Developing ColdFusion MX Applications with CFML Previous Up One Level Next Home Search
Using Query of Queries


About Query of Queries

After you have created a record set with a tag or function, you can query the record set in several dependent queries. A query that retrieves data from a record set is called a Query of Queries. A typical use of a Query of Queries is to retrieve an entire table into memory with one query, and then access the table data (the record set) with subsequent sorting or filtering queries. In essence, you query the record set as if it were a database table.

Note:   Because you can generate a record set in ways other than using the cfquery tag, the term In Memory Query is sometimes used instead of Query of Queries.

Benefits of Query of Queries

Performing a Query of Queries has many benefits, including the following:

Performing a Query of Queries

There are four steps to perform a Query of Queries.

To perform a Query of Queries:

  1. Generate a record set.

    You can write a normal query using a tag or function that creates a record set. This is sometimes called a master query. For more information, see "Creating a record set".

  2. Write a detail query-a query that specifies dbtype="query" in its cfquery tag.
  3. In the detail query, write a SQL statement that retrieves the relevant records. Specify the names of one or more existing queries as the table names in your SQL code. Do not specify a datasource attribute.
  4. If the database content does not change rapidly, use the cachedwithin attribute of the master query to cache the query results between page requests. This way, ColdFusion accesses the database on the first page request, and does not query the database again until the specified time expires. You must use the CreateTimeSpan function to specify the cachedwithin attribute value (in days, hours, minutes, seconds format).

The detail query generates a new query results set, identified by the value of the name attribute of the detail query. The following example illustrates the use of a master query and a single detail query that extracts information from the master.

To use the results of a query in a query:

  1. Create a ColdFusion page with the following content:
    <body>
<h1>Employee List</h1>
<!--- LastNameSearch (normally generated interactively) --->
<cfset LastNameSearch="Doe">

<!--- Master Query --->
<cfquery datasource="CompanyInfo" name="master"
  cachedwithin=#CreateTimeSpan(0,1,0,0)#>
  SELECT * from Employee
</cfquery>

<!--- Detail Query (dbtype=query, no data source) --->
<cfquery dbtype="query" name="detail"> 
  SELECT Emp_ID, FirstName, LastName
  FROM master
  WHERE LastName=<cfqueryparam value="#LastNameSearch#" 
cfsqltype="cf_sql_char" maxLength="20"></cfquery>

<!--- output the detail query results --->
<p>Output using a query of query:</p>
<cfoutput query=detail>
  #Emp_ID#: #FirstName# #LastName#<br>
</cfoutput>
<br>

<p>Columns in the master query:</p>
<cfoutput>
  #master.columnlist#<br>
</cfoutput>
<br>

<p>Columns in the detail query:</p>
<cfoutput>
  #detail.columnlist#<br>
</cfoutput>
</body>
  2. Save the page as query_of_query.cfm in the myapps directory under the web_root.
  3. In your browser, enter the following URL to display the queryresults:

    http://127.0.0.1/myapps/query_of_query.cfm

    The following figure shows how the output appears:

    Using the results of a query in a query.

Reviewing the code

The master query retrieves the entire Employee table from the CompanyInfo data source (the CompanyInfo database). The detail query selects only the three columns to display for employees with the specified last name. The following table describes the code and its function:
Code
Description
 
cfset LastNameSearch="Doe"
Sets the last name to use in the detail query. In a complete application, this information comes from user interaction.
 
<cfquery datasource="CompanyInfo" name="master"
  cachedwithin=#CreateTimeSpan(0,1,0,0)#>
  SELECT *   from Employee
</cfquery>
Queries the CompanyInfo data source and selects all data in the Employees table. Caches the query data between requests to this page, and does not query the database if the cached data is less than an hour old.
 
<cfquery dbtype="query" name="detail">
  SELECT Emp_ID, FirstName, LastName
  FROM master
  WHERE LastName=<cfqueryparam
  value="#LastNameSearch#" 
  cfsqltype="cf_sql_char" 
  maxLength="20"></cfquery>
Uses the master query as the source of the data in a new query, named detail. This new query selects only entries that match the last name specified by the LastNameSearch variable. The query also selects only three columns of data: employee ID, first name, and last name. The query uses the cfqueryparam tag to prevent passing erroneous or harmful code.
 
<cfoutput query=detail>
  #Emp_ID#: #FirstName# #LastName# <br>
</cfoutput>
Uses the detail query to display the list of employee IDs, first names, and last names.
 
<cfoutput>
  #master.columnlist#<br>
</cfoutput>
Lists all the columns returned by the master query.
 
<cfoutput>
  #detail.columnlist#<br>
</cfoutput>
Lists all the columns returned by the detail query.

Displaying record set data incrementally

If your database is large, you can limit the number of rows displayed at one time. The following example shows how to use the currentRow query variable of a Query of Queries to do this. For more information on query variables, see "Getting information about query results".

To display record set data incrementally:

  1. Create a ColdFusion page with the following content:
    <html>
<head>
<title>QoQ with incremental row return</title>
</head>

<body>
<h3>QoQ with incremental row return</h3>
<!--- define startrow and maxrows to facilitate 'next N' style browsing --->
<cfparam name = "MaxRows" default = "5">
<cfparam name = "StartRow" default = "1">

<!--- master query: retrieve all info from Employee table --->
<cfquery name = "GetSals" datasource = "CompanyInfo">
  SELECT * FROM  Employee
  ORDER BY LastName
</cfquery>

<!--- detail query: select 3 fields from the master query --->
<cfquery name = "GetSals2" dbtype = "query">
  SELECT   FirstName, LastName, Salary
  FROM     GetSals
  ORDER BY LastName
</cfquery>

<!--- build table to display output --->
<table cellpadding = 1 cellspacing = 1>
  <tr>
    <td bgcolor = f0f0f0>
    <b><i>&nbsp;</i></b>
    </td>

    <td bgcolor = f0f0f0>
    <b><i>FirstName</i></b>
    </td>

    <td bgcolor = f0f0f0>
    <b><i>LastName</i></b>
    </td>

    <td bgcolor = f0f0f0>
    <b><i>Salary</i></b>
    </td>
  </tr>
  
<!--- Output the query and define the startrow and maxrows
     parameters. Use the query variable currentRow to
     keep track of the row you are displaying. --->
  <cfoutput query = "GetSals2" startrow = "#StartRow#" maxrows = "#MaxRows#">
  <tr>
    <td valign = top bgcolor = ffffed>
      <b>#GetSals2.currentRow#</b>
    </td>

    <td valign = top>
      <font size = "-1">#FirstName#</font>
    </td>

    <td valign = top>
      <font size = "-1">#LastName#</font>
    </td>

    <td valign = top>
      <font size = "-1">#Salary#</font>
    </td>
  </tr>
  </cfoutput>
<!--- If the total number of records is less than or equal to 
  the total number of rows, provide a link to the same page, with the 
  StartRow value incremented by MaxRows (5, in this example) --->
  <tr>
    <td colspan = 4>
    <cfif (startrow + maxrows) lte getsals2.recordcount>
      <a href="qoq_next_row.cfm?startrow=<cfoutput>#Evaluate(StartRow +
MaxRows)#</cfoutput>">See next <cfoutput>#MaxRows#</cfoutput> 
      rows</a>
    </cfif>    
    </td>
  </tr>
</table>
</body>
</html>
  2. Save the page as qoq_next_row.cfm in the myapps directory under the web_root.
  3. In your web browser, enter the following URL to display the query results:

    http://127.0.0.1/myapps/qoq_next_row.cfm

    The following figure shows how the output appears:

    Displaying record set data incrementally

Using the cfdump tag with query results

As you debug your CFML code, you can use the cfdump tag to quickly present the value of your query. This tag has the following format: 

<cfdump var="#query_name#">

For more information on the cfdump tag, see CFML Reference.

Using Query of Queries with non-SQL record sets

A Query of Queries can operate on any CFML tag or function that returns a record set; you are not limited to operating on cfquery results. You can perform queries on non-SQL record sets, such as a cfdirectory tag, Verity searches, a cfldap tag, and so on.

The following example shows how a Query of Queries interacts with the record set of a Verity search. This example assumes that you have a valid Verity collection, called bbb, which contains documents with a target word, film, or its variants (films, filmed, filming). Change the name of the collection and the search criteria to as appropriate for your Verity collection. For more information on Verity, see Chapter 24, "Building a Search Interface".

To use Query of Queries with a Verity record set:

  1. Create a ColdFusion page with the following content:
    <html>
<head>
<title>QoQ and Verity</title>
</head>

<body>
<!--- master query: retrieve all documents from the bbb collection 
that contain 'film' (or its stemmed variants); change values for 
collection and criteria as needed for your Verity collection --->
<cfsearch name = "quick"
  collection="bbb" 
  type = "simple"
  criteria="film">

<h3>Master query dump:</h3>
<cfdump var="#quick#">

<!--- detail query: retrieve from the master query only those 
documents with a score greater than a criterion (here, 
0.7743) --->
<cfquery name="qoq" dbtype="query">
  SELECT * from quick
  WHERE quick.score > 0.7743
</cfquery>

<h3>Detail query dump:</h3>
<cfdump var="#qoq#">

</body>
</html>
  2. Save the page as qoq_verity.cfm in the myapps directory under the web_root.
  3. In your web browser, enter the following URL to display the query results:

    http://127.0.0.1/myapps/qoq_verity.cfm

    The following figure shows how the output appears:

    Using a query of queries with a Verity data set

    Note:   This figure shows a collapsed master query output and an expanded detail query output. Click an output to expand or collapse it.

    The first cfdump tag shows the master query, which retrieves all records. The second cfdump shows the Query of Queries results.

    Tip:   Adjust the score criterion of the detail query to reflect the contents of your collection.

The next example shows how a Query of Queries combines record sets from a cfdirectory tag, which is limited to retrieval of one file type per use.

To use Query of Queries to combine record sets:

  1. Create a ColdFusion page with the following content:
    <html>
<head>
<title>Images Folder</title>
</head>

<body>
<h2>Image Retrieval with QoQ</h2> 
<!--- set the images directory --->
<cfset dir = ("C:\pix\")>

<!--- retrieve all GIFs --->
<cfdirectory name="GetGIF"
  action="list"
  directory="#dir#"
  filter="*.gif">

<!--- retrieve all JPGs --->
<cfdirectory name="GetJPG"
  action="list"
  directory="#dir#"
  filter="*.jpg">
  
<!--- join the queries with a UNION in a QoQ (cfdirectory 
      automatically returns the directory name as "Name") --->
<cfquery dbtype="query" name="GetBoth">
  SELECT * FROM GetGIF
  UNION 
  SELECT * FROM GetJPG
  ORDER BY Name
</cfquery>

<!--- display output in a linked, ordered list --->
<cfoutput>
  <p>The <strong>#dir#</strong> directory contains #GetBoth.RecordCount#
images:<br>
  <ol>
    <cfloop query="GetBoth">
      <li><a href="../images/#Name#">#GetBoth.Name#</a><br>
    </cfloop>  
  </ol>
</cfoutput>

</body>
</html>
  2. Save the page as qoq_cfdirectory.cfm in the myapps directory under the web_root.
  3. In your web browser, enter the following URL to display the query results:

    http://127.0.0.1/myapps/qoq_cfdirectory.cfm

    The following figure shows how the output appears:

    Using query of queries to combine record sets

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