rep</em>,
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and <em>Year</em> before they can be selected or omitted.</dd>
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<dt class="dlterm">2</dt>
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<dd>All records are compared with the <em>Year</em> field. Then, the records
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before 1988 must be compared with the <em>St</em> and <em>Rep</em> fields.</dd>
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<dt class="dlterm">3</dt>
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<dd>All records are compared with the <em>Rep</em> field. Then, only the few
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for JSMITH are compared with the <em>St</em> field. Then, the few records that
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are left are compared to the <em>Year</em> field.</dd>
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</dl>
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<div class="p">As another example, assume that you want to select the following items: <ul><li>All records for departments other than Department 12.</li>
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<li>Only those records for Department 12 that contain an item number 112505,
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428707, or 480100. No other records for Department 12 are to be selected.</li>
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</ul>
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</div>
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<p>If you create the preceding example with a sort sequence table, the select/omit
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fields are translated according to the sort table before the comparison. For
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example, with a sort sequence table using shared weightings for uppercase
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and lowercase, <samp class="codeph">NY</samp> and <samp class="codeph">ny</samp> are equal. </p>
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<p>The following diagram shows the logic included in this example:</p>
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<br /><img src="rbafo504.gif" alt="Example of a record" /><br /><p>This example is coded as follows using the DDS select and omit functions:</p>
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<pre>|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
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A S DPTNBR CMP(NE 12)
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A S ITMNBR VALUES(112505 428707 480100)
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A</pre>
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<p>It is possible to have an access path with select/omit values and process
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the file in arrival sequence. For example, a high-level language program can
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specify that the keyed access path is to be ignored. In this case, every record
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is read from the file in arrival sequence, but only those records meeting
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the select/omit values specified in the file are returned to the high-level
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language program.</p>
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<p>A logical file with key fields and select/omit values specified can be
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processed in arrival sequence or using relative record numbers randomly. Records
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omitted by the select/omit values are not processed. That is, if an omitted
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record is requested by relative record number, the record is not returned
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to the high-level language program.</p>
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<div class="p">The system does not ensure that any additions or changes through a logical
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file will allow the record to be accessed again in the same logical file.
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For example, if the selection values of the logical file specifies only records
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with an A in <em>Fld1</em> and the program updates the record with a B in <em>Fld1</em>,
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the program cannot retrieve the record again using this logical file. <div class="note"><span class="notetitle">Note:</span> You
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cannot select or omit based on the values of a floating-point field.</div>
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</div>
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<p>The two kinds of select/omit operations are: access path select/omit and
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dynamic select/omit. The default is access path select/omit. The select/omit
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specifications themselves are the same in each kind, but the system actually
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does the work of selecting and omitting records at different times. </p>
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<p>You can also use the Open Query File (OPNQRYF) command to select or omit
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records. </p>
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</div>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="rbafoapso.htm">Access path select/omit</a></strong><br />
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With the access path select/omit operation, the access path only contains keys that meet the select/omit values specified for the logical file.</li>
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<li class="ulchildlink"><strong><a href="rbafoapdyn.htm">Dynamic select/omit</a></strong><br />
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With the dynamic select/omit operation, when a program reads records from the file, the system only returns those records that meet the select/omit values. That is, the actual select/omit processing is done when records are read by a program, rather than when the records are added or changed.</li>
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<li class="ulchildlink"><strong><a href="rbafoapopn.htm">Use the Open Query File (OPNQRYF) command to select and omit records</a></strong><br />
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Another method of selecting records is using the QRYSLT parameter on the Open Query File (OPNQRYF) command. The open data path created by the OPNQRYF command is like a temporary logical file; that is, it is automatically deleted when it is closed. A logical file, however, remains in existence until you specifically delete it.</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafodaplf.htm" title="Use one of the ways to describe access paths for logical files.">Describe access paths for logical files</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="../dds/kickoff.htm">DDS concepts</a></div>
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</div>
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</div>
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